07 December, 2006

Hold your Martian horses

I share Emily’s slight disquiet over the spin applied to the latest (and possibly last) results from Mars Global Surveyer. This is not to say that pictures like this (source) don’t get me very excited:

However, let’s be clear on what we can say for sure. The deposit which appears in the later photo on the top right, and is magnified in the bottom right image, has features very suggestive of fluid-like flow: it appears to be diverted around elevated topography, and fans out at the end. And we strongly suspect that any water on Mars will have very high concentrations of mineral salts, meaning that the lighter colour of the new feature might plausibly be explained by these salts being left behind on the surface when flowing water sublimes away in Mars’s puny atmosphere. However, in the absence of spectrometry to analyse the composition, there are still alternatives to water, such as liquid carbon dioxide, or dust flows.

In the midst of all this excitement I wondered whether there was any more data coming in from MARSIS, the radar instrument on Mars Express. It’s been more than a year since I blogged about preliminary indications of subsurface water ice beneath Chryse Planitia (at mid-latitudes, like the crater gulleys being scrutinised by NASA). Very little seems to have been released since; the only reference I found was a throwaway remark in this nice summary article:

"MARSIS has shown that many of the upper layers of Mars contain water ice," says Jeffrey Plaut of the Jet Propulsion Laboratory, Pasadena, who is the co-Principal Investigator on the MARSIS experiment.

Interestingly, most of the article is discussing spectrometry results which show how early standing water on Mars seems to have been fairly quickly locked away in clay minerals, sulphates, and red ferric oxides. But pure water may still be around, in the form of subsurface permafrost which in some places may be quite close to the surface. The Global Surveyor pictures hint that this arrangement may not be completely stable. And dynamic processes, even on a small scale, are exciting; change leads to disturbances from equilibrium, and interesting chemistry, which are the sorts of things that life thrives on.

Either way, results like this show exactly how the decision to send a succession of probes to Mars to place it under close, continuous observation is paying real scientific dividends.


Iknownotwhattodo said...

I know zero about geology or mars but it seems just simple wishful thinking to declare this a water. What is the temperature of the surface of mars.Wouldn't it all be ice? How can they discount dust/scree flow which from a great distance would look like fluid flow wouldn't it?
Also images like that are a 'deliverable' for the NASA boys, bet you there is a funding round looming. Call me a cynic.

Lab Lemming said...

Would liquid CO2 be more or less cool than liquid H2O?

CJR said...

Actually, that was a foolish thing for me to say - you only get liquid CO2 at pressures above 5 bars, otherwise it's straight sublimation. So if CO2 were involved, it would have to be some sort of gas/rock density flow.

Indeed, this might also be the case if it's water - according to this
the average temperature on Mars is 218K /-55C (range from 140-300K) and surface atmospheric pressure is about 7 millibars; in which case, this phase diagram suggests you'll get straight sublimation under increased heat or pressure (although this may change if the water contains substantial mineral impurities)

Iknownotwhattodo - the argument is that these gullies occur only on the sides of craters away from the sun, and at mid-low latitudes, which suggests a temperature control on their formation, and they also seem to be located where faults and fractures make the underlying rock more permeable. If they were just landslides it seems unlikely that things would be quite so clear cut (see here or here).

The real problem is that if this is evidence of water, we have a bit of a paradox on our hands. There doesn't appear to be a water cycle on Mars, so when it gets to the surface the water just escapes into the atmosphere and is lost into space. That being the case, even small flows like this will quickly (geologically speaking) deplete any water just below the surface, and the flows will cease. This makes our seeing them suspiciously lucky...

Iknownotwhattodo said...

ok I see - thanks for that I feel humbled your geologicalness ;)
Interesting stuff.

Lab Lemming said...

There is too a hydro cycle- the north polar cap is mostly water ice.

And if a brine is in the sublimation field, it will quickly lose water vapour and increase the salinity until it reaches stability. I wonder if anions are identifiable by remote sensing...

CJR said...

True enough - but the exchange between ice caps and atmosphere will not lead to any water lost at lower latitudes being replenished.

MRO and Mars Express both have spectrometers, so may provide some clues if they can be aimed right.

Anonymous said...

I know nothing about geology or mars either but like many find any off earth discovery fascinating.

Incidently even if it were an outright con and a simple bit of photoshop had taken place its still all good, I mean Interplanetary exploration should be high on everyones list certainly more so than bombing areas of the middle east back into the stone age...sorry wasnt going to mention the war.

Adam said...

Point of fact, but water's triple point is at 0.1 C and just shy of 612 Pa - thus water frequently evaporates and doesn't sublime on low latitude Mars when it's warm enough. Problem is the air is so damned dry that water evaporates almost instantly, is carried off and frosted onto anything that's sufficiently cold. And that is sublimation in action - albeit of the depositing kind.