The Iapetus Beat

LPSC 41 Recap, Part 1: South Pole-Aitken Basin

2010-03-11T00:31:00.001-06:00

The Moon was the major topic at the 41^st Lunar and Planetary Science Conference (LPSC) last week. Several sessions comprised results from Lunar Reconnaissance Orbiter (LRO), Chandrayaan-1, Selene, and Chang’e-1, and others combined remote sensing data coupled with lunar petrology and experimental work. There was also a session dedicated to lunar water. All abstracts are available through the Program with Abstracts on the website. The Planetary Science Blog posted a good overview of much of the LRO content.

I caught many of the lunar talks but I spent some time in Mars, terrestrial impact, asteroid, and Mercury MESSENGER sessions, too. Regarding the last, there are few new data and not much new interpretation since I wrote about Mercurian volcanism and crustal composition in November, so the session was a bit of a tease. The Mercury MESSENGER orbital insertion is scheduled for March 18, 2011, and there should be reams of great data soon after.

Of the lunar sites discussed, the South Pole-Aitken basin (SPA) received a good dose of attention. SPA is on the lunar farside, extending ~2500 km from the South Pole to Aitken crater. It’s the largest and deepest impact structure on the moon and one of the largest in the solar system. The highest mountains on the moon are remnants of SPA’s uplifted rim and the lowest points lie near its center. The impact excavated so deeply that the bottom of the basin contains melt sheets from lower lunar crust or even the upper mantle. Because of this, SPA represents an entirely different lunar composition from the highlands or mare samples.

The South Pole-Aitken basin is the roughly circular anomaly on the southern far side.

Sasaki et al. studied crustal structure of SPA with Kaguya (Selene) data. They used laser altimetry to refine the topography and the first precise gravity data to model depth to the Moho (the crust-mantle interface of a rocky planet is known as the Mohorovocic discontinuity, or Moho). When crust is thin, dense mantle will rise to compensate and maintain isostatic equilibrium. The Sasaki study’s model interprets the Moho to be at a depth of 60-80 km around the basin and around 35 km in the basin. Thus, they suggest that that lower crust, but not mantle, is exposed in SPA.

Even if the Moho is well below the surface, mantle melting may have contributed to the impact melt sheet in the basin center, as suggested by Potter et al. based on computer impact modeling with a resolution of 30 km/cell. Depending on the degree of mantle melting, the melt composition will vary from near-mantle compositions to basaltic, and the commensurate density could mimic a range of mantle-lower crust-crust mixtures. Since gravity models only "see" density and thickness, they can't constrain composition in such cases.

(Both the Sasaki et al., and Potter et al. studies compliment the past work in their fields, and I’ll leave it to the interested reader to follow up on the references in their abstracts.)

One of the papers I found most interesting was the Petro et al. study on the Apollo Basin, a 480 km crater in the northeast section of SPA. They investigated compositions in Apollo using spectral data from the Moon Mineralogy Mapper (M³) – a NASA instrument aboard Chandrayaan-1 – and compared the Apollo lithologies to the rest of SPA and to the far side highlands outside of SPA. The high resolution (1.4 km/pixel) and quality of the data allowed them to recognize: (1) “anorthositic” rocks in the uplifted center rim of Apollo (anorthosite is a rock that composes the upper lunar crust and contains more than 90% plagioclase feldspar, and Petro et al. call the Apollo rocks “anorthositic” because they approach the composition of anorthosite but contain more mafic [Mg-Fe] minerals); and (2) noritic materials exposed in some of the deeper craters within Apollo (norite has a composition intermediate between anorthosite and a mafic or a ultramafic [mantle] composition). The crust in Apollo is very thin, and because it lies near the edge of SPA the overlying impact melt sheet is relatively thin as well. Petro et al. interpret the anorthositic material to be a layer in the crust below the true anorthosite higher in the section, and the noritic lithology to be an even deeper level of preserved lunar crust.

Apollo crater from Petro et al.'s Figure 1. (A) Albedo image with dashed lines showing inner (240 km) and outer (480 km) crater rings. (B) Long wavelength image showing topography. (C) False color image: deep blue, as in the southwest inner crater rim, is anorthositic material; deep green, as in crater between the NW rims, is noritic. Yellows are basaltic impact melt.

If Petro et al. are correct, this adds to the reasons why SPA is an ideal site for a sample return mission. Not only might it have preserved crustal stratigraphy not exposed anywhere else, but it is far from the nearside mare basalts and impact ejecta that are mixed into our previous samples (U.S. Apollo 11-17 and Russian Luna missions). The other primary reason to target SPA is its age. It is known to be the oldest lunar impact basin based on mapping of ejecta blankets and on crater counts, but its absolute age is unknown. A precise radiometric age would be an anchor to all of the other relative lunar ages and would place an upper limit on basin-forming impacts in the inner solar system. An SPA impact melt age would constrain the early history of the Moon and, to a significant extent, of the Earth.

Exterior of lunar meteorite Dho 961, a glassy matrix regolith breccia. Ticks are in mm's. (Image from Washington University)

One of the three proposed New Frontiers NASA missions is a sample return from SPA called MoonRise. I personally hope it goes through, though all three missions are worthwhile. In the near-term, there are some exciting developments in lunar meteorites that might allow for study of SPA without a mission. Based on its composition and mineralogy, the group at Washington University have identified meteorite Dhofar 961 as possibly coming from SPA. They presented some of this work at the 2009 LPSC and, this year, Korotev et al. and Zeigler et al. both presented evidence that several other lunar meteorites are directly related to Dho 961. It’s an exciting hypothesis and they’re building a good case. Many scientists (including some in my group) are waiting for pieces of these samples to date them.

A Review of the Chicxulub impact extinction link

2010-03-08T07:20:00.006-06:00

There's a new Science paper by Schulte and others espousing the link between the Chicxulub impact event and the mass extinction at the end of the Cretaceous (K), ~65.5 million years ago, and it has received significant media attention. The forty-one (!) pro-bolide scientists review the theory and evidence that has accumulated since the seminal Alvarez paper in 1980, which proposed a link between an extraterrestrial impact and the extinction, and the Hildebrand et alia (1991) study, which reported the discovery of the buried Chicxulub crater on the Yucatán Peninsula. As mentioned elsewhere, its list of authors does not, as far as I know, include recently converted anti-bolidists and does not represent the practical dissolution of debate, regardless of the media’s representations. I think, however, that the argument for the Chicxulub-impact model is strong and getting stronger.

Map of gravity anomalies in the Chixculub crater. The circular portion is ~180 km across. The white line is the superimposed coast of the Yucatán Peninsula. Image from the Geological Society of Canada.

There are plenty of scientists who disagree with the Chicxulub-extinction link. One of the most prominent is Gerta Keller from Princeton, whose work has been in the media in recent years (see Science Daily, for example). In 2004, the Geological Society of London hosted a written debate between Keller and some pro-bolidists (found here) – and she has advanced her case since then. Keller disputes the causation based largely on sedimentary deposits in the crater itself, in northern Mexico and Texas, and elsewhere. Keller and her coauthors report locales where the interpreted Chicxulub impact ejecta layer significantly underlies the last occurrence of Cretaceous microfossils; thus, they interpret as much as 300,000 years between the impact and the mass extinction. Strictly speaking, Keller’s work, especially pre-2009, supports either multiple impacts or the flood basalt volcanism of the Deccan Traps as likely culprits, but in her recent work she more directly invokes the volcanism. Schulte and coauthors cite a number of sedimentological and paleontological studies that disagree with Keller’s interpretations: for example, Bralower, one of the 41 authors, leads a new Geology paper providing suggesting that the Keller work grossly underestimates sedimentation rate in the deposits proximal to Chicxulub; Schulte et alia also cite studies that interpret some of Keller’s deposits to be reworked and therefore not representative of primary depositional relationships.

Unsurprisingly, opponents of the Chicxulub-extinction link, like Norman Macleod (quoted here), suggest that the authors don’t address key counter evidence even while the authors claim they’ve cinched the case. I’m not in the best position to evaluate the sedimentological arguments, and there are other lines of arguments I won’t address here, but rest assured they are plentiful. My reading of the literature suggests that most scientists now agree that there was a large impact near the time of the extinction, and the disputes tend to be over the exact timing and the effects, and whether the effects of volcanism superseded or added to the impact’s effects. Likewise, most scientists who agree with Schulte and others probably recognize that extinction events are complicated – for instance, that dinosaurs were fading out for millions of years before the end of the Cretaceous. As the media is prone to do, they’ve exaggerated the finality of this paper’s assertions, and we'll hear much more about the topic. In the meantime, I find Schulte study’s conclusions to be convincing, namely that while there are environmental models that need work:

…alternative multi-impact or volcanic hypotheses fail to explain the geographic and stratigraphic distribution of ejecta and its composition, the timing of the mass extinction, and the scale of environmental changes required to cause it.

