[X] My Assistant

[Guest_BruceMoomaw_*]

This is inevitably a subjective list of the stuff that happened to strike me, personally, as especially interesting -- but here goes:

#2401: MER-B's analysis of the composition of the "cobblestones" that have intrigued the scientists for so long. They turn out to be varying mixtures of the sulfate-processed stuff seen elsewhere with less modified basalt, suggesting that they are local crater ejecta from lower layers of the deposit that were exposed to less acidic water.

#1312: Some nice sharp photos of Phobos by Mars Express from different angles.

#1592: Mars Express' OMEGA analyses of White Rock prove, once and for all, that there's nothing mineralogically unusual about it -- it's just indurated dust with nothing at all distinguishing it from the composition of the surrounding surface, and in particular no signs of water modification.

#2283: THEMIS maps of the southern polar cap's overlying CO2 layer suggest that -- contrary to MGS' earlier photos of the growing "Swiss cheese" holes -- it is actully GROWING in overall areal extent right now from one summer to the next, rather than shrinking due to slowly warming polar summers. Hard to know what to make of this.

#2376: Latest analyses of Mars Odyssey's ner-polar gamma-ray and neutron studies suggests that the dust layer over the near-solid underlying ice is no more than 4-6 cm thick, which "may present difficulties for those investigations interested in seeing gradients as a function of depth in the dry soil" -- and which my explain the mission's recent increase in emphsis on properly sampling and analyzing the permafrost itself, which has led to the addition of a rotating "ice shredder" on the rear of the sampling scoop. (See Deborah Bass' blog: http://phoenix.lpl.arizona.edu/features/we...eborah_bass.php .)

#2070, 1648 and 1739: the debate over the nature of the "Type 2" rock seen by MGS' TES covering the rocky parts of the nothern lowlands (as comapred to the regular "Type 1" basalt in the southern highlands) continues. Odyssey's GRS (#2070) shows a higher abudance of both potassium and thorium in the Type 2 rock, which suggests strongly that it is NOT just water-modified basalt, but some volcanic rock that was different in composition from the first, such as andesite, or at least basalt from "compositionally distinct mantle sources". However, OMEGA and THEMIS maps show VERY close local proximities of some deposits of Type 1 and Type 2 rock -- so close as to make it hard to see how they could be flows of seriously different types of lava, and thus suggesting again surface water modification of Type 1 basalt to make the Type 2 rock. This one has yet to be settled.

#2035: Strong visual evidence of eskers (ridges of sediment deposited by streams of meltwater underneath glaciers) in Isidis Planitia, "strongly indicative of a widespread ice cover across the basin at some stage in the past, even at the low latitude of Isidis Planitia."

#1242: Examination by TES and THEMIS of the phyllosilicate clay deposits found by OMEGA suggest that the clay component is actually quite dilute: "So far, it is not clear from unmixing results that the putative clay-rich deposits have significantly higher abun-dances of modeled clay minerals. Overall, these preliminary results suggest that the putative clay-bearing deposits are composed of igneous materials in large part. The thermal infrared spectral character of these deposits is not consistent with the expected spectral signature from extremely clay-rich materials such as bentonite layers...The deposits are not dominantly clay – they are com-posed of igneous materials with a limited clay component." Not all that much liquid water even under the most favorable circumstances in the Noachian?

#1342: Results of the latest survey of Mars for alluvial fans. 25 have now been found (still out of 40,000 craters), but there's a puzzle: "One fan has been found in the walls of Valles Marineris. It is the only fan not found originating in a crater rim. This is surprising because Valles Marineris provides an excellent topographic setting for the formation of alluvial fans (i. e. an abrupt topographic dictomy), so it might be expected that many more fans should be present. Future modeling and analysis will focus on why there are so few fans and why they form in specific locations along the canyon walls."

#2011: A suggestion that the dramatic (and recent) floods that have carved the features in Cerberus Plains may have been driven by the pressure of underground CO2, which should please Nick Hoffman no end.

