Basaltic Sediments, rethinking Mars - again? Options

[ngunn]

This blockbuster article from Emily seems to justify a new discussion topic:
http://www.planetary.org/blogs/emily-lakda...le-of-gale.html

So many thoughts, so many questions, I don't know where to start. What about Gusev? Were those basalts actually sandstones too?

[jmknapp]

From the conclusion:

With Curiosity's results, I feel like the "water on Mars" argument is bifurcating; Mars was never wet like Earth, but it is not dry like the Moon. Water is crucially important to Mars' story, and yet Mars is drier than the driest conceivable environment on Earth.

Before the ESA livestream on Siding Spring they reran a webcast titled something like Mars Express: Ten Years at Mars. The speaker made the statement that if all the water in the polar caps was melted, water would cover the surface of Mars to a depth of three meters. I think the figure must refer to average depth, so in reality any Mars oceans would be patchy. It's really not a lot using Earth as a standard--but how much has been lost to space?

The average ocean depth on Earth is close to 3000m, so it's about a 1000:1 difference.

[Gerald]

Thus far, we've just seen a few data points, with respect of time, and location.
Just think of 500 million years of geologic history on Earth (the latest about 11% ). So many things can happen. We don't even know the distance between Mars and Sun, three or four billion years ago, nor the number of planets / moons / planetesimals in our solar system at that time.
One possibility combining abundant water with low chemical weathering could be low temperatures with occasional melts, be it due to volcanism, impacts, chaotic obliquity, orbital instability, solar activity peaks, whatever.
Or think about the lower gravity with respect to Earth, slowing down e.g. separation of water/ice and rock.

Investigating more layers of Mt. Sharp should reveal more detail about the currently just very crudely known geologic history of Mars, or Mt. Sharp, at least.

With respect to the hard sandstone: What about impact metamorphism?

[Julius]

Whereas it is true that we require more time and more landing site in situ investigation the likes of gale crater, meridian and gusev crater, I feel that the overall picture of the water story on Mars is somehow limited to volcanic, tectonic, hot spring activity . The next landing site for investigation i believe should be selected based on the presence of chlorides and carbonates rather than clay and sulphates.

[elakdawalla]

The basalts in Gusev are definitely basalts -- Spirit demonstrated that conclusively. Ken's point is that he's having a hard time telling from orbit which dark, erosion-resistant deposits are seds and which are igneous anymore.

[SFJCody]

What would sedimentary rocks deposited by liquid CO2 be like? Not that I really think that's the case, just wondering if there's some peculiarly Martian way to get these to form from basaltic source material without significant alteration.

[Gerald]

My first association would be carbonates or HCO3 salts, if at least some water is present to form H2CO3.

[craigmcg]

What would sedimentary rocks deposited by liquid CO2 be like? Not that I really think that's the case, just wondering if there's some peculiarly Martian way to get these to form from basaltic source material without significant alteration.

to make CO2 liquid you need pretty high pressure, greater than 5 atm.

[nprev]

Here's my best guess at a brief history of Mars with respect to this subject:

1. The LHB just pounded the hell out of Mars. Specifically, there were not only more impactors (presumably due to the planet's probable proximity to the Asteroid Belt) but also many more smaller ones reached the surface due to the fact that the atmosphere, while still much thicker than today, was nevertheless far thinner than currently thought...maybe 0.25 bar or less, maybe even just above the critical value for liquid water. This resulted in the formation of finer basaltic pieces than is normal on Earth.

2. Episodic flooding due to melting of subsurface ice from various causes (impacts & vulcanism, primarily) caused formation of sedimentary strata. However, due to the low (and dropping) atmospheric density standing bodies of water did not persist for long periods of time at most locales, usually evaporating before basaltic minerals finished transforming into clays.

3. In between flooding events (as in the present), aeolian erosion continued, further grinding down basalt into fine material. Subsequent floods produced fine-grained basaltic sandstones.

4. As the atmosphere continued to thin, small impactors assumed an increasingly influential role in soil mixing and generation of basaltic fines, similar to lunar processes albeit augmented by aeolian erosion.

That's pretty crudely painted, but it seems like a good fit for the observations. Clay-rich sites & sulfate plains like Meridiani are examples of places where standing water persisted longer than normal, presumably as a consequence of geothermal activity. However, even there the evaporation rate was rapid. Most of the water ultimately ended up at the poles except for the buried ice deposits we've seen, which eventually will be melted via impacts over time.

Of course, the pros will come up with something a lot simpler and more plausible, which is why they're the pros.

[Don1]

What would sedimentary rocks deposited by liquid CO2 be like?

To get liquid CO2 you need at least 5.2 bar of pressure. For it to be stable enough to be geologically relevant would require over 10 bar.

Among pure compounds the only remotely plausible alternative to water looks to be ammonia. For liquid ammonia you need over 61 millibars, which is about 10 times current Martian pressure. At 300mb ammonia will melt at about -78C and boil at -58C, giving a 20 degree temperature range for the liquid phase.