Bryan at In Terra Veritas discusses the Schulte study in the first relevant post to appear on Research Blogging. While he’s open to the Chicxulub hypothesis, he objects to parts of the paper and to the media’s coverage of the debate based on [1] the fact that there’s nothing new in the paper, [2] statistical issues with the fossil record, and [3] the problem of assigning causation to correlation. (He makes other lucid points and I encourage you to read his post and not rely only on my simplified rendition. To his credit, he also includes some evidence that would convince him of the bolide-extinction link). His point [1] is trivially correct because it’s a review paper, but I don’t think that diminishes its value as there is probably enough work in this field to support a yearly review. On Bryan’s points [2] and [3], I think he may be correct in the strict sense but I differ on the practicalities. There are sampling problems with the fossil record such as the Signor-Lipps effect Bryan invokes, but I don’t accept that this means we can’t build a case for an extinction cause that is valid to a reasonable degree of scientific certainty (especially if the model invokes a single cause finishing the event and allows for earlier effects to set the stage). And it’s true that [3] correlation doesn’t prove causation – that’s something scientists remind each other frequently. Moreover, we could go back to Karl Popper to remember that, in terms of the rigid philosophy of science, we can only disprove causations and never prove them. It all depends how rigorously we use the word “prove”. The equivalent of “proof” in our everyday scientific parlance is the accumulation of evidence that supports a theoretical framework and the falsification of other causes. Part of that framework, of course, has to include mechanisms that tie a cause to an effect. These frameworks are always subject to re-evaluation, but strong and multiple correlations with plausible linking mechanisms are how we demonstrate causality in geology. We may not be there yet with Chicxulub, but it looks to me like the burden of evidence shifted towards the skeptics some time back.

Articles cited:

Alvarez, L., Alvarez, W., Asaro, F., & Michel, H. (1980). Extraterrestrial Cause for the Cretaceous-Tertiary Extinction Science, 208 (4448), 1095-1108 DOI: 10.1126/science.208.4448.1095

Bralower, T., Eccles, L., Kutz, J., Yancey, T., Schueth, J., Arthur, M., & Bice, D. (2010). Grain size of Cretaceous-Paleogene boundary sediments from Chicxulub to the open ocean: Implications for interpretation of the mass extinction event Geology, 38 (3), 199-202 DOI: 10.1130/G30513.1

Hildebrand, A. R., Penfield, G. T., Kring, D. A., Pilkington, M., Camargo Zanoguera, A., Jacobsen, S. B., and Boynton, W. V. (1991). Chicxulub Crater: a possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico. Geology, 19 (9), 867-871.

Keller, G., Adatte, T., Juez, A., & Lopez-Oliva, J. (2009). New evidence concerning the age and biotic effects of the Chicxulub impact in NE Mexico Journal of the Geological Society, 166 (3), 393-411 DOI: 10.1144/0016-76492008-116

Article reviewed:

Schulte, P., Alegret, L., Arenillas, I., Arz, J., Barton, P., Bown, P., Bralower, T., Christeson, G., Claeys, P., Cockell, C., Collins, G., Deutsch, A., Goldin, T., Goto, K., Grajales-Nishimura, J., Grieve, R., Gulick, S., Johnson, K., Kiessling, W., Koeberl, C., Kring, D., MacLeod, K., Matsui, T., Melosh, J., Montanari, A., Morgan, J., Neal, C., Nichols, D., Norris, R., Pierazzo, E., Ravizza, G., Rebolledo-Vieyra, M., Reimold, W., Robin, E., Salge, T., Speijer, R., Sweet, A., Urrutia-Fucugauchi, J., Vajda, V., Whalen, M., & Willumsen, P. (2010). The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary Science, 327 (5970), 1214-1218 DOI: 10.1126/science.1177265

LPSC XLI

2010-02-26T16:19:00.000-06:00

I'll spend next week at the 41st Lunar and Planetary Science Conference outside of Houston. I've long been a regular attendee of GSA and I've been to the occasional AGU, but this is my first time at LPSC. It's the big conference in planetary sciences and it's purported to be kind of a zoo. Should be fun.

The extended abstract for each talk and poster presentation is posted online and can be searched by author or by session, for the interested reader. Some of the sessions relevant to topics I've blogged on that I'll try to attend are: LCROSS, Chandrayaan, and Chang'e-1 results; terrestrial impact craters; Mercury MESSENGER results; these two sessions on Martian igneous processes and geochemistry; and lunar petrology. Talks on volatiles in the Earth and Moon are scattered throughout these and other sessions. I expect that I'll be running from session to session and missing at least half of what I should see, but that's the way these things go. I'm giving a poster presentation so my stress is light (now that it's completed and printed).

I may post during the conference, depending on how harried I am, or I might even consider using my virtually untouched twitter account at IapetusBeat. But I may just write up a retrospective afterwards.

Cheers.

Phobos

2010-02-26T12:29:00.001-06:00

Color view of Phobos from Mars Reconnaissance Orbiter (Credit: NASA / JPL / U. Arizona)

The European Space Agency’s Mars Express Orbiter has been examining Mars since late 2003. It’s currently undertaking a series of 12 flybys of the larger of Mars’s two Moons – Phobos. ESA has posted a detailed schedule of the flybys and the experiments conducted for each. They’re also hosting a blog to follow the events. The closest approach of 50 km will occur on March 3^rd.

Phobos is an irregularly shaped (non-spheroidal) body only 27 km across at its longest point. Based on spectral and density data, Phobos and Deimos (Mars’s other moon) seem to have very primitive compositions similar to carbonaceous chondrite meteorites. Asteroids matching that composition compose most of the asteroid belt, and it is suspected that Phobos and Deimos may be captured asteroids.

Either Phobos or Deimos is a possible target for the Flexible Path option raised by the Augustine Commission (PDF, see section 3.5). This alternative would place humans in orbit around Mars and/or on the surface of a Martian moon. From there they could sample Phobos or Deimos directly and teleoperate rovers on the Martian surface. (Teleoperation would be much more efficient than driving rovers from more than 170 million kilometers away). One of the biggest energy expenses and engineering obstacles in manned exploration is getting back off of whatever body you land on – assuming you want to come back – and the larger the body’s gravity, the harder it is. That’s why visiting a small Martian moon is attractive – you could even have a craft return samples from the Martian surface to astronauts in orbit, since getting a small robotic craft off of the surface avoids the added weight and safety constraints that come with human passengers.

Kaidun meteorite (image from the Vernadsky Institute)

It would be cool to sample Phobos directly, but it’s possible that we already have an indirect sample in the form of the Kaidun meteorite, which fell in Yemen in 1980. It is suspected to be a piece of Phobos that was ejected from the moon’s surface by an impact. (See an Astrobiology Magazine story here and an abstract on it here.) This is one of my favorite meteorites (though I’ve only seen pictures and read papers) because it contains pieces of alkaline igneous rock – a subclass of magmatic rocks that is one of my research interests as well as a geeky fixation. Alkaline rocks are rare on Earth but almost non-existent in meteorites except for two 3-4 mm clasts in Kaidun and in one other meteorite.

People vs. the Rover (and some New Mexican Geology)

2010-02-10T18:32:00.001-06:00

Before my last post, I had a week away from blogging. My laptop died and I was doing fieldwork so there was no chance to fix it (and I was too tired after dinner and drinks to write anyway). We were in New Mexico, along the transition between the Rio Grande Rift and the Colorado Plateau, testing operational strategies for planetary rovers by comparing the data and maps generated by field geologists to the geologic interpretations of a simulated rover. I was on the human team, and to gain value from the exercise we documented all of the small steps, observations, and shifting hypotheses that make up what passes for intuition in experienced fieldworkers. New Mexico is beautiful and I learned a great deal from my field partners, each of whom is an expert volcanologist or planetary scientist or both. I wish all my work was like that.

Cabezon Peak seen from the truck.

On day one, we mapped part of a maar volcano. Maars are low, flat volcanoes that form when magma explosively interacts with water (usually groundwater or permafrost), and the craters commonly host shallow lakes. The aerial photos showed this one to be about 800 meters across, though we only mapped one side. The rim and lower units were inward-dipping, loosely consolidated volcanic material overlain by debris flows and relatively flat-lying lake sediments deposited in the crater. I’ve read about maars and even taught about them, but I’d never before been close enough to see the structures and the outcrop-scale features. As a petrologist, I’m usually focused on the magmatic rocks, but in phreatomagmatic systems like this one the basalt composes only 5-10% of the material, and most of it’s hydrothermally altered. The bulk of the material is old country rock, sediment, and mud brought up and churned by the eruption.

From inside a maar crater. The dark layers in the foreground are continuous with the ridge in the right of the photo (though this interpretation was somewhat contentious). The cliffs to the back left are a massive layer of volcanic debris topped by thinly-bedded crater lake deposits.