Now, finishing up with a few abstracts from the new EGU, COSPAR and Astrobiology Science conferences:

http://www.cosis.net/abstracts/EGU06/06134/EGU06-J-06134.pdf and http://www.cosis.net/abstracts/EGU06/09673/EGU06-J-09673.pdf : Werner and Neukum say that an analysis of the size-frequency function of secondary craters proves that (contrary to William Hartmann) they are NOT making us serious fouling up our crater-rate estimates of the age of various terrains on the surfaces of other worlds by overestimating their age -- and particularly not for Mars. (Neukum, however, withdrew his paper from the EGU meeting for some reason.)

http://www.cosis.net/abstracts/COSPAR2006/...006-A-00693.pdf : Matt Golombek says again that the MER observations confirm that "a dry and desiccating environment similar to today’s has been active throughout the Hesperian and Amazonian (since ~3.7 Ga). By comparison, erosion rates estimated from changes in Noachian age crater distributions and shapes are 3-5 orders of magnitude higher and comparable to slow denudation rates on the Earth (>5 micron/yr) that are dominated by liquid water. The erosion rates from Gusev as well as those from Viking 1 and Pathfinder strongly limit this warmer and wetter period (recorded in the Meridiani evaporites and Columbia Hills) to the Noachian, pre-3.7 Ga and a dry and desiccating climate since."

http://abscicon2006.arc.nasa.gov/agenda-session.php?sid=23 , paper #34: Krasnopolsky has a whole series of interesting remarks regarding Martian methane. (He has, however, said many of the same things in two recent Icarus articles: http://www.ifa.hawaii.edu/~meech/NAIJC/pap...y_CH4onMars.pdf and http://www.ifa.hawaii.edu/~meech/NAIJC/pap...y_SO2onMars.pdf .)

[Bob Shaw]

Bruce:

Very interesting! Heroic reportage, as ever

One that particularly interests me is #2035: Strong visual evidence of eskers (ridges of sediment deposited by streams of meltwater underneath glaciers) in Isidis Planitia, "strongly indicative of a widespread ice cover across the basin at some stage in the past, even at the low latitude of Isidis Planitia."

Is that one online anywhere? Inded, are they all online?

Bob Shaw

[tty]

Is that one online anywhere? Inded, are they all online?

Bob Shaw

http://www.lpi.usra.edu/meetings/lpsc2006/...6.download.html

or for that particular abstract:

http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2035.pdf

tty

[Bob Shaw]

Excellent!

Bob Shaw

[Guest_BruceMoomaw_*]

If they weren't online, I'd never have seen them at all. I've had a stack of the things sitting on my desk for months, while I went through them trying to decide what was worth mentioning or not. (You'll recall that I've been sprinkling mentions of other interesting LPSC abstracts through my comments here for some time.) But -- by the same token -- everyone else is free to try to find the stuff that interests them; it comes out online as early as January before each conference. Just be warned that there are hundreds of abstracts to slog through, although I find myself skipping past most of the very large fraction that involve meteorite analyses.

[Guest_BruceMoomaw_*]

Some more on two subjects:

First (in this entry): Like the Gully Origin Wars that I mentioned in an earlier thread, the war over the origin of the layered deposits in the bottoms of the Valles Marineris canyons rages on. There's a veritable swarm of theories, briefly listed in L.R. Gaddis' abstract (#2076). Trying to follow this debate makes my head spin -- damn it, Jim, I'm a writer, not a sedimentologist! But:

R. Stesky suggests (#2013) that the way the layers are draped over underlying bedrock features suggests that they were not laid down by forceful fluvial flow or by glaciers. Neil Coleman suggests (#1879) that the discovery of outflow channels on a Martian plateau fully 2.5 km above the Martian datum (the average altitude) requires a very high-altitude water table, and that the logical supplier would be lakes almost filling the very deep and high-altitude Valles Marineris canyons. However, other analysts (#2076 and 2022) conclude that the layers slope too much, following underlying bedrock, to be lake-bottom sediments, and are more likely to be some kind of volcanic ashfall.

