[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 .)

[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