Slump block of layered volcaniclastic material in the massive debris layer.

Prismatically jointed bomb (PJB), formed when molten basalt blobs land in water or wet mud.

Day two was a little more familiar. It was a volcanic neck that came up through Mesozoic sedimentary rocks. We worked from the base with an aerial photo so the only volcanic material available to examine was brought down in long debris flows (a very realistic scenario for a rover). There were reddish boulders of vesicular cinders and scoria in a matrix of altered basaltic glass (palagonite) and there were blocky flows of extraordinarily fresh aphyric basalt. We interpreted the former as deriving from early cinder cones or rim deposits and the latter as a crater-filling lava lake or flows outside the crater. Both rocks contained beautiful and abundant mantle xenoliths. Because such xenoliths occur almost exclusively in alkali basalts and related rocks (these magmas typically form from very deep melting and have a low enough viscosity to ascend quickly) we could infer the basalt composition in the field even though it lacked phenocrysts.

A volcanic neck. The tan layers in the mid-slope are Cretaceous sedimentary rocks. The top is capped by dark red-brown scoriaceous basalt and darker blocky basalt. The boulder in the foreground is the former and the latter litters the ground and fills the far drainages.

Looking down from the base of the sedimentary cliffs. Blocky basalt in the foreground.

An aside: xenoliths are our primary source of knowledge about the Earth’s mantle. Although sections of uppermost mantle are occasionally tectonically emplaced in ophiolite complexes, alkali basalts can sample much deeper material – from greater than 60 km. (Xenoliths in kimberlites provide the deepest mantle samples we have, from depths down to 200 km, and the great pressures are why kimberlites are the primary diamond-bearing rocks). Here’s a pretty good paper on some xenoliths in the area we were mapping.

Two mantle xenoliths in basalt. The front sample is a pyroxenite (mostly clinopyroxene) visible on a slightly weathered surface. The rear sample is a lherzolite (mostly olivine + pyroxene) seen in a freshly broken surface.

I don’t have any operational experience with rovers and I haven’t (yet) been on a rover science team, so the experience was edifying. A rover is limited by time, tools, and energy expenditure. A two-legged geologist can cover hundreds of square feet in minutes, breaking rocks to expose fresh surfaces and holding them up for examination with a hand lens. If a rover even has an arm (and many designs don’t), it would be a major decision to take the time and energy to turn over a rock, and even the best rover cameras are limited in field-of-view and height as to what they can image. It’s reassuring that both of our teams came to similar general interpretations at both sites; however, it took a rather long time for the rover team to spot the xenoliths, which would be a major find on Mars or the Moon. On the human side, we seemed to have a bias that the layered rocks would be volcaniclastics (not surprising, given our backgrounds) or even pyroclastic surge deposits. The lake sediments in the maar undoubtedly had some reworked volcanic input but the sandstones and shales from the neck contained little if any obvious volcanic material. This was much clearer when we could examine them up close, but a rover team doesn’t always have that option. I think this shows how important these analog studies can be in identifying the pitfalls of rover geology, which is the first step in avoiding them or compensating. I’m pleased to have been a part of it.

The Space Program: The Lack of a Goal and a Specious Argument Against a Moon Mission

2010-02-09T14:47:00.003-06:00

The New York Times published an editorial (registration required) today on Obama’s changes to the space program. I don’t claim that the NY Times editorial board has great scientific insight (though their paper’s science journalism is among the best in the country), but they express a few misgivings that I’ve heard elsewhere, especially from NASA folks and others in the planetary science community.

If done right, the president’s strategy could pay off handsomely. If not, it could be the start of a long, slow decline from the nation’s pre-eminent position as a space-faring power.

We are particularly concerned that the White House has not identified a clear goal — Mars is our choice — or set even a notional deadline for getting there. The National Aeronautics and Space Administration and Congress need to keep the effort focused and adequately financed.

Yes, on both points. The complaint I’ve heard most is that administration’s approach may be sound, but WHAT IS THE GOAL? The press release had grandiose rhetoric but no stated exploration goal. Are we going to Mars? Are we stopping off at the Moon, even if we’re delayed by a few years? Are we visiting near-Earth asteroids or the moons of Mars (the objects with low gravity wells suggested in the Augustine Report)? Any of these would be OK with me, but without a plan, it all sounds suspiciously like the Obama administration wants to change Bush’s plan for the sake of doing so (which is far from unprecedented) or that they don’t hold science beyond climate and medicine to be much of a priority.

The editorial makes another point I’ve been hearing, but less often from the planetary science people as from science blogs and news sites, and that is that the Moon return mission as planned (or any Moon mission) would have limited value. On this I disagree. Another quote:

A lunar expedition would be of some value in learning how to live on the Martian surface but would not help us learn how to descend through Mars’ very different atmosphere or use that planet’s atmospheric resources effectively. Nor would it yield a rich trove of new scientific information or find new solutions for the difficulties of traveling deeper into space. [italics mine]

Like I said, if you don’t want to go back to the Moon, fine – there are good arguments to be made there; even within NASA and the larger community there were many who thought we should have set sights directly on Mars or near-Earth asteroids. I think, however, that to say the Moon won’t “yield a rich trove of new scientific information” is just wrong. For some background, you can browse the online edition of The Scientific Context of Exploration of the Moon, a publication of the National Research Council. The Moon has no plate tectonics and no weathering processes (other than the “space weathering” I wrote about here as it relates to asteroids) and so it contains the most complete picture of 4.5 billion years of solar system history. This includes records of impact flux which can refine (or disprove) the Late Heavy Bombardment model (concerning a peak of impact activity around 3.9 billion years ago) or the late veneer hypothesis (concerning the early arrival of water on the Earth and Moon by meteorites or comets). The same impact record coupled with frozen volatile deposits detected by LCROSS and Chandrayaan can tell us something about later impact flux and the composition of the impactors. This record has been almost entirely wiped from the Earth’s surface, and the arrival of water and the occurrence of cataclysmic impacts have a huge relevance to the origins of life. Furthermore, the Moon preserves its original crust, however fractured, and its study can illuminate our understanding of planetary differentiation and crustal formation.

The public hasn’t been sufficiently exposed to the wealth of information gleaned from returned Apollo samples. These materials are still under intense study of all kinds, but all Apollo landing sites were centered on the Procellarum Region – an interesting but not wholly representative part of the Moon. Rare as they are, lunar meteorites are another rich source of information, but they lack context because we don’t know their original location. Remote sensing by orbiters has expanded our knowledge of the Moon’s composition away from the Apollo landing sites, but these data open up whole new frontiers of questions that are best answered by careful directed fieldwork. Robotic rovers and sample return missions would be a huge step in that direction, but with foreseeable technology they fall far short of a single trained human being.

There are many reasonable arguments for skipping the Moon and going straight to Mars or elsewhere, and I’m happy to listen to them. But the Moon’s lack of scientific value is absolutely not one of them.

Changes in US Space Policy: No Manned Moon Mission?

2010-01-30T15:29:00.000-06:00

We’ll know more when Obama presents his 2011 budget on Monday, but it appears that the Vision for Space Exploration (pdf) is doomed and that NASA’s Constellation program won’t survive intact. That doesn’t mean that the U.S. is done with space exploration or even with manned space exploration, but changes and delays are in store.

You can read aggregations of this news from Knight Science Journalism Tracker, stories from Space.com here and here, or an earlier take by the NYT here. I won’t rehash it all, but most of the news comes from anonymous sources and early versions of the report, so there’s a lot of uncertainty. There seems to be agreement that the plans for a return to the Moon by 2020 are off, that the International Space Station’s life will be extended until around 2020, and that the shuttles will be still be retired, as planned, this year. Everyone seems to agree that some of the new craft design and construction will be handed over to industry – an approach supported, in general, by some of the voices at NASAwatch.com and elsewhere. Loud dissenting voices come from former NASA head Mike Griffin and congressmen and women from Texas and Florida, which house NASA centers.

I’m disappointed about the delay or disappearance of the Moon mission (though, like many, I suspected it would happen). There are many proponents who rightfully point out how much more science you can get for your money through unmanned than through manned exploration. That’s true, but no matter how good the rovers become, there’s no substitute for having a trained scientist conducting fieldwork, and a long-term Moon base would undoubtedly have yielded immense scientific returns. However, as the Augustine Commission (pdf) found, we weren’t achieving what we needed with the money NASA was given, so something had to change. The early news reports suggest that science at NASA as a whole will remain well funded, and may even prosper, under the new budget. I'll revisit this when we have more real information.

Minor Housekeeping Changes

2010-01-26T12:37:00.000-06:00

I'm decided to try to keep this blog at least tangentially about science, so I've exported a few older posts to Daily Xmas. Thanks.