Brian Lucchitta (#1952) claims to have found evidence that the deposits at the bottom of west Candor Chasma are the result of a subsurface water layer well up above the bottom of the canyon, which at some point broke through the canyon's side and swept large amounts of rocky debris down to the bottom of the canyon. But E. Hauber and a large number of his Mars Express co-workers (#2022) claim that OMEGA found water-altered minerals only in "very deep portions" of Hebes Chasma -- not on the sides of that canyon -- suggesting that the alteration was done only by groundwater at the canyon's bottom, on volcanic ashfall. Gaddis, in #2076, elaborates on that idea by suggesting that the Candor Chasma deposits may have been due to volcanic ash eruptions that perhaps occurred actually "in shallow water" at the canyon's bottom. And Thomas McCord and the Mars Express HRSC team (#1757) conclude that the camera's color data define two main different-colored materials -- light red and darker less-red -- all over Mars, and that these two general types of different-colored materials intertwine within the Valles Marineris canyons in strange and complex ways which don't really match up well with ANY obvious theory. One can hope that MRO's very high-resolution photo and composition maps will finally solve the mystery of the Valles Marineris floor layered deposits -- just as I think it likely that MRO will be able to use these (along with its SHARAD data) to solve the gully mystery.

[Guest_BruceMoomaw_*]

My second additional point concerns the fact that a lot of researchers have concluded recently that Gebne Shoemaker was right in guessing back in the 1960s that a great many of the smaller craters (< 1 km) on worlds throughout the Solar System are actually secondary craters -- produced not by meteoroids, but by hunks of ejecta thrown out by the huge meteoroid impacts that produced much bigger craters -- and that this explains the fact that there seems to be an excess of these smaller craters above a simple inverse-square relationship between the diameter of craters and their number, such as exists with bigger craters. If so, then it's risky to try to estimate the ages of the youngest terrains on Mars -- such as the slopes of the Tharsis giant shield volcanoes, or the Cerberus Plains -- because they're so young that not enough big craters exist on them for dating estimates; the only way to crater-date them IS by counting such tiny craters on them. And if a lot of the supposed small primary craters on them are really secondaries produced on them by big chunks of ejecta thrown from giant impacts elsewhere on Mars, then they may be a lot younger than thought -- meaning that both volcanic and weather-related resurfacing on recent-day Mars may be more energetic than had been thought.

The advocates of a lot of sub-kilometer secondary cratering on the various worlds of the Solar System are numerous -- Al McEwen, Clark Chapman, William Bottke, N. Artemieva -- and they had a whole session to themselves at last year's LPSC ( http://www.lpi.usra.edu/meetings/lpsc2005/pdf/sess11.pdf ). McEwen, however, has come up with a peculiar twist on the idea, to the effect that our uncertainty about dating using small craters may actually prove that a lot of supposedly young Martian terrains may actually be a lot OLDER than thought. http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2111.pdf : "What is the maximum age we can assign to terrains free of any craters larger than 300 m? We expect the number of primary craters ≥ 300 m/km[2]/Ma to be ~3.5 x 10[-5]... over the past 3.4 billion years. Therefore, the maximum cratering age is a function of the area of a crater-free unit. For terrains covering ~100 square km, like sets of gullies and debris aprons within a large crater, the upper age limit is ~300 million years, greater than the few Ma upper limit suggested by Malin and Edgett. Mustard et al. stated that the absence of craters larger than 100 m on the mid-latitude debris mantle indicates a maximum age of 150,000 years... Recalculating based on the absence of craters larger than 300 m increases the maximum age to 10 million years. If we assume a power-law slope of -3 for craters from 0.3-1 km diameter, then the maximum age increases to ~30 million years. Since many gullies cut this debris mantle, their age limit is also ~10 to 30 million years. There has been geologically recent activity and climate change on Mars, but we cannot justify correlations with very recent (order 100,000 yr) obliquity cycles given our current state of understanding."

