Loss of Martian atmosphere

Mars seem quite massive enough to have held a substantial atmosphere, as have Earth and Venus.  That it has barely any is a major puzzle.  One possible reason is that Mars has a tiny magnetic field.  A strong magnetic field on Earth serves to deflect the solar wind, a stream of charged particles emitted by the outer part of the Sun.  Undeflected in this way, the solar wind would gradually strip off an atmosphere.  Currently, Mars has so little atmosphere that photosynthetic life that combines water and carbon dioxide to build carbohydrate is impossible, despite the fact that most of what little air there is  comprises CO2.

In the great chattering about prospects for Martian life at some time in the planet’s past, a central issue is the timing of atmospheric loss.  It is inconceivable that Mars never had an atmosphere, because it possesses the largest volcanoes in the Solar System which must have vented mantle gases.  If its magnetic field slowly dwindled, that gives ample time for life to have emerged.

Unsurprisingly, one of the tasks of NASA’s Mars Global Surveyor Mission has, for the last two years, been a global survey of the Martian ionosphere.   That is a proxy for regional variation in magnetic field strength.  A recent meeting of the Mars Global Surveyor team revealed the maps and their implications to the public.  The oldest terrains – those showing the greatest density of impact structures, as in the Lunar Highlands – show evidence of remanent magnetism.  Those affected by the youngest major impacts – analogous to the 4 billion-year old lunar maria – do not.  This suggests that Mars lost its magnetic field some time in its first half billion years, and thereby any substantial atmosphere.  One possible reason for this loss is that Mars has long been a geologically sluggish planet.  It is turbulent motion in the Earth’s liquid outer core that generates a magnetic field.  That turbulence is probably kept in motion by convective heat transfer in the mantle – it is a companion of terrestrial plate tectonics or any kind of regular mantle overturn.  Mars’ mantle does not do that, either by tectonics or through plume activity (unlike Venus), so its core may well be devoid of motion.

Exactly when magnetism stopped, with the attendant effect of the solar wind on any atmosphere, is crucial for estimates of how long life might have had to appear and begin evolving.  The results certainly rule out evolution beyond the most primitive life forms.  However, establishing that date must await future Mars landers, either staffed or robotic, on which the most important experiments will aim at detecting signs of former of extant life.  The magnetic data are not encouraging for exobiologists.

(Source:  Samuel, E.  2001.  The day the dynamo died.  New Scientist, 10 February 2001 issue.)

And now, Martian glaciers

Readers will have seen scornful comments in Earth Pages, regarding the desperate search for evidence of liquid water on modern Mars.  That water once was there seemed cut and dried from the giant valleys scoured across the Red Planet’s surface.  It was said that vast volumes of deep-seated ice catastrophically melted to flood from large impact sites.  Like the supposed evidence for active watery emissions in recent time, that for past flooding which cut the large valley systems rested on interpretation of the landforms themselves.  Re-examination of the valleys shows that they almost exactly mimic features revealed by sonar sounding on the sea floor surrounding the Antarctic ice sheet.  The Antarctic features probably formed during increased flow regimes when sea level stood at its lowest during glacial maxima.  Such surges can flow uphill, and sure enough the valley systems on Mars do have uphill tracts.

Baerbel Luchita of the US Geological Survey applied work on structure beneath the Ross Ice Shelf to Mars, suggesting that impact-melted water froze on emergence at the surface to flow in a more or less glacial fashion.  Undoubtedly, ice flow is far more capable of large-scale excavation than an equal volume of water, but to form the 1000 km long systems on Mars implies a considerable head.  Also its branching nature forces the assumption of many coalescing glaciers over a very large area.  That meets problems in imagining a widely distributed source of energy that caused the melting.  Impacts are at points, so perhaps yet another mechanism, such as seismicity, will need to be invoked.

(Source:  Hecht, J.  Sliced by ice.  New Scientist, 27 January 2001 issue)


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