Ron Greeley and his crew of grad students and postdocs have systematically studied eolian sedimentary physics on other planets, starting with clues from Mariner 9 images and building the Mars Wind Tunnel (to start with). We have a pretty good idea of the physics of different sized (and density) particles on Mars, Venus (I presume they've been doing Titan) etc in those "planets" gravity, surface pressure, mean atomic weight, temperatures, wind-regimes, etc. etc. etc.

Mars dust is probably a lot less of a problem than lunar dust, due to chemical weathering, though acid sulfates and peroxides could be a problem. Lunar dust is full of utterly sharp glass microshards and unweathered mineral grain fragments, plus reactive nanoscale iron and vapor deposited products. It's not super toxic. Biological tests with Apollo 11 and 12 (mostly) samples showed that, but it is nasty.

Good info, Ed. Thanks!

I'd sure like to see their Titan studies, if for no other reason than their model might set some composition constraints on the dune material(s).

Not to nitpick but, the full quote is

"The same physics applied to Mars predicted the smallest particles capable of saltation will be four times larger---about 200 microns---than on Earth, and that, analogous with Earth, the mean particle size for Martian dunes should be several times greater still (i.e., coarser than is typical for terrestrial dunes). However, recent field evidence collected by MER is inconsistent with these predictions, revealing well-formed, active ripples of 100 micron basaltic sand".

Observation trumps theory. What has formerly been noodled/theorized has been proven wrong by what the rovers have seen. Bang goes another theory!

Replicating a dynamic 7.5 Millibar CO2 atmosphere accurately on Earth is outrageously difficult even on a small scale. It's not like we have warehouse-sized rooms to test dune formation in. Sure the Mars Surface Wind Tunnel takes up a warehouse, but only about a cubic metre's worth of that is useful.

Dust on Mars goes well below 20 microns, and it "looks" (apologies for being so normative) as though particles of that size are well integrated into Meridiani dunes. Whether this resulted from a saltation process or a suspension-settling process combined with saltation of larger grains doesn't really matter. There is plenty of very fine dust in dunes on the surface of Mars, and Opportunity has not just seen it but gotten stuck in it.

As we've said before, processes on Mars are similar to those on Earth, but the details are different. Until we get the composition and a particle size distribution on these dunes we won'y know anything for sure.

--Bill

There are several ways that dust can be incorporated into dunes without violating the fact that the dune forms themselves are constructed of material that was tranported and deposited as sand sized particles.

The dust particles can be transported as sand sized aggregates of clay-sized particles. You can get interstratifed sand from salatation and dust from fall out. Sand grains can also be converted into much finer grains post depositionally. Dust aggregates may fall apart, sulphate grains can disinegrate from hydration-dehyration reactions.

None ofthis changes the fact that dunes are constructed primarily by the transport and deposition of sand grains.

Jon

The boundaries of phenomena certainly change in sublte and interesting ways. The abstract linked by an earlier poster illustrates this nicely. But the fact that dunes are constructed by sand is either subtle nor a detail. This is something that can be safely predicted without getting close to them. Determining whathe dunes are actually made of, their detailed size fraction, grain structure and composition, and post depositional modification (if any) is what will require up close observation.

Jon

Agree that Martian dust is less likely to be a problem from anengineering perspective, partly because it is chemically weathered and also extensively tranported. Acidity should not be a problem either there are plenty of environments with acidic soils and other regolith materials with similar pHn Earth than can be dealt with provided people are aware of the issue. As for peroxides, does anyone have any details on the concentrations? The only source I have brave enough to give a concentration suggests that the levels are actually quite low.

Lunar fines are nasty, but whether they are very much nastier than say fine volcanic ash remains to be seen. Volcanic ash is also full of glass microshards and unweathered mineral grain fragments, although it does lack reactive nanoscale iron and vapor deposited products.

Aerospace engineers are just going to have to get used to the fact that their nice clean spacecraft are going to get dirty.

Jon

No, it doesn't.

One of the "surprising" -- at least somewhat to me -- discoveries of the rovers is that while micrometer sized dust is everywhere, it doesn't stay micrometer sized except as thin layers on stuff. There are no thick dust deposits that the microscopic camera has looked that that are unresolved dust particle accumulations. There's been plenty of ideas that there are dust-aglomeration (nice word... I can probably work that into a conversation once a decade, if I spelled it right...) processes that build up dust pellets that can be strong enough to act like sand. It's entirely possible that light toned dunes found in many areas are dust-pellet-sand dunes. We just don't know yet.

Though everybody wants a bit, sophisticated sampling rover, there still is an enormously great scientific case for a absolutely minimal cost "grab-bag" Mars sample return. I'd be entirely satisfied with a Viking-like lander with crawler tracks like the proposed Viking III mission... Just enough mobility to get 1 gram samples of different soils and rock-chips from a 10 meter radius of a landing spot. (I would NOT trust a landing to not come down in a sand ripple bed or something like sleepy hollow, and not be able to move a minimal amount.)

We see this on Earth as well. Clay-sized particles in parna often form aggregates (agglomerates if you prefer, although that term is already used with respect to coarse-grained volcanic rocks formed near vents) up to 60 microns. Sand sized aggregates of clay and silt make up many dunes, especially source bordering ones. the really interesting questions are to do with what does this tell us about the soil forming processes on Mars. On Earth such aggregates are mostly deflated from bare smectictic arid soils, dry lake beds, and flood plains.



Definitely. Even a single scoop of the globally distributed fines (~100 g) would tell us about the global crustal composition of Mars and by inference the mantle and perhaps the core. It would also give lots of information of soil-atmosphere exchanges, weathering, granulometry and thus wind behaviour, allow a range of geochronological techniques to be carried out and detailed surfaces for the missing organics and the putative super oxides. Lastly understanding the physical and chemical properties of the fines would be very useful for designing future missions, both robotic and crewed. It would be very expensive but is long over due.

Jon

Well, Purgatory sure looked to me to be made predominantly of grains about the size of fine wheat flour -- whatever size that is. Imprint something smooth on it, and it retains a very smooth surface. It looked just like when I used to play with flour as a kid.

Scott

Have we noticed that there appears to be another area of ripple dune deposits outside the main area in the centre of Victroia...

http://marsrovers.jpl.nasa.gov/gallery/all...FP1997L0M1.HTML

Kenny

I saw those too. They show up as little "wrinkles" in the HiRise image and seem to be of a different orientation that the more prominent dunes.

They look quite nice looking down from the edge...

Certainly, Purgatory might well consist of flour sized particles now , but if it was a dune then they must have been deposited as sand grains. If the majority of the dune sand was composed of sulphates and/or aggregates then diagenesis/pedogenesis since they were deposited could resulted in major changes in grainsize.

Jon