Well. Gerhard Neukum and S. Werner are advocates of the idea that there really IS an excess of small impactors in the Asteroid Belt above the inverse-square slope that exists for bigger impactors, and they've hit the ceiling. #2379: "Our method making use of small craters was criticized by McEwen et al. As pointed out already by [William] Hartmann, McEwen et al.’s critique is not well-based, but essentially erroneous and contradictory in itself. McEwen et al.’s arguments which are mainly theoretical have recently been thoroughly analyzed by Werner et al. on the basis of mainly hard empirical data. We have been able to demonstrate in these papers on top of Hartmann’s arguments that McEwen et al.’s reasoning is totally invalid." Their evidence was summarized in those two EGU abstracts I mentioned last night -- although my memory was faulty in saying that Hartmann was one of their archenemies; they actually regard him as a supporter. But I overlooked one LPSC abstract from this year in which they go into more detail on their argument (#1595).

Their argument is simply this: if the excess of craters below 1 km diameter is due to the fact that a lot of those craters are really secondaries produced by ejecta from bigger craters, then -- on older terrains -- you would also expect an excess of somewhat BIGGER secondary craters produced by the really huge chunks of ejecta thrown out by the occasional really huge primary-crater impact events. But this isn't the case; no matter how old the terrain is, the excess of craters turns up ONLY when the craters are below 1 km diameter. Therefore there really must be an excess of smaller objects in the Asteroid Belt -- from which most inner-world impactors since the Solar System's earliest days have come -- due to the nature of collision and fragmentation processes going on there. (They also say that the distribution of small craters seen by Galileo on the asteroid Gaspra --where even McEwen admits that "secondaries must be estremely rare" -- backs this up.) By their calculations, only about 10% of the craters on Mars and the Moon that are less 1 km wide can be secondaries, so their effect in causing mis-estimations of terrain age are relatively small.

But Hartmann, at last year's LPSC, actually made a more general attack on McEwen ( http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1427.pdf ) that's a lot harder to shake off than Neukum and Werner's technical argument. Namely, that -- even if McEwen IS right -- it doesn't have as scientifically earhthsking an effect as he seems to think, because McEwen's reasoning that the unreliability of age-dating young Martian surfaces using small craters could mean that they're a lot OLDER than Neukum and Hartmann think is flat-out absurd. "The effect of unrecognized secondaries is not equivalent to a normal uncertainty or error bar, as implied [by McEwen], but can only REDUCE our reported [estimated surface] ages... an intriguing fact in itself." That is, if he, Neukum and Werner are wrong, then Mars' recent rate of volcanic and/or climatic surface change is even higher -- and thus more interesting -- than they've estimated.

There is, however, yet another twist to the story: namely, that if secondary craters really are major contributors to the population of small craters on Mars, then their naturally lumpy distribution over the surface -- given that they're thrown out in ray patterns only by the biggest impacts on Mars, as on the Moon -- makes it extremely hard to estimate the RELATIVE ages of different types of younger terrain on Mars). McEwen also brings up this point, as do Clark Chapman ( http://www.lpi.usra.edu/meetings/earlymars2004/pdf/8028.pdf ) and Jeff Plescia ( http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2171.pdf ). Plescia is especially forceful: "Craters in the size range of meters to hundreds of meters can not be reliably used for either relative or absolute chronologies on Mars. Processes not well understood result in significant differences in the frequency and statistics of impact craters on surfaces that should be approximately the same age" -- and points out that, if you try to use small craters to age-date the overlapping Olympus Mons calderas, you get the exact reverse of their relative sequence of ages as shown by their cross-cutting! (McEwen added back in 2003 that "If Head et al. have the right explanation for the age variations of Martian meteorites" -- that is, that only chunks of hard, usually recent surface igneous lava flows survive getting blasted into space by big impacts, while softer regolith-mantled surface material crumbles into dust -- "then impacts into young lava plains with little regolith must produce many more distant secondary craters than impacts into older regolith-mantled terrains. Hence the secondary cratering rate in the southern highlands must be less than that in the northern plains." http://www.lpi.usra.edu/meetings/sixthmars2003/pdf/3268.pdf )

Okay. Clearly -- especially when you add the difficulty in using small craters to try to age-date the original volcanic FORMATION of terrains on Mars, given that the planet's winds have often caused them to be covered or re-exhumed -- we need other techniques to age-date Martian surface material. (Not just its original solidification out of lava -- but also the duration of time over which it's been most recently exposed to the surface, using such techniques as radioluminescent dating). And since Martian sample-return missions will always be few and far-between, the importance of trying to develop halfway reliable in-situ age-dating instruments for Mars landers is indeed great if we are ever to have a halfway decent understanding of the planet's overall geological history.