A Nod to ResearchBlogging.org and to a Great Archaeology Website

2010-01-25T10:26:00.000-06:00

I sometimes post reviews of journal articles here, and when I do, I usually submit them to Research Blogging. Those who follow science blogging probably already know, but Research Blogging aggregates content that reviews and cites articles in the literature across the natural and social sciences (and Philosophy). The bloggers submit the citation and the post and Research Blogging links to it, the idea being that most bloggers (like me) post on a range of topics of varying seriousness and scientific content, but through Research Blogging you can follow a blog or browse by field and get only the more rigorous posts (ideally, anyway). The four posts I’ve sent to them have accounted for well over half of the total visits to Iapetus Beat and my Toba supervolcano post was an Editor’s Selection, so I’m grateful. Also, I’ve had issues using their html with my blogger site and thus I haven’t posted their icon, so this recognition is me giving back.

One of my discoveries from Research Blogging is an archaeology/anthropology site concerned with Chaco Canyon and related southwestern archaeology called Gambler’s House. Blogger teofilo has submitted 15 posts so far to the aggregator site and has many more at Gambler’s House. It’s not really for casual skimming – it’s comprehensive and in-depth, and he reviews evidence from isotopic analysis, geochronology, linguistics, and the like, and unfailingly provides good synthesis and context. On top of that, it’s lucidly written and the photographs are very nice.

Pueblo Bonito in Chaco Canyon. Photo from Gambler’s House.

When in grad school, I taught four seasons on two related cross-disciplinary field classes that covered North American geology, archaeology, and ecology/environmental science, in that order of concentration (UGA-IFP and Geojourney). The trips changed a bit year-to-year, but we always spent a lot of time in the Southwest. The major cultural sites we visited were Chaco Canyon, Mesa Verde, Wupatki, and Bandelier. We went pretty deep, considering it was a traveling class for undergrads, and I really enjoyed discussing what is and isn’t known about those sites with the Archaeology Instructors and Professors. Migrations and settlement patterns, Mesoamerican influence, the rise of the Kachina cults… Gambler’s House covers all of that.

The Unreddening of Asteroids

2010-01-24T18:52:00.014-06:00

Richard Binzel from MIT is the lead author of a new Nature paper titled Earth encounters as the origin of fresh surfaces on near-Earth asteroids (abstract only). They address a long-standing problem in meteoritics: why does the color of meteorites found on Earth so rarely match that of asteroids in the Main Belt? Binzel and his colleagues demonstrate that it is exactly the proximity to Earth that causes changes in the color of the meteorite parent bodies – a tidy solution to a puzzle, and it bears on a few of the most important processes on planetary bodies and the tools we use to interrogate them.

First, some background. The surface composition of a planetary body determines the color of the visible, infrared, and ultraviolet light it reflects. We observe this light, with ground telescopes or spacecraft instruments, measure and quantify it using reflectance spectrometry, and try to deconvolve the spectra to pull out compositional information*. Of course, our solar system is a dynamic place, and the color of a surface changes with time from exposure to solar and cosmic radiation and meteorite impacts (both large and microscopic). This is known as space weathering**, or maturation of the surface. Mature planetary surfaces look darker to the naked eye, and this is referred to as reddening because of the shift in dominance to the larger wavelength light in the infrared.

Consider our Moon. There are two crustal domains visible to the naked eye: the highlands are light due to the abundance of plagioclase feldspar and the mare are largely dark basalt. These are primary compositional differences. (When you examine broader spectra including nonvisible light, much more compositional variation is evident.) However, within a given lunar crustal domain, the craters and the rays that extend from them will be lighter in color. This is because the impacts excavate and expose the less mature, and thus lighter and less red, regolith. Understanding how mineralogical and chemical composition controls reflectance and how to distinguish that from the superimposed effects of maturation is a major undertaking in planetary science.

The Earth’s Moon. The craters and the rays that radiate from them are lighter than their host regions because they excavate relatively fresh material.

Over 80% of the meteorites we’ve collected on Earth are ordinary chondrites (which are more-or-less primitive samples of the early solar system). There are some relatively unweathered asteroids that have similar spectral characteristics to ordinary chondrite meteorites, and they are known as Q-types. However, Q-type asteroids are so far undetected in the Main Belt and compose only a small minority of the near-Earth asteroids. This is the “ordinary chondrite problem” that Binzel et alia address. It’s important to distinguish that “ordinary chondrite” is a petrological classification based on mineralogy, texture, and bulk chemistry – properties we can only measure on meteorites in hand, while Q-type is a classification based on the spectral properties which we can measure on asteroids and meteorites. In fact, the NEAR Shoemaker mission landed on 433 Eros, a non-Q-type near-Earth asteroid, in 1998 and found the bulk chemistry to be similar or identical to ordinary chondrite meteorites. This raised the possibility that it is only the unweathered nature of meteorites that is underrepresented in the solar system, and not the primary composition.

The relative reflectance spectra of ordinary chondrite meteorites, Q-type asteroids (which are relatively unweathered) and increasingly weathered Sq- and S-type asteroids. The more weathered asteroids have spectra more reflective in the infrared (above 0.7 µm in wavelength). The dips in the spectra at 1 µm and 2 µm are due to the absorption of Fe-bearing minerals olivine and pyroxene, respectively. These two absorption bands become more subdued with increasing weathering/maturity. From Binzel et al. (2010), Figure 1.

Binzel and his colleagues tested the hypothesis that near-Earth encounters cause the partial or complete resurfacing of those asteroids which become or shed meteorites. They examined 95 asteroids whose paths cross that of Earth or Mars, and limited their sample group to asteroids of diameter 0.2 to 4 kilometers. They had spectral data for each asteroid, and 20 of the 95 were Q-type asteroids. They also had orbital data, with which they calculated the Mean Orbit Intersection Distances (MOIDs) – a measure of the potential closeness between each asteroid and Earth. Asteroidal orbits can be perturbed by the gravity of any of the eight planets, so the authors used planetary orbits as input and modeled MOID for each asteroid for the last 500,000 years, with an output for every 50 years of model time. It’s known from lunar and asteroidal studies that the processes of maturation take less than one million years – a rapid pace, geologically speaking, but a longer period than the modeled time span. They then examined the closest calculated MOID for each of the 95 asteroids.

They found that 75 of the 95 objects could have come within one Earth-Moon distance in the last half million years. All 20 of the Q-type asteroids fell in this group and, statistically, there is only a 0.9% chance of this occurring randomly. Recall that the MOID calculates possible closest distances so that a low MOID doesn’t mean that the object actually came within that distance; thus, it isn’t necessarily problematic that 55 of the modeled orbits had low MOIDS but weren’t Q-type asteroids. So the correlation of fresh surfaced asteroids with proximity to Earth is robust at 99.1%.

No one has yet done the detailed calculations, but the proposed mechanism is tidally induced seismicity on the asteroid. This seismicity is envisioned to shake and stir the surface regolith, exposing fresh material. Density measurements taken by NEAR Shoemaker and from the ground suggest that some asteroids may be as much as 50% empty space; this data has led to the rubble pile model for asteroids that suggests they are mechanically weak agglomerations held together by gravity. Such loose material would presumably be particularly susceptible to shaking and resettling. Furthermore, events as relatively small as landslides in asteroid craters have been observed to expose fresh regolith. Although there is insufficient data for 0.2-4 km Main Belt asteroids to compare directly to this near-Earth dataset, the lack of any detected Q-type Main Belt asteroid strongly suggests that proximity to a large body, i.e., a planet, is necessary to unredden (my word, not theirs) an asteroid.

The S-type asteroid 433 Eros as photographed by the NEAR Shoemaker probe (NASA-JPL).

*Iron has disproportionate effects on the spectra of geologic materials. Beyond space weathering effects**, the Mg/Fe of the mafic minerals also exhibit control over the subtleties of absorption bands. Lunar plagioclase contains, on average, more FeO than terrestrial plagioclase, though still generally less than 1.5 wt.%. This allows lunar plagioclase abundance to be more easily measured by spectral techniques than it otherwise would.

**The surfaces of planets with atmospheres are shielded from most meteorite impacts, and the surfaces of those with magnetospheres are protected from most effects of the solar wind. Among the primary effects of space weathering are (1) mixing and churning of regolith at a range of scales by meteorite impact; (2) implantation of solar wind ions, largely H⁺, into the particle surfaces; and (3) very localized melting, i.e., glass formation, on the scale of microns to millimeters by micrometeorite impact. The localized melting of (3) in the presence of implanted H⁺ from (2) results in glass with reduced iron spherules. This glass coats other particles with micron-thick rims. The metallic iron spherules the rims contain are often nanometers in size, and they contribute to the reddening of the mature regolith.

