In Joseph Heller’s Catch 22, Hungry Joe is noted for ‘…snorting, stamping and pawing the air in salivating lust and grovelling need’. That is a close metaphor for reactions among some scientists (and astronauts) to observations that seem to support the notion that indeed, there is life on Mars. Remember the meteorite ALH84001? In 2004, a spectrometer carried by ESA’s Mars Express probe detected methane in the Martian atmosphere above areas that probably carry sub-surface water ice. Many exobiologists attributed this to exhalations by methanogen bacteria perhaps living in the ice, which seemed plausible. Sadly, it seems that hydrous alteration of the mineral olivine, which is widespread at the Martian surface, to serpentine is even more likely. The reaction can yield hydrogen, which generates methane by reducing carbon dioxide. Exobiologists are keeping their options open…. Meanwhile, it is not implausible that hydrogen from this simple reaction might be used to resolve global warming: olivine is the most abundant mineral in the rocky planets. Incidentally, it is serpentinisation of ultramafic rocks that best explains methane exhalation from the deep ocean floor and from crystalline basement, which Thomas Gold thought had a deep-mantle origin and was responsible for all hydrocarbon deposits.

Source: Schilling, G. Martian methane: rocky birth then gone with the wind? Science, v. 309, p. 1984.

Where do impactors come from?

All the rocky bodies in the Solar System (the Moon, Mars, Mercury, Venus, Earth and moons of the giant planets) preserve to some extent the signs of collisions with errant bodies. One period stands out dramatically: the Late Heavy Bombardment or LHB (4.0-3.8 Ga) that produced the lunar maria, and left its signature in Archaean rocks on Earth (see Tungsten and Archaean heavy bombardment, August 2002 EPN). The planet Venus was entirely resurfaced about 500 Ma ago, and its plains record the later flux of impactors in much smaller more widespread craters, as do the lunar maria, parts of Mars and to a very limited degree the Earth. The LHB stopped abruptly, having appeared equally out of the blue. The influence of astronomical collisions on planetary histories may be an established fact, but is still something of a mystery as regards its pace and intensity. High resolution images of large rocky bodies sustain a thriving cottage industry of measuring, counting and dating craters; the latter from stratigraphic evidence of relative age, such as craters that have been cratered, and ejecta mantles that bear signs of impact themselves.

Hidden inside such statistics are clues to the astronomical processes that lead to impacts (Strom, R.G. et al. 2005. The origin of planetary impactors in the Inner Solar System. Science, v. 309, p. 1847-1850). The crater-size distributions for the early events and those after 3.8 Ga are very different. Those of the later generation show features very like the size distribution of objects whose orbits intersect that of the Earth (near-Earth Objects or NEOs) and largely reflect the element of chance in a more or less stable late Solar System. The LHB pattern extends to craters more than an order of magnitude larger than the younger one, and resemble the size distribution of bodies that now orbit quite happily in the Main Belt of asteroids. It seems that during the period between 4.0 and 3.8 Ga, some main belt asteroids were flung out of their orbits to enter the Inner Solar System in large numbers. The analysis by Strom et al. suggests that the gravitational disturbance during that period might have been due to gradual migration of the giant Outer Planets before they took up their present stable orbits.