[Bob Shaw]

Bruce:

In some repects I've always thought that 'crater counting' ought to be a certifiable disease.

Bob Shaw

[CosmicRocker]

Bruce:

Very interesting! Heroic reportage, as ever...

An understatement if ever there was one.


Thank you Bruce, for bringing us this news summary. I'm not well-versed in crater counting, but that gave me some new things to consider.

The Southern polar cap is actually growing? Is that politically correct to mention in mixed company?

Eskers? I could swear I saw some drumlins once. I could be way off base, but I always thought that the discussion of glacial phenomena on Mars was a neglected (but less so recently,) subject.

[Guest_BruceMoomaw_*]

It may be premature to say that the southern CO2 polar cap is "growing"; all we can say right now is that the measurements of its total area by OMEGA seem to show this, at the same time that the "Swiss cheese" holes in it shown by the camera on MGS seem also to be growing. Given the strange processes that occur in that CO2 ice layer, there may be no contradiction between the two.

Thanks very much for the compliments, by the way. I have quite a few more recent tidbits on more scattered subjects for you guys (I tend to let these things stack up on my desk, until I have to make a spasmodic effort to clear them off again). But I'm also reexamining right now the stuff I have on primary vs. secondary craters -- I'm not entirely sure that the interpretation I offered you earlier was correct in every way. More soon.

[Bob Shaw]

Eskers? I could swear I saw some drumlins once. I could be way off base, but I always thought that the discussion of glacial phenomena on Mars was a neglected (but less so recently,) subject.

Tom:

I need help *not* seeing glacial features - I want other people to see them as a sanity check. I see glacial features all over the place (well, not at home, or at work, or whatever, I don't mean they're following me around, oh no. Come to think of it, maybe that explains the lumps in my lawn... ...thought they were molehills!). I see many more glacial than fluid flow features (except where there's been catastrophic flooding).

Bob Shaw

[tty]

I see glacial features all over the place (well, not at home, or at work, or whatever, I don't mean they're following me around, oh no.

I also see glacial landforms all over the place, which after all is not surprising since I live in an area that was glaciated until about 10,000 years ago. However I have difficulty in finding any on Mars.

Concerning this putative esker, I am rather dubious. It has some distinctly un-esker like characteristics. It is too sinuous for one thing and rather too continuous. Eskers are normally fairly straight and often have short breaks, often combined with slight offsets. To me this looks more like an inverted channel.

However it would be interesting to have high-definition images of the "break" and the "kinks". If there is no evidence for faulting it would strongly support the esker hypothesis since river channels don't have breaks in them.

If it really is an esker it has some quite interesting implications. Eskers only form in front of a retreating melting (not sublimating) ice-sheet. The ice-sheet must be fairly thick (since a considerable hydraulic head is required) and it must calve into fairly deep water (eskers form subaquatically, though they may be modified by wave action subsequently).

Incidentally the pingos mentioned in the abstract are not ice-cover related, rather they indicate permafrost with an intermittently melting active layer.

tty

[Guest_BruceMoomaw_*]

My reexamination of the recent abstracts on secondary cratering suggests that, when push comes to shove, the recent belief that there may be more fewer small primary craters on Mars and more small secondary ones than we thought doesn't really change all that much where actually dating wide areas of the Martian surface is concerned -- because the same principle surely applies to the Moon's surface, where age-dating of the returned Apollo and Luna samples has allowed us to use the total number of accumulated craters to date the surface pretty accurately, no matter what kind of craters it turns out we've actually been counting there.