Binzel RP, Morbidelli A, Merouane S, Demeo FE, Birlan M, Vernazza P, Thomas CA, Rivkin AS, Bus SJ, & Tokunaga AT (2010). Earth encounters as the origin of fresh surfaces on near-Earth asteroids. Nature, 463 (7279), 331-4 PMID: 20090748

New Year's on the Yucatán

2010-01-11T15:41:00.009-06:00

I’ve had a long absence. I traveled around state to state to get hooded, for Christmas with my family, and then for holidays with my girlfriend’s family, and then we took a vacation to the Yucatán.

Map of gravity anomalies in the Chixculub crater. The circular portion is ~180 km across. The white line is the superimposed coast of the Yucatán Peninsula; the white dots are mapped cenotes. Image from the Geological Society of Canada.

Geologically, the Yucatán Peninsula is a big slab of limestone – an interesting enough rock; I like caves and karst as much as the next guy, but it’s not really my cup of tea. Of course, the Yucatán is the site of the great Chicxulub impact that occurred at the Cretaceous-Tertiary (K-T) transition and which may have killed the dinosaurs, the ammonites, etc. You can see it from satellite and you can image it by geophysical methods, but the only surface manifestation is a concentration of sinkholes in the karst around the rim. These water-filled sinkholes are called cenotes (dzonot in Mayan) and were important to the ancient Maya as sources of water and as ceremonial entrances to the underworld. We saw a few cenotes and swam in one, and that was my only geological undertaking on this trip, except for identifying onyx, limestone, and obsidian on the souvenir tables.

The Cenote Sagrado at Chichén Itzá. It was dredged twice in the 20th century; in it were found bodies of infants and adults along with jade and other precious items.

We had a good mix of ruins, cities, and beaches. We stayed in Cancún (the city, not the monstrous beach hotel zone), Vallodalid, Mérida, Ticul, and Tulum. We spent New Year’s eve and day in Mérida – a beautiful colonial city with a strong Mayan influence (like all of the Yucatan that we saw). We drove some back roads, which was an adventure in itself. We stayed on the beach in Tulum to finish our vacation with a bit of luxury after budget hostels and inns elsewhere. We ate great food everywhere – my favorites of the local delicacies were cochinita pibil (meltingly succulent pork cooked in banana leaves and served with pickled onions) and sopa de lima (“lime soup”). I also had some wonderful posole (hominy stew with pork, chicken, and chiles), but it’s more of a Jalisco than Yucatán dish. Though I love tamales, they are seldom served in restaurants and I had only one, bought at a street market and taken as a package lunch to Ek’ Balam. Along the Caribbean coast we had great seafood in Italian- and Thai-styles dishes that incorporated varying fusion with Mexican cuisine. This might seem odd, but all three traditions – Thai, Italian, and Yucateac – prize seafood, fresh vegetables, and chiles, and the latter two especially prize tomatoes, which of course the Yucatecs had before the Italians. I’ve never seen prettier tomatoes than in the markets of the Yucatán.

Shrimp and octopus pizza on the beach in Tulum.

We saw ruins in Tulum, Ek’ Balam, Chichén Itzá, Uxmal, Kabah, Sayil, X’lapek, and Labna. Uxmal was our favorite – a truly spectacular site nestled in the Puuc hills. Uxmal, along with Kabah, Sayil, X’lapek, and Labna, defines a particular Puuc style of Mayan architecture marked by elaborately decorated protruding facades also evident in the Classic-era (pre-850 AD) Chichén Itzá buildings. Later buildings at Chichén Itzá (and some at Uxmal) show more influence from central Mexico Toltec culture. There doesn’t seem to be much agreement yet as to whether this purported Toltec influence was from invasion or cultural osmosis, as it’s known there was widespread trade throughout Mesoamerica. But it’s interesting that the Maya “collapse” in the highlands and southern lowlands precedes a flourishing in the northern lowlands (including all of the area we visited) that is accompanied by increased Toltec influence. Though human sacrifice was apparently practiced throughout ancient Mayan history, it peaked in the post-classic, Toltec-influenced period (after about 850-900 AD). Though drought is often invoked as a cause of the Mayan decline, the northern lowlands where the resurgence occurred are the driest part of the Mayan world, so it’s more complicated than sometimes portrayed. (I should move on, because I’m straying dangerously far from my expertise.)

View of Uxmal from the platform of the House of Turtles. The ball court is in the center and the Pyramid of the Dwarf is in the distant right.

During the trip I read Michael D. Coe’s Breaking the Maya Code about the decipherment of the Mayan hieroglyphics. It’s a very good book and I recommend it; if you want a detailed history of the Mayan cultures you may want to start elsewhere, as this is really an intellectual history of the investigation. Coe is a Yale Professor Emeritus and an important Mayanist himself, having written one of the foremost textbooks on ancient Mayan civilization and playing a part in the decipherments.

Puuc architecture at Kabah. This is a wall of more than 250 Chaac (a rain god) masks. Some still have their protruding noses intact.

Coe is quite critical of the Mayanists who he feels delayed progress by clinging to romantic notions of Mayan mystical symbolism or by underestimating Mayan practicality and sophistication. Either way, many workers refused to believe that the Mayan hieroglyphics have a phonetic basis and record a spoken language. In the end, Mayan writing yielded to the same techniques used to crack Egyptian, Assyrian, and Hittite scripts, though it took much longer. Coe’s book documents the failures of science that stem from the human flaws of its practitioners – be they racism, romanticism, or just stubbornness and vanity, but it also shows the truth winning out in the end.

Iapetus, Light and Dark

2009-12-15T12:06:00.001-06:00

I would be remiss if I didn’t mention the recent papers on Saturn’s moon Iapetus – a namesake for this blog along with the paleo-ocean and the Greek Titan, for which both the moon and the ocean are named. Iapetus the moon has a striking surface dichotomy of light and dark regions first recognized when Giovanni Cassini discovered the moon in 1671. The leading hemisphere of the moon is dark and most of the trailing hemisphere is light. The geometry of the regions led to speculation that Iapetus was collecting material as a fine rain of dust from other Saturnian moons that coated portions of Iapetus’ otherwise bright icy surface.

Iapetus (NASA/JPL/Space Science Institute)

High-resolution images taken by the Cassini spacecraft since 2004 provided further detail of the regions. They show that small, fresh craters in the dark regions are bright, indicating the dark mantle is only 10’s of centimeters to meters thick. They also show that the transition zones between the dark and light regions are mottled with smaller-scale regions of dark and light material, rather than having a gradational transition of grays. In the mottled transition zones, the equator-facing walls of craters, which receive the most sunlight, are dark while the poleward-facing walls, which receive less sunlight, are light-colored.

Portion of the transition zone on Iapetus. The dark material is concentrated on the equator-facing sides of the craters (in this picture, North). (NASA/JPL/Space Science Institute)

Two papers published online in Science last week synthesize the Cassini data into a model for the dark-light dichotomy. The abstracts can be found here and here.

(1) As to the source of the dark mantle on Iapetus: it shares spectral characteristics with Saturn’s moon Phoebe (itself a fascinating body), and the recent discovery (Nature abstract here) of a large diffuse ring coincident with Phoebe’s orbit provides a likely source for the dark material. Whether Phoebe itself is feeding the ring by shedding material during impacts or if the ring is the detritus of another Phoebe-like body, now destroyed, is uncertain.

(2) As to the mottled pattern: Iapetus has a very slow rotation of 79 Earth days and it therefore has large temperature differentials between the long days and nights. Exacerbating this is the effect of albedo -- areas that have some dark material absorb more sunlight (i.e., they have a lower albedo) and are warmer while lighter areas reflect more sunlight and stay colder. This provides a feedback mechanism wherein those areas with some dark mantle are warmer, thus more ice sublimates to gas and migrates towards colder (i.e., brighter) spots. There should be some critical amount of dark material that perpetuates this feedback for any given light regime, so that areas with that much or more mantle become increasingly dark, while nearby areas with less than the critical amount of mantle may host deposition of ice and become increasingly light. The leading edge receives a lot of dust and is dark; the trailing hemisphere receives little dust and is light; the transition zones receive some dust and the local albedo depends on the sunlight budget which in turn depends on topography.

This story was reported last week in Science Daily, the New York Times, and elsewhere.

At Long Last

2009-12-10T16:13:00.001-06:00

I just submitted my (and my three coauthors') extended abstract to the 41st Lunar and Planetary Science Conference, to be held in March just north of beautiful Houston, Texas. LPSC is probably the single biggest meeting for the planetary crowd, especially, it seems, for the lunar folks. I've never been, and I almost didn't submit but I was encouraged to, so I stayed up all night (just like grad school) getting it together. My first postdoctoral abstract.

So maybe now I can find some time to post again, if I can work it around my Christmas (Holiday) party this weekend. There has been some good planetary science news after a bit of a lull.

Cheers.

