NASA's Mars rover, Curiosity, has determined that a river from ankle to knee deep once flowed on Mars for thousands or even millions of years. The next question is whether it ever supported life. Per the article:


Less than two months after touching down inside a giant impact basin near the planet's equator, Curiosity has returned clear evidence of flowing water, scientists told reporters during a conference call Thursday....Curiosity scientists are quite sure the gravel was deposited by a vigorously flowing stream, one that was between ankle and knee-deep and likely flowed for thousands or even millions of years. 

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Congratulations to NASA on an extraordinary scientific achievement.

I have to wonder why the planet became barren.

The water must have evaporated. Could it be because of lack of air pressure that creates a vaccuum like condition?

I don't know. Maybe the water source was somehow internally generated and became exhausted for lack of a cycle to replenish it.

Small size and tidal isolation leads to internal cooling.  Internal cooling leads to the iron core ceasing to convect or spin.  This means no magnetic field.  No magnetosphere means the solar wind pounds the surface and blasts away volatiles such as water vapor, co2 and other gasses.  Low mass means weaker gravity and easy loss of material.

There's a succinct answer, well put. 

Thanks for shedding light on the matter. With Mars having become barren in this manner do you know whether the nature of its surface dust can be used to logically back into what kind of atmosphere it must have had well before it went barren?

Well, no magnetic field means you lose free hydrogen in atmospheric water vapor, and the oxygen probably combines with iron on the surface.  On Earth, that hydrogen follows the magnetic field to return to the surface, on Venus and Mars, it just goes by-by.

Martian craters (really old ones) have properties that indicate that the atmosphere was denser when they were made (something called bomb sag, and of course the ejecta patterns indicate this, as well as ejecta crusting).  How much this has to do with the composition of the aerolith/regolith on the surface at this point and time, I have no idea.  I have a good idea that such is not beyond the scope of science by any means, but planet-wide geosurveys must be done for "big picture" data of the formation and evolving processes on Mars so that single samples or even regional studies can be evaluated to provide data on the atmospheric composition at any particular point in time.   

  I have no idea when or if Mars was ever in a stable state when young for any geologically (aerologically?) meaningful amount of time before it sort of bottomed out to something like what we see today.

Don't moons also have a great deal to do with stripping away an atmosphere?  Our moon, for example, is one of the major things that make the difference between Earth and Venus.  Venus has an atmosphere so dense that you can't see to the surface, although I'm sure a lot of that has to do with the chemical composition.

The indication of a past atmosphere that was denser might mean that Mars picked up its two moons long after it's formation.

Very enlightening Art.

The difference between Earth and Venus is the temperature of the surface (leading to no plate tectonics, just lots of vulcanisim) it's composition (in subtle ways), and the important fact that it's day is longer than it's year.  Essentially it is tidally locked to the sun (or even slightly retrograde).  Convection still makes a weak magnetic field, but the biggest influence is locked.

Our moon, while Waaaay out compared to phobos and deimos, is a planetoid.  Mars' moons are just rocks.  They do not have that much influence.  Mars gets a denser atmosphere every time a large impact occurs.  In the Noachian (sp?) age, there were a lot of those.  Dust settles, gasses freeze out, cosmic rays / solar wind blasts the bits away, and the gasses ooze out into space.

  Important thing to remember about Earth, Venus, and Mars.  At an altitude of say 50 miles above the surface, the atmospheric density of all three are about the same!  This has to do with gravity (which decreases with the sqrrt of distance, relative mass, and the partial pressures involved.  Most of Venus' atmosphere (or Earths) is in the troposphere, or bottom few miles.




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