Glaciers on Mars - a safe landing for hitchhiking microbes from Earth
by Michael Paine, The Planetary Society Australian Volunteers, mpaineATtpgi.com.au
31 July 2005
A possible mechanism for transfer of life between planets is via rocks ejected by major asteroid or comet impacts. The term "transpermia" was coined by Oliver Morton to describe the transfer of lifeforms by this method and to distinguish it from the more general concept of panspermia.
For a description of the mechanism of transpermia and the odds of successful transfer of life between planets see "Estimate flux of rocks bearing viable lifeforms exchanged between Earth and Mars".
An icy landing
There is plenty of excitement (and controversy) about the possible discovery of extensive glaciers, or ice seas, on Mars. While most of the discussions centre around the chances of finding native life on Mars the discovery, if confirmed, is another factor in favour of transpermia.Beyond Mars
Extensive glaciers on Mars could resolve one of the difficulties that Earth-life would have establishing itself on that planet. It is quite likely that Earth rocks with live bacteria have reached the surface of Mars but the odds of finding favourable conditions to flourish seemed very remote. They would be stranded within a rock on an extremely cold and dry surface.
However, if an Earth meteorite landed on a Mars glacier it might eventually sink to the bottom of the glacier, where there is likely to be liquid water, due to the pressure of the overlying ice (the same mechanism that creates a liquid film under ice skates). Under these icy conditions some types of Earth-life might thrive.
Although the chances of "hospitable" rock transfers are substantially less, the same mechanisms may have delivered microbe-bearing Earth rocks to Jupiter's moon Europa . It is thought that Europa has a thick water ocean covered by a crust of ice. Therefore, if a life-bearing Earth rock reached the surface of Europa intact the impediments to colonisation might be less than those on present day Mars. A major difficulty, however, is the lack of an atmosphere on Europa. Collisions with the icy crust would usually take place at interplanetary speeds and the meteoroid, and any hitchhikers, could be expected to be vaporised in an impact explosion. [note 1]
Saturn's tiny icy moon Enceladus displays evidence for active ice volcanism and so is another contender for transpermia. However, it also lacks an atmosphere and therefore also has the problem of hyper-velocity impacts.
Note 1. Although very thin compared to the Earth, Mar's atmosphere is dense enough to slow meteorites sufficiently so that they do not explode on impact with the surface. As with Europa, a lack of atmosphere also appears to make it unlikely that Earth-life would colonise the Moon by transpermia.