Martian Meteorites

2009-12-03T16:52:00.000-06:00

I'm still working on The Rock From Mars as I mentioned in my last post. I'm also learning two new probe/SEM software packages and IDL (interactive data language) to better work with my own Martian meteorite data. One of the projects I've undertaken is looking at the impact glass composition of EETA79001, which is the sample by which the group of SNC meteorites were determined to be from Mars. Bogard and Johnson (1983) analyzed the noble gas abundances trapped in the meteorite and found them to be almost identical to the Martian atmospheric composition measured by Viking.

Figure from Pepin's (1985) paper showing the correlation of gas compositions from EETA79001 and Mars's atmosphere. Pepin's work built on and expanded that of Bogard and Johnson (1983).

In the interim, Andrew Moseman summarizes responses to the new Mars meteorite ALH84001 study at 80beats.

The Most Super Eruption of a Supervolcano

2009-12-02T12:56:00.000-06:00

About 73,000 years ago in what is now Sumatra, Indonesia, Mount Toba exploded in the largest eruption since the appearance of Homo sapiens. This enormous eruption (a supereruption of a supervolcano in the new parlance) spewed 2800 km³ of pyroclastic material from a 100 by 30 km caldera. That is a hell of a lot – almost three times more than Yellowstone 600,000 years ago and even more than the giant (super) Yellowstone Huckleberry Ridge eruption of 2 million years ago. Toba was probably the biggest single explosive eruption in tens of millions of years.

Lake Toba, Sumatra. The white dashed line marks the extent of the 73 ka caldera, which is 100 km long and 30 km wide. Landsat image from NASA.

Martin Williams of the University of Adelaide is the lead author of an upcoming paper on the eruption’s environmental impact. Stanley Ambrose of the University of Illinois at Urbana-Champaign, a co-author on the study, suggested in a 1998 paper that the eruption correlates to a catastrophic drop in human population. There is strong genetic evidence that such a bottleneck occurred and reduced the human population to perhaps 1,000-10,000 breeding pairs during the Middle Stone Age; the time constraints are broad but they allow the Toba eruption as a cause. From my reading of the literature, this idea is fairly well accepted, though there is by no means a consensus.

The eruption covered much of the Indian subcontinent with 10-15 cm of ash and released huge amounts of aerosols like hydrogen sulfide and sulfur dioxide into the atmosphere. There’s good evidence from geochemical markers in ice cores that the eruption had the immediate short-term effect of causing a 6-year volcanic winter – a more extreme version of the years following the 1815 Tambora, 1883 Krakatau, and 1991 Pinatubo eruptions. This 6-year period marks the beginning of the coldest 1,800 years of the Pleistocene, although the relationship between the eruption and the longer term cooling is controversial.

The main contribution of the Williams et al. (2009) study is that it directly demonstrates environmental changes from the eruption across the Indian subcontinent. By examining pollen in a marine core from the Bay of Bengal and the isotopic signature of soil carbonate minerals from across central India, they show a sudden and lasting cooling and drying coincident with the ashfall. The pollen counts from the core showed a reduction in trees and wet-adapted ferns. The carbonate isotopic signatures indicate almost pure C3 habitats (like forests) below the ash layers and show a large component of C4 habitats (grasslands and mixed forested grasslands) within and above the ash layers.

The authors present a strong temporal correlation between their findings and data from ice cores in Greenland, deep lake sediment cores in Africa, and fossil soils in China, all of which indicate global cooling, falling lake levels, and the disruption of monsoons. There doesn’t seem to be much doubt that the Toba eruption played a large role in this. The longer-term cooling, however, is still problematic. In the short term, aerosols reflect and scatter solar energy and widespread ash cover increases the albedo of the surface, also reflecting sunlight. In the longer term, you need to invoke increased albedo from snowfall in high latitudes, changing of ocean currents, or other feedback mechanisms that are plausible but not as well constrained.

(I was alerted to this study by an article on the Science Daily website.)

Williams, M., Ambrose, S., van der Kaars, S., Ruehlemann, C., Chattopadhyaya, U., Pal, J., & Chauhan, P. (2009). Environmental impact of the 73ka Toba super-eruption in South Asia Palaeogeography, Palaeoclimatology, Palaeoecology DOI: 10.1016/j.palaeo.2009.10.009

Ambrose, S. (1998). Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans Journal of Human Evolution, 34 (6), 623-651 DOI: 10.1006/jhev.1998.0219

Martian Life Revisited

2009-12-01T23:18:00.006-06:00

It has been the longest break in the short life of this blog. After a great Thanksgiving I cleaned out my grad student office and drove the load back to my new home. I've been organizing papers for a few days now.

One of the famous ALH 84001 SEM images, showing the chain-like series of crystals which McKay et al. (1996) suggested were biogenic.

In the world of planetary science, there are two new publications on the possibility of Martian fossils in meteorite ALH 84001. You may recall that in 1996 Dave McKay and his coauthors revealed possible evidence for past life in the sample. They found carbonate and nanophase magnetite (Fe₃O₄) mineral grains which, based on the minerals' occurrence, texture, and association with organic molecules, they suggested may have been deposited by bacteria. This was contentious, and still is -- many argued that the minerals had an inorganic origin -- but the hypothesis was never disproved. The new papers, found here and here, argue that the primary alternative hypothesis -- that the unusual magnetite textures formed during the impact shock that sent the rock Earthwards -- is not viable.

I read about the new work at New Scientist and The Martian Chronicles but I'm still working through the journal articles. It all came as quite a coincidence, because I just started reading Kathy Sawyer's book The Rock From Mars which details the escalation of the scientific debate that began in 1996, and it's enough for now to say that it quickly got bitter and personal. I was going to post about the sometimes all-too-human workings of the science world when I finished reading it, but now I feel I should hurry up and put it all in context with the new papers. I'll get on it -- in the meantime, there's a huge amount of information on the topic online beyond what I've linked to above, if you're interested.

Links on Extinctions and Enceladus

2009-11-23T08:09:00.001-06:00

I'm slow getting around to this, but Jacquelyn Gill of UW-Madison and others had an important paper in the November 20th Science on megafauna extinction-Younger Dryas-Clovis culture-cometary impact research. (I touched on the topic in the introduction to this post.) Gill et al.'s abstract is here, and you can find an excellent summary of the popular press coverage at the Knight Science Journalism Tracker, here. If you're not familiar with that website, they provide consistently thorough and usually insightful reviews of popular science journalism. On the research itself: Gill and her coauthors use spore counts and other paleobiological data from lake sediment cores to posit that the megafauna went into a steep decline 800-1900 years before the proposed impact and climate cooling. Yet again, the search for a single smoking gun for a mass extinction comes up short.

Plumes on Enceladus. (Credit to NASA/JPL/SSI, mosaic by Emily Lakdawalla, from her post here.)

In other news, The Planetary Society blog posts some amazing images from Cassini's second flyby of Enceladus -- here, here, and here. The resolution of the vapor/ice plumes is amazing. I link to that site often (we bloggers are a parasitical bunch) so if you're interested in this kind of think you should follow it directly.

The Oldest Crust and the Earliest Water on Earth

2009-11-22T22:05:00.003-06:00

I’ve written a few posts now on the distribution of water on the Moon, the formation of the Earth-Moon system, and the origin of its volatiles. I've been wading through the literature for a few months now and I'm only now starting to pull it together. It's worthwhile, though, because one of the things that makes Earth special is that it's wet.

Water is intrinsic to all parts of the plate tectonic cycle: when the oceanic crust forms at mid-ocean ridges it incorporates water; when this same crust is recycled into the mantle in subduction zones, metamorphic reactions at depth cause the slab to give up water, and this causes the volcanism that creates continental crust. A trace of water deep in the Earth lowers the mantle’s viscosity enough that it can flow and convect, and this drives plate movement. Plate tectonics regulates Earth’s climate, replenishes its surface nutrient supply, and is probably responsible for Earth’s long-lived magnetic field, all of which are conducive to, if not necessary for, the evolution of complex life. Clearly, understanding how water gets into a planetary system is important in determining how Earth-like planets, and complex life, come to exist.

The oldest known zircon. The spot of the ion microprobe analysis is circled. From John Valley's website.

About eight years ago, there was a surprising discovery of just how early large amounts of liquid water appeared on Earth. The proto-Earth accreted by 4.55 billion years ago and the giant impact formed the moon about 4.5 billion years ago. Until recently the early stage of Earth’s history was thought to have been terribly inhospitable: largely molten and dry. It was named the Hadean eon for its presumed hell-like conditions. There is now very good evidence for surface water by 4.4 billion years ago. Although we don’t have rocks that old, we do have individual crystals of the mineral zircon (ZrSiO₄).

