I don’t know. The short answer is it doesn’t have much in the way of ionosphere and the solar winds rip all the gasses off the planet. But that’s the thing… It used to have a decent ionosphere. I think. What happened to the planet?
I was under the impression that Mars has some atmosphere, thin though it may be. Didn’t we see a photo album shot by the first Rover showing a pale vaguely reddish sky (as opposed to a pitch black sky, as a truly atmosphere-free planet would have)?
ETA: Article with photo. Do am image search for Martian sky photography to find many more. (More ETA: The linked site is a gallery of 29 Martian images!)
It’s got some atmosphere, true, but it doesn’t have jack shit in the way of an ionosphere to keep those gasses stuck. Yet it’s got an iron core just like Earth does.
I suppose my fundamental question is: What happened astro-geologically to Mars to make the ionosphere so weak?
NASA’s MAVEN mission is still investigating this.
Here is a lectureby an eminent planetary scientist. (60 minutes long, you can start at 14.30)
Many probes used a parachute to descend to the surface, there are dust devils, so there is some atmosphere.
But unlike earth, is not Mars interior thought to be totally solid unlike Earth’s solid core floating in liquid, and hence no magnetosphere?
Mars is less differentiated than Earth. That’s why the surface is red, because there’s significant amounts of iron still in the crust. But that just pushes us back to another question: Why is Mars less differentiated? Its lower mass (compared to Earth) may be part of it, but Mercury is even lighter, and is still well-differentiated.
The standard answer is the lack of a Martian magnetosphere so perhaps the question really is what no magnetosphere around Mars? This article addresses that plus the lack of vulcanism resulting in CO2 being locked up in carbonates in the soil.
I’ll raise you 27,489 images: High Resolution Imaging Science Experiment (HIRISE)
But not all atmosphere focus.
Because it doesn’t have Aaron Eckhart and Hilary Swank to dive into the inner core in an unobtainium vessel and restart it spinning with nuclear weapons. Pathetic little planet, really. I’d rather go to Titan any day.
In Astronomy weird things are usually explained with “and then there was a giant impact”! It’s surprising how often this is the answer.
One hypothesis for why Earth has a strong magnetosphere and a more molten interior stems back to the impact with Theia (http://en.wikipedia.org/wiki/Theia_(planet)#Theia) that created the moon. This impact would have completely melted the proto-Earth giving us the differentiation in layers Chronos alluded to. This hypothesis also explains why we have a thinner crust which allows for more volcanism which replenishes our atmosphere as well. Mars is a less active planet - with no magnetic field and no active volcanos Mars is in a steady decline.
It’s interesting to note that some people think that a magnetic field doesn’t have much to do with preventing the solar wind from stripping away the atmosphere:
(of course, Mars has a much thinner atmosphere, but I don’t think this really matters since the density of Earth’s atmosphere is similar at high altitudes)
The catch with a magnetic field is that while it deflects the solar wind, it isn’t actually completely deflected; rather, it is directed into the polar regions, leading to a similar overall stripping effect:
As for the actual loss rates, even Mars should have only lost the equivalent of about a foot of water over the past 3.5 billion years at present loss rates, and about 4 inches for Earth and Venus (Mars’ lower mass appears to be a bigger factor since Venus also doesn’t have a dipolar magnetic field), thus atmospheric replenishment from volcanoes is unlikely to have had much of an impact (although on Earth, some gasses like carbon dioxide, which reacts with rock and gets biologically locked up, are replenished this way).
Big asteroid smashed into Mars and blew away half its crust into space? Blew away most of the atmosphere along with it? That’s what I heard somewhere. Could an asteroid of proportionally bigger mass do that to the earth?
Merely posing queries.
How much of the Martian atmosphere (oxygen) is locked up in the iron oxide on Mar’s surface? There were several SciFi stories written about how an oxygen atmosphere cold be re-introduced, by reducing all of that rust.
I think you’re getting it backward: It’s Earth’s relatively thin atmosphere that’s often explained by an impact (presumably, the same one that formed the Moon). It’s negligibly likely for something like that to happen nowadays, though, since the Solar System is much less crowded now.