Mars' Missing Air Might Just be Hiding
By Ker Than
Staff Writer, Space.com
posted: 25 January 2007
02:01 pm ET

This refers to the Barabash et al. paper ("Martian Atmospheric Erosion Rates") being published in the January 26, 2007, issue of Science.

I don't buy the impact theory, at least as it is quoted. 10 km diameter is about the size of the Chicxulub impactor which apparently had no appreciable effect on the Earth's atmosphere (except for a brief CO2 spike from vaporised limestone).
Unless there is a misquote I think they've mislaid at least one zero somewhere in their calculations.

Another releated release:

- Hints of huge water reservoirs on Mars

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Its [MEx ASPERA-3] measurements suggest the whole planet loses only about 20 grams per second of oxygen and CO2 to space, only about 1% of the rate inferred from Phobos-2 data.

If this rate has held steady over Mars's history, it would have removed just a few centimetres of water, and a thousandth of the original CO2.
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OK, turn it on it's head: look at Earth and Venus and explain *their* atmospheres. I share the view that there's a decimal point gone walkabout.


Bob Shaw

I don't buy the impact theory either. The bodies that created Hellas & Argyre were undoubtedly much larger than 10 km/dia, and there are literally hundreds if not thousands of craters from hits in the quoted range, so why is there any air at all?

I know it's just a summary writeup of a paper, but also note that there's no mention of the effects of volcanic outgassing...the Tharsis Bulge alone must have produced prodigious amounts of gas during its heyday (IIRC, on the order of 10 bars)...where is it?

I saw that in the Space article and my first thought was, of course it does at the present atmospheric pressure and density. I can't imagine however that such a rate would remain constant with a larger, warmer, denser atmosphere. More atoms bouncing around with a whole lot more energy would equate to a substantially greater rate of escape in the ancient past. What am I missing?

Yes - one would assume such things drop off at a rate dictated by where you draw a vertical line on a bell curve, and as the total ammount decreases, so does the ammount above the line...perhaps that was already figured into their calculations though?

Doug

Thing is, though, Mars' atmospheric density in all likelihood did not follow a normal distribution curve (density vs. time) over its history. Again looking at Tharsis, there may have been vulcanism & massive outgassing there as recently as 100 My ago.

I might buy repeated impact/loss scenarios. MGS recently showed us that little rocks hit with almost alarming frequency. Due to its proximity to the Belt & the influence of Jupiter, maybe Mars is unfortunate enough to encounter substantial impactors often enough to blast its atmosphere away over and over again.

Hmm...maybe we need a dedicated "NMO" search to assess this!

Another thought to ponder: Mars' average density is something like 3.5 g/cc vs. over 5 g for Earth, so obviously Mars must have a substantially larger fraction of lighter (& presumably volatile) constituents; even the core is thought to consist of iron sulfides in most models.

I guess I come from a slightly different science background, but I have serious problems when authors state "These rates can be propagated backward over a period of 3.5 billion years". How well have they constrained the numbers presented in the article--I don't see any error bars on the numbers in the abstract? (I'm at home, so I don't have access to the article.)

Details of the exact model seem lacking, and my initial reaction was very similar to that of ElkGroveDan.


In terms of bulk properties of Mars, I don't think you need a substantially higher fraction of volatiles to account for the lower the bulk density. Evidence supports that Mars underwent a high degree of differentiation after accretion. Iron and other metals sunk to the center, silicates rose to the mantle, and the volatiles made their way to the surface. And Mars’ bulk density is not much greater than the silicates (olivine-rich) in our own mantle. Include a smaller core with generous amounts of sulfur and you have a very plausible scenario.

But I don’t expect the volatiles in Mars’ crust to have any noticeable effect on the total density. You would need to move farther away from the sun for that.

In the paper, Barabash et al. state: "Propagating the measured rates backward in time, one can estimate the total amount of carbon dioxide and water, ΔM(CO2) and ΔM(H2O), lost through this particular channel [i.e., solar wind interaction] over 3.5 gigayears (Gy)." They refer the reader to the Supporting Online Material for the paper, which, I believe, is freely available to non-subscribers. See page 3 under the heading Total escape over the planetary history for a more thorough explanation of their methodology.