Zircon is a very robust mineral; it can survive in sedimentary, metamorphic, or igneous environments and remain relatively unchanged. The ancient zircons in question were found in metamorphic rocks of the Jack Hills of Western Australia. They originally crystallized in magmatic rocks that were later eroded, and many of the grains were deposited in river deltas. These sedimentary deposits turned to sedimentary rocks (conglomerates) and were later metamorphosed to become metaconglomerates. The zircons have ages as old as 4.4 billion years and are the oldest continental material yet found. Moreover, their oxygen isotope signatures indicate that their magmas formed by melting of material that had interacted with low-temperature water. Two seminal papers on these zircons are Mojzsis et al. (2001) and Peck et al. (2001).

Outcrop in the Jack Hills, western Australia, from which the oldest known zircons were collected. These minerals provide evidence for continental crust and liquid water by 4.4 billion years ago. From John Valley's website.

This means that several oceans worth of water accumulated on the Earth within 50-100 million years after the giant impact (if you agree with Albarède and his sources that support the late veneer model). The impact flux was much higher then – about one million to one billion times higher than now. The Moon is much smaller than the Earth and has less gravity, so it would have experienced fewer impacts and more of an impactor’s volatiles would escape upon collision with the Moon. It’s possible that one test of the late veneer model may be whether the Moon’s volatile budget meets these expectations; data from LCROSS, LRO, and future missions could help evaluate this. Of course, this is complicated by the later impacts which overprint the earliest history. (Post-late veneer impacts will be a topic of a future post). It’s not clear if it will be possible to unravel the earliest portions.

Learn How to Process Space Pics

2009-11-19T16:01:00.002-06:00

Rock layers in the Home Plate outcrop on Mars. Taken by Spirit's Pancam; false color mosaic. (NASA/JPL/Cornell)

Emily Lakdawalla over at the Planetary Society has been posting a series of tutorials on downloading and processing images from NASA. She held the second live webex conference today, but you can stream the classes and download powerpoint presentations from the tutorial section of their site.

Today, she used examples from the Mars Exploration Rovers (the MER program) and Cassini. Check it out, but remember that the Planetary Society is a non-profit organization and would appreciate donations.

Speaking of MER, attempts to free Spirit aren't going so well.

The Source of Water on the Earth and Moon

2009-11-17T09:58:00.013-06:00

A protoplanetary disk rotating around a young star. (NASA)

Francis Albarède published a review paper in the October 29^th Nature on the source of water and other volatiles in the Earth-Moon system. I read it when it came out but the LCROSS results prompted me to revisit it. Here’s a portion of Albarède’s abstract:

Accretion left the terrestrial planets depleted in volatile components. Here I examine evidence for the hypothesis that the Moon and the Earth were essentially dry immediately after the formation of Moon – by a giant impact on the proto-Earth – and only much later gained volatiles through accretion of wet material delivered from beyond the asteroid belt. This view is supported by U–Pb and I–Xe chronologies, which show that water delivery peaked ~100 million years after the isolation of the Solar System. Introduction of water into the terrestrial mantle triggered plate tectonics, which may have been crucial for the emergence of life…

Here are some possible scenarios:

Wet accretion: Water was incorporated during accretion of the Earth. A giant impact ejected the material that formed the Moon, but this process caused escape of much of the volatile component (water, etc.) from the Moon.
Dry accretion: The Earth-Moon system did not include a significant amount of water until after the Moon’s formation. Impactors, i.e., asteroids and comets, delivered water and other volatiles.
Wet accretion + devolatilization: The Earth-Moon system was accreted wet but lost water to degassing during an early period of global magmatism. Impactors delivered water and other volatiles.

Albarède argues for dry accretion by reviewing evidence that the inner planets, including Earth, are too close to the center of the solar system for water to have condensed during their formation. Water was added later.

Our solar system formed from a solar nebula, which was hottest towards the center (now the sun). As the nebula cooled, elements began to condense from their gaseous to their solid states. Refractory elements, including platinum group elements, Si, Al, Fe, Mg, and Ca, condensed first; next were alkali elements like Na and K, and eventually, as temperature continued to fall, the volatiles like C and water solidified. Our sun was much more violent during its early stages (the T Tauri Phase) and it produced strong solar winds which swept much of the gaseous nebula outwards. Less refractory elements that hadn’t condensed at high temperatures were pushed away from the sun. This model fits observations nicely: the rocky inner planets are composed of refractory elements and alkalis, while the gas giants and icy planets formed farther out. The nebular model also predicts that there was a “snow line” at 2 to 2.5 AU (AU = Astronomical Units, or the average distance from the Earth to the sun); any water closer to the sun than this wouldn’t have condensed before being cleared by early solar radiation. The prediction of a snow line necessitates the dry accretion model: there shouldn’t be water on the inner planets.

A proposed timeline of Earth's accretion and some of the isotope systems that constrain it from Albarède’s Figure 5. During its T-Tauri phase, the sun cleared the inner solar system of volatiles. Runaway growth marks the peak accretion of Mars-sized bodies that form the cores of giant planets and the proto-inner planets. The Hf-W system constrains core formation on Earth. The giant impact forms the Earth's Moon. The late veneer deposits water and siderophile elements.

But of course there’s water on Earth, and quite a lot: ~1.34*10⁹ km³ in the oceans, and perhaps 1-3 times that much in the mantle. If Earth accreted dry, then this water may have been added as part of the late veneer, when formation of the giant planets like Jupiter upset the orbits of planetesimals and asteroids and sent many of them hurtling into the inner solar system. This extraterrestrial material would be incorporated into the Earth’s mantle by foundering of early surface rocks or by the onset of subduction. Icy comets seem like a likely source, but the isotopic composition of Earth’s water is a better match for carbonaceous chondrites, which are concentrated in the outer part of the Main Belt, just past the snow line. They contain up to 10% water by weight.

Albarède’s paper draws on the recent literature, including much work on the H, O, Pb, W, and I-Xe isotopic systems, to make his case. He synthesizes what seem to be (based on my literature searches) the newest widely accepted findings across a number of fields. It is not, however, a consensus view. There are arguments against the standard late veneer model based on W isotope compositions and the Earth’s abundances of siderophile elements (elements that include the platinum group, Mn, Co, and Au which behave like Fe and should follow Fe during geochemical processes). We should be able to reconcile the abundances of elements deposited on Earth as part of the late veneer – not just the water, but the siderophile elements, too – and we’re not quite there yet. See, for example, Alex Halliday’s 2008 paper that discusses some of these problems.

Those issues need to be resolved, and research on siderophile element behavior during core formation is addressing them. For now, the nebular model as I understand it all but requires the late veneer to explain Earth’s water, but there’s plenty I don’t understand (yet). We’ll see.

*****

My post on the LCROSS results.

A post on Chandrayaan’s lunar water discovery.

Albarède, F. (2009). Volatile accretion history of the terrestrial planets and dynamic implications Nature, 461 (7268), 1227-1233 DOI: 10.1038/nature08477

LCROSS Found Water Ice

2009-11-13T12:36:00.003-06:00

Mosaic of the Lunar south pole from Clementine images (NASA).

The LCROSS Science team held a briefing at 9AM PST today with results from the October 9th impact. Here's the summary of the content. They found ice. Previously, Lunar Prospector found hydrogen, Cassini detected water and hydroxyl (OH), and Chandrayaan found significant amounts of both water and OH present globally in hydrous minerals and as monolayers adsorbed on the surface of grains in the regolith. Those instruments were only able to examine the top few millimeters of the lunar surface. The LCROSS impact excavated to a depth of several meters in a permanently shadowed crater, and the science team says they've found large amounts of water in the impact plume -- too much, they think, to be solely due to water on the surface of mineral grains. It looks like the Moon may have a hydrologic cycle of sorts, with low concentrations in the regolith that increase towards higher latitudes and culminate in ice trapped in the permanently shadowed craters at the poles.

I posted on lunar water last month, but I'll recap. Lunar water may come from impactors, especially comets. It may come from the solar wind interaction with regolith. It is possible that water comes from degassing of the lunar interior -- we know that some water is present in the lunar interior because it has been found in volcanic samples returned by Apollo missions. My money is on the comets providing the bulk of it.

The science team is working to constrain the possible abundances of ethanol, hydrocarbons, and other chemical species. Apparently, ground-based observers of the LCROSS plume detected Na (Na is generally depleted on the Moon because it's volatile, so it makes sense to find it with water and other volatiles). We'll see detailed news reports later today and journal articles are on the way.

Impacts, Ocean Mixing, Iron, and Life

2009-11-11T11:19:00.006-06:00

Banded Iron Formation near Timmons in Northern Ontario; 2.7 billion years old. From Laurentian University.

