Summary

Various mechanisms have been suggested for the depletion of volatiles from the atmosphere of Mars. Thermal escape has been found to be satisfactory for helium and hydrogen, whereas non-thermal mechanisms such as dissociative recombination are capable of ejecting atoms of nitrogen, carbon, and oxygen. Solar wind sweeping is effective in removing any charged particles present in the ionosphere. Multiplying the present escape fluxes for these mechanisms times the age of the Solar System, the amount of atmosphere that has been lost by these known processes was found to be probably at least several times larger than the present atmospheric pressure. Explosive blowoff can raise this factor to 100 or more without introducing observable isotopic fractionation effects.

The fundamental issue of the primordial volatile content of Mars remains unresolved by the available evidence. No one disputes that the present atmospheric inventory is small; what is at issue is whether Mars has outgassed thoroughly and whether massive loss of volatiles has taken place. The bulk density of the planet and its rotational moment of inertia reveal a high degree of oxidation, a property that, in chondrites, is always correlated with a high volatile content.

The suitability of Mars for the origin of life during its earliest history is far from obvious, and the impossibility of producing and accumulating organic matter in the present highly oxidizing photochemical regime is manifest. There remain a number of urgent needs in the study of the evolution of Mars and its atmosphere. First and foremost among these, and at the top of the list in order of expense, is the dating of Martian geological and tectonic processes. Valuable insight into the chemical evolution of the planet may also be gained by compositional mapping of the surface, a goal that may be achieved at far lower cost than a Mars sample return mission.

Interesting Atmosphere Observations

Image taken from NASA without permission

Hubble Captures A Full Rotation Of Mars

These pictures were taken during three Hubble Space Telescope orbits that were separated by about six hours. This timing was chosen so that Mars, with its 24-hour 39-minute day, would rotate about 90 degrees between orbits. This imaging sequence therefore covers most of the Martian surface. These observations will be combined with others planned for March 30 to provide complete coverage.

During each orbit, Mars was observed in nine different colors spanning the ultraviolet to the near infrared. The specific colors were chosen to clearly discriminate between airborne dust, ice clouds, and prominent Martian surface features. The Martian north pole is at the top (near the center of the bright polar cap) and East is to the right.

These images show the planet on the last day of Martian spring in the northern hemisphere (just before summer solstice). The annual north polar carbon dioxide frost (dry ice) cap is rapidly sublimating, revealing the much smaller permanent water ice cap. This polar cap remnant, along with a few nearby detached regions of surface frost

are most obvious in pictures taken through ultraviolet, blue, and green filters. These filters also show numerous bright water ice clouds. The brightest clouds are in the vicinity of the giant volcanoes on the Tharsis Plateau (to right of center on left image), and in the giant impact basin, Hellas (near bottom of right-hand image), but a diffuse haze covers much of the Martian tropics as well.

The familiar bright and dark markings on the Martian surface are most obvious in images taken through red and near-infrared filters. These images clearly reveal the large, dark, circular "sea" of sand dunes (Olympia Planitia) that surrounds the north pole, as well a number of other familiar features, including the giant Tharsis volcanoes. The 16-mile (27 km) high Olympus Mons is near the center of the left-hand image, with Arsia, Povonis, and Ascraeus Mons forming a south-west to north-east line just to its right. The volcano, Elysium Mons is near the center of the middle image. The prominent dark feature just below the center on the disk on the rightmost image is Syrtis Major Planitia.