Al McEwen admits this, in a kind of sideways manner, in his 2004 LPSC abstract ( http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1756.pdf ): "The evidence for fewer small primary craters
[on Mars] than expected is surprising because previous workers have concluded that Mars and the Moon are cratered by the same population of small bodies." One of his explanations is indeed that a lot of small craters thought to be primaries are actually secondaries on the Moon as well. His other possible explanations for the (apparent) disparity are singularly unconvincing, as he himself admits. One is that "The flattening of the size-frequency distribution reflects the actual population of small asteroidal fragments near Mars, as suggested for the main asteroid belt." But it's hard to see why this principle would apply just to small fragments in the Main Asteroid Belt and not to those objects that peppered the Moon as well -- and, sure enough, William Bottke et al ( http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1489.pdf ) conclude from their analysis of the size distributions of near-Earth asteroids with those in the Main Belt that small fragments in both do follow the same size curve, and therefore that "relatively few small craters on the Moon and Mars were formed by primary impacts...[M]ost small craters on the terrestrial planets are secondary impacts generated by ejecta from large craters."

BUT: the squabble over the primary/secondary ratio for small craters DOES have very important implications in trying to date smaller areas on any world, because the distribution of small secondaries thrown out by big primary impacts is naturally going to be very spotty and uneven from one small place on the surface to another. McEwen and Jeff Plescia remain correct on this point, and William Hartmann -- while defending our current estimates of the age of overall big regions on Mars ( http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1427.pdf ) -- admits it as well for smaller ones (as I pointed out in my earlier message above). It may be that, where the Moon is concerned, we've just been lucky so far in getting back all our radioactively age-dated Apollo and Luna samples from a relatively small collection of places where the local clustering of small secondary craters HASN'T fouled up our crater-based age-dating. Or it may be, as Hartmann suggests, that this problem is less important for the Moon than for Mars because all of the Moon's surface is so old, and has thus accumulated so many small secondary craters from big impacts, that they aren't all that unevenly distributed there -- whereas using counts of small craters to try to date small areas on the more recently resurfaced parts of Mars can be very risky.

[dvandorn]

The Moon is in a cratering equilibrium at most size scales, and the lunar highlands are in equilibrium at large scales, as well. That is to say, as craters are destroyed by erosion, new craters take their place. The lunar maria are not in equilibrium at large (1 km and larger) scales, because larger impactors have been much rarer since the maria were formed than they were prior.

So, when you're talking about 1m to 1km sized craters, crater counting on the Moon doesn't give you all that good a concept of age. Counting craters of a certain sharpness (i.e., that have undergone only up to 'x' amount of erosion) is a better age indicator.

The problem with crater counting on Mars as opposed to the Moon is that small (1m to 10m, especially) secondaries are pretty well erased by aeolian erosion in relatively short time scales. For example, Sleepy Hollow at Spirit's landing site is obviously a 1m to 2m secondary crater that has been eroded and filled in over millennia. It's not even recognizable as a crater in the CProto MOC images of the landing point. And yet, its origin and subsequent erosion are quite apparent when you look at it, from the ground, at a distance of less than 10 meters.

So, while on the Moon cratering reaches an equilibrium because the only real erosion process is additional cratering, the same thing does *not* happen on Mars, because the aeolian erosional processes are much more effective (i.e., have a far greater effect), over all time scales, than impact erosion processes.

And remember, when it comes to dating based on crater counting on the Moon, we have the following data points: 1) counts of craters of varying size and apparent freshness, and 2) geophysical dating of returned samples that identify absolute rock ages from given locations. It is only from those two data points that we have *interpolated* frequency of cratering events and size of impactors as a function of time.

The fact that the lunar surface is in a cratering equilibrium accounts for why we get the same crater counts of given sizes over different stretches of terrain -- it's simply a function of the lack of any other major erosional processes, so craters of most sizes, over billions of years, have reached equilibrium. The fact that the maria are less heavily cratered than the highlands makes a statement about the crater flux rate before and after the creation of the visible maria, and that's about it. It doesn't necessarily tie a given flux rate to a given timeframe. If you change the number of craters resulting from primary impacts, all you do is change the interpolated impactor flux rate -- you don't change the age indicated thereby.