[Don1]

Virtually all the knowledge about how rocks weather is for an oxygen atmosphere. Most water on earth has dissolved oxygen in it, and that influences the chemistry. If you put a steel nail in a test tube and add water, the nail will corrode as expected. If you boil the water to drive out the dissolved gases, and add a cap of melted wax to seal it from the atmosphere, then the steel nail doesn't corrode. The point of the experiment is that you require both water and oxygen for corrosion to occur.

Dissolved gases and salts have a big impact on chemistry. I don't know anything about rock weathering reactions, but it wouldn't surprise me if weathering worked differently in atmospheres without oxygen.

[ngunn]

Here's my best guess at a brief history of Mars with respect to this subject

That's a pretty good story, for me. A few comments and queries I would add:

Comments first. I can't help thinking back to Don Burt's descriptions of rapid sediment emplacement following impacts. Bang, slosh, swish, and a whole sedimentary complex with many varied structures both dry and watery is in place all at once. Later aqueous alteration episodes could be similarly brief. Forming gypsum veins this way is no problem. Only clay takes more time, and as you say that seems to have happened just in certain special places.

But - shouldn't liquid water be stable at some depth in the crust, perhaps below the buried ice deposits already known? Why doesn't alteration of the basalt minerals happen there? Or maybe it does but this material has never been excavated by sufficiently large impacts. Is there a buried wet clay-rich world down there still awaiting exploration?

Queries: We know there's ice in contact with the rocks in many places. Are basalt minerals completely immune from hydration as long as the water remains frozen? A 'Yes' would help here. Basalt chunks on the surface: In the absence of obvious weathering is there any way other than by finding a source outcrop to prove the basalt was formed where we find it rather than being carried in by a sedimentary process? Martian water verus terrestrial water: Could the former be less efficient than the latter at hydrating basalt minerals? Absence of some crucial ingredient peculiar to Earth?? (EDIT Don1 got to dissolved oxygen as I was typing. That's the kind of thing I had in mind.)

[nprev]

Good questions, and I'll defer to someone with actual knowledge of geology to answer them. My fields or expertise are well known and somewhat disreputable.

[dburt]

Here's my best guess at a brief history of Mars with respect to this subject: ...

Nice summary. Agree with you regarding stages 1 and 2. Disagree somewhat beginning at 3.

3. Aeolian erosion yes, but water flood deposition to deposit basaltic sandstone, probably not. Not needed, given your Stage 4 that episodic impact erosion, transport, and deposition has continued at a reduced level up to the present. Erosional scouring, plus transport and deposition can then occur via clastic density currents, the impact explosion equivalent of the pyroclastic density currents that can make explosive volcanism so dangerous. These, like water, are fluids.

Mars rovers can only be landed and operated at relatively safe, flat sites and, lacking significant drilling capability, are only able to observe relatively young bedded rocks at the surface, so it is not surprising that all three rovers have observed relatively young "blast beds" (as I have recently called them) of basaltic composition on the surface, covering whatever older, water-deposited beds might underlie them. Inside large impact craters, such as Gusev and Gale, the sediments have been observed to be to be generally coarser, as topographic lows attract the denser, coarser portion of density-driven flows, as well as other types of mass movements (e.g., debris flows and landslides, a major component of terrestrial alluvial fans).

The atmosphere of Mars, although way too thin to support the continued existence of liquid water at the surface, is more than capable of transporting basaltic sediment in clastic density currents (think desert dust storms), especially if these currents are temporarily beefed up by steam and other gases generated by energetic impacts into brines, ices, hydrous salts, clays, or carbonates.

This simple hypothesis, first presented at the Geological Society of America annual meeting exactly 10 years ago to explain initial Opportunity observations at Meridiani, readily explains the observation of fresh basalt in sediments, the problems of which were so ably summarized by Emily. Incidentally, the dry acid salts observed by the first two rovers provide even better evidence than fresh basalt that liquid water cannot have been present for long, if at all, during and after deposition because liquid acids will chew into and alter everything, especially basalt rocks (owing to their low silica content).

ngunn: Thanks for the mention. Yes liquid water should have been stable at depth, especially near cooling impact craters or volcanoes, or if it contained significant quantities of dissolved salts to suppress the freezing point. Basaltic impact glasses, which should have been exceptionally common on early Mars, are trivially easy to alter to clay minerals, especially smectites, and especially if this alteration occurred by liquid water away from the surface. No need to call on special chemical properties of Martian ice. Note also, as I mentioned here when they were first encountered by Opportunity, that the mysterious spherules called "newberries" probably formed by devitrification of basaltic impact glass, based on their matrix and on the their radiating fibers imaged at high magnification. Other spherules observed at all three sites probably owe their existence directly to impacts.

I've been given to understand that discussion of this particular simple hypothesis on impacts, although it is now fully time- and rover-tested, is not particularly welcome at this site, so newcomers to the idea should probably consult our various publications (mainly by Burt, Knauth, and Wohletz).

Don Burt

[djellison]

For those who want to read tens of thousands of words by Don on that issue - read this thread http://www.unmannedspaceflight.com/index.php?showtopic=4308