A Review of the Chicxulub impact extinction link

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

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

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)

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

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?

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

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

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

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