-the other Doug

[Guest_BruceMoomaw_*]

I've alswo reinspected the 2006 LPSC abstracts on puzzle of the Type1/Type 2 Mars rock dispute. You'll recall that the spectra from the TES on MGS identified two distinct types of rock in the dark, dust-free parts of Mars' surface. Type 1, which fills much of the southern highlands, is no puzzle -- it's classic unweathered basalt. But Type 2, which fills some parts of the northern hemisphere such as Syrtis Major, is one. In 2001 there were three interpretations of it:

(1) Not basalt, but silica-rich andesite. This was the initial favorite, especially since the APXS measurements from the Pathfinder rover seemed to confirm high silicon content -- but andesitic rock would require that the magma oozing up in Mars' northern hemisphere encountered a great deal of deeply buried subsurface water or ice.

(2) Regular basalt extensively weathered by exposure to water.

(3) Basalt with an unusually large amount of volcanic glass in it (maybe weathered rather than silica-rich).

Well. Mars' surface has now been mapped by three more orbiting compositional instruments. Unfortunately, the OMEGA on Mars Express turns out to be unlikely to enlighten us. LPSC abstract #1648 notes that the THEMIS camera on Odyssey -- some of whose channels were set to look into the puzzle -- has provided detailed maps of some small, sharply bordered deposits of Type 1 and Type 2 rock immediately bordering each other in Nili patera -- but they turn out to look identical to OMEGA.

Odyssey's gamma-ray spectrometer, however -- with its ability to peer beneath the surface or rocks to analyze their element makeup -- is a very different matter. #2070 shows that it has conclusively driven a stake through the heart of the andesite theory -- Type 1 and 2 rocks contain almost exactly the same percentage of silicon (and, indeed, Mars' entire surface has a very even distribution of it).

But it has found one very important different in the two rock types: Type 2 is almost half again richer in both potasium and thorium. The question is still what this means. On Earth, K is easily moved from one place to another by neutral water, but Th is not. However, highly acidic water of the sort which now certain to have existed in siginficant amounts on or near early Mars' surface turns out to also dissolve Th easily out of rocks (especially from phosphate minerals), raising the possibility that the high levels of both elements in Type 2 rocks might be due to aqueous processes after all.

But -- as another twist -- Jeffrey Taylor, in an upcoming piece in "JGR-Planets", will say that acid water actually dissolves and moves Th far more easily than K -- in fact, he made this observation back in 2003 ( http://www.psrd.hawaii.edu/Jan05/SixthMarsConf_TaylorGRS.pdf ), as another possible reason for why there are significant differences in the ratios of the two elements in other places on Mars. But Karunatillake et al say in abstract 2070 that such weathering is doubtful as an explanation of the Type 1/Type 2 difference, because both elements are found in comparably greater amounts in Type 2 rock. Moreover, Hahn et al say in abstract #1904 that K and Th also show a similar drop in levels on Amazonian parts of the Martian surface compared to the older parts -- which is hard to explain by greater aqueous weathering, since the Amazonian parts of Mars are precisely those unweathered by water in any form.
This suggests to both groups that Type 1 and Type 2 really are two different forms of basalt that were different in composition from the very first -- differing in their K and Th content, though not in their silicon content. Quoting #1904: "Earlier terrains derived from a relatively undepleted mantle would show an enrichment in K and Th compared to younger terrains produced by resurfacing from a more evolved relatively depleted mantle source." #2070 suggests that the Type 1/Type 2 difference may instead be due to actual differences in the makeup of the mantle magma in Mars' northern and southern hemispheres due to its great hemispheric dichotomy.

As one final frustrating twist: Newsom et al say in #1427 that parts of Mars' surface that are more soil-mantled seem richer in K and Th, suggesting that they have been moved into the soils by weathering -- but #2070 flatly disagrees from its own measurements. So, while the first major theory as to the origin of Mars' Type 2 rock has now been disproven, the other possibilities remain open.