John Slack and William Cannon, two USGS geologists based in Reston, Virginia, have a paper in this month’s Geology that suggests a link between a major asteroid impact and the cessation of banded iron deposition 1.85 billion years ago. Scientists love to correlate impacts with major geological events. We’ve recently seen research suggesting an extraterrestrial impact or impacts 12,900 years ago led to the onset of the Younger Dryas, the last ice age extinction, and the end of the Clovis Culture in North America. (And a skeptical take on that position from RealClimate.org). Though most people accept that the Chicxulub impacter was related to the K-T extinction, there are reasonable contentions that it’s not the sole cause (read the Science Daily article and the abstract).

Map of gravity anomalies in the Chixculub crater. The circular portion is ~180 km across. The white line is the superimposed coast of the Yucatán Peninsula; the white dots are mapped sinkholes. Image from the Geological Society of Canada.

Here’s Slack and Cannon’s abstract:

In the Lake Superior region of North America, deposition of most banded iron formations (BIFs) ended abruptly 1.85 Ga ago, coincident with the oceanic impact of the giant Sudbury extraterrestrial bolide. We propose a new model in which this impact produced global mixing of shallow oxic and deep anoxic waters of the Paleoproterozoic ocean, creating a suboxic redox state for deep seawater. This suboxic state, characterized by only small concentrations of dissolved O₂ (~1 μM), prevented transport of hydrothermally derived Fe(II) from the deep ocean to continental-margin settings, ending an ~1.1 billion-year-long period of episodic BIF mineralization. The model is supported by the nature of Precambrian deep-water exhalative chemical sediments, which changed from predominantly sulfide facies prior to ca. 1.85 Ga to mainly oxide facies thereafter.

Some background:

Banded Iron Formations (BIFs) are chemical sedimentary deposits composed of thin alternating layers of Fe-rich minerals and silica. The Fe-minerals are usually oxides such as magnetite (Fe₃O₄) or hematite (Fe₂O₃), but they may be Fe-sulfides like pyrite (FeS₂) or Fe-carbonates such as siderite (FeCO₃). The silica is usually microcrystalline or amorphous chert or jasper. They’re found all over the world but most of them are limited to rocks between 3.5 and 1.8 billion years old, from the Archean eon and the Paleoproterozoic era. There’s a group of huge Paleoproterozoic BIFs around Lake Superior in Minnesota and Michigan, many of which are mined for Fe ore.

Some context to understand the Slack and Cannon paper:

Oxidized, or ferric, iron (Fe³⁺) is relatively insoluble in seawater and will precipitate out. Ferrous iron (Fe²⁺) is more soluble in seawater.
The ocean was layered until ~1.8 billion years ago, with an anoxic (oxygen-depleted) deep ocean at >600 meter depth and an oxic (relatively oxygen-rich) shallow ocean on the continental shelves.
Although Superior Province BIFs formed on the continental shelves, the chemical signature, particularly the abundances of rare earth elements, indicates that the source of the Fe was deep ocean hydrothermal vents (Klein, 2005, abstract). Magmatically heated water convects through the newly formed basaltic crust and sequesters Fe, which is then pumped out into the seawater.
The Fe from the deep oceans remained in solution under anoxic conditions, but came out of solution on shallow continental shelves where the oxic conditions promoted deposition of Fe³⁺-rich sediments, forming the Superior Province BIFs.
At around 1.8 billion years ago, the oceans became mixed, resulting in some oxygen (suboxic conditions) in the deep oceans. Deep ocean sediments record strong evidence for this mixing: prior to 1.85 billion years ago deepwater chemical sediments were largely sulfide-rich, afterward they were oxide-rich (Slack et al., 2007, abstract only).
This mixing meant that more Fe was oxidized and precipitated in the deep ocean and so it wasn’t transported in solution up to the continental shelves. Thus ended the Superior-type BIFs.

The Slack and Cannon paper suggests that the Sudbury impact was responsible for stirring the oceans and delivering oxygen-rich material to the deep ocean by underwater landslides. This oxygenated the deep ocean and led to the cessation of BIF formation in the Superior Province and globally. In an upcoming paper, Cannon and others give detailed evidence of an impact-deposited layer of rock that immediately overlies the major Superior BIFs. They tie this to the 1.85 billion year old Sudbury impact ~600 kilometers east of the Superior Province – at 250 kilometers diameter, it’s the second largest known impact event. (Sudbury is a fascinating topic in itself; the impact led to the formation of one of the world’s largest ore deposits. The Geological Society of Canada has nice compilations of geologic maps and cross-sections and other images here and here.)

Geologic Map of the Sudbury impact structure from the Geological Society of Canada. The crater is elongate due to later deformation.

Previously, the ocean mixing event was thought to be related to more gradual processes. Possibly just the incremental accumulation of photosynthesizing organisms, or terrestrial environmental changes. Slack and Cannon make a good case and their models seem sound. Often, the arguments against these correlations come from timing: as people gather more stratigraphic and radiogenic dates stories can fall apart (this is what Keller and others have posited with regard to the K-T extinction, though I remain slightly skeptical). Almost all of the BIF articles I’ve linked to here will affirm how important these mixing and oxygenation events are to the evolution of life, so I expect we’ll hear more about this.

(I found this story through my subscription to Geology, but there were good articles in Science News and Wired Science yesterday.)

Slack, J., & Cannon, W. (2009). Extraterrestrial demise of banded iron formations 1.85 billion years ago Geology, 37 (11), 1011-1014 DOI: 10.1130/G30259A.1

Mars

2009-11-09T10:49:00.000-06:00

You can find an amazing array of images of the Martian landscape here. Each picture has links to the High Resolution Imaging Science Experiment (HIRISE) at the University of Arizona, complete with details and context.

Thanks to Bad Astronomy for the link.

Basaltic sand dunes in Abalos Undae region of Mars; picture is 1.2 km across. (from HIRISE)

Commentary on the Schisms

2009-11-06T14:33:00.003-06:00

Dan Jones had an article yesterday in The New Statesman on the religious-creationist-atheist-accomodationist schisms. It’s short and worth reading, but there’s one section I found particularly interesting which references Steven Pinker’s opinions about Francis Collins, the evangelical Christian who now serves as the head of the National Institute of Health. Here’s the excerpt:

Steven Pinker, a Harvard psychologist and an atheist, has voiced grave misgivings over Collins's appointment - not just because of his religious beliefs, but because of his "public advocacy" that "atheistic materialism" must be resisted. Collins believes in an interventionist God who, in his own words, "gifted humanity with the knowledge of good and evil (the moral law), with free will, and with an immortal soul".

Although, in principle, religious beliefs need not affect one's day-to-day science, in practice, they might. Take research on the foundations of human sociality and ethics, currently one of the hottest areas in behavioural science. Researchers are probing these questions with evolutionary theory, comparative primate studies and neurobiology, among other approaches, but no one invokes non-natural or non-material explanations. Are these instances of atheistic materialism to be resisted?

How would Collins's views affect the priority he might give to funding such research, if his prime belief is that ethics and the moral law are God-given? It is perfectly possible that he would accept the materialistic explanation of morality, and just add that everything was set up by God in such a way that naturalistic processes were bound to produce a big-brained moral species. Time will tell if, and how, NIH funding changes under his leadership. It would be unfair to prejudge the case.

Pinker raises a reasonable question and I’ll admit to some discomfort with Collins’ outspoken religiosity. But, Collins has proven himself to be a good scientist and administrator (he’s an MD/PhD who led the Human Genome Project). I’ve heard and read interviews with him and though I haven’t read his book, I’ve read the summary of it and looked at the website he founded. He believes in the science of evolution. He deserves the benefit of the doubt.

What drew me to this article was Chris Mooy’s mention of it in his blog, The Intersection. (Now I have at least two degrees of meta-commentary going on.) Chris has found himself on the accomodationist side of the schism (which you are by default if you don’t fall on the New Atheist side). I very much liked his closing comments, from which I will liberally excise:

Throughout all of this, there has been a longstanding perception in much of America that science and religion are inimical to one another, and contradictory. This is a perception that the creationists have fanned for a long time (hence the “secular humanism” business), and that the New Atheists now also explicitly support. In a sense, it works to the advantage of both groups, at the expense...of the middle.

This “conflict narrative” or “conflict thesis” [Mooney’s link] also happens to be a historically misinformed perspective, in my view, and one that is questionable on other fronts as well–but I think it is a dominant perception, and constantly reinforced by the mass media.

For those of us critical of the New Atheists, then, it is not because we think they have emerged and dramatically upset the culture war stalemate over the teaching of evolution in some way. Rather, it is because they are likely [to] alienate the middle ground and aren’t a constructive response, in the present moment, to the need to defuse longstanding tensions over science and religion in America.

Well said.

MESSENGER Link

2009-11-05T20:16:00.002-06:00

As always, Emily Lakdawalla offers a good overview, this time through the Planetary Society News instead of the blog. She covers most of the good stuff from MESSENGER's 3rd flyby. Especially valuable is the description of how data from the neutron spectrometer is interpreted, which I didn't have the strength to tackle.