Dreams of Planetary Oceans Dry Up
Astrobiologists like oceans. The vision of life evolving on Earth in a primordial soup drives the quest to find liquid on other worlds. It doesn’t have to be liquid water: just liquid that stimulates the imagination with visions of exotic life. Two solar system bodies once considered prime candidates for ocean front property, though, have recently dried up substantially, reducing their value as astrobiological real estate.
- Titan: A JPL press release painted a rosy picture for realtors looking at Titan’s lakefront property, after Nature1 wrote up the scientific results of a July flyby by Cassini (see also the comments by Christophe Sotin2). Though Sotin boasted “Titan’s lost seas found,” those who know the history of the region must sigh over the long-lost global ocean that was once thought to exist on this huge Saturnian moon (see Science 12/16/1983).3 In 1983, Titan expert Jonathan Lunine and team “proposed that Saturn’s satellite Titan is covered by an ocean one to several kilometers deep consisting mainly of ethane.” Cassini found, on the contrary, that the bulk of Titan is crisscrossed by sand dunes (05/04/2006). Scientists are pretty sure now that the lakes, only found between the 70th and 83rd parallel north, have some liquid in them. As to what happened to the vast oceans of liquid ethane and methane thought to have accumulated over 4.5 billion years, they can only speculate that it went underground or glommed onto solid particles (10/18/2006). Cassini is incapable of directly confirming that possibility.
- Mars: Few probably heard the whimper about Mars water expressed in Science last week.4 In the heyday of early science results from the Mars Exploration Rovers, scientists trumpeted the possibility of extensive and persistent oceans of water in the Martian past (12/20/2004, 08/06/2004, 04/12/2004, 01/03/2004). These days? Here’s what was overheard about “a nastier early Mars” at last month’s meetings of the American Geophysical Union (AGU):
When the Opportunity rover sent back signs of water early in martian history, the usual descriptor was “shallow salty seas.” Sounded nice and cozy for any early martian life. But at a press conference at the meeting, rover science team leader Steven Squyres of Cornell University made a point of spelling out the team’s best current understanding of early Mars, which is much less encouraging. “At the surface, this was primarily an arid environment,” he said. Only occasionally, here and there, would puddles of salty, acidic groundwater form between dunes of salt sand. As the team’s latest paper puts it, “dominantly arid, acidic, and oxidizing” environmental conditions would have posed “significant challenges to the origin of life.”
Jupiter’s moon Europa remains as a prime candidate for a vast under-ice ocean that might harbor exotic life. The other large Galilean satellites might have deep water, but it’s probably too deep to encourage astrobiologists. The geysers of Enceladus might not require liquid water (see 12/15/2006, last par.). As waterfront real estate shrinks, the stakes for astrobiology marketing skyrocket.
1Stofan et al., “The lakes of Titan,” Nature 445, 61-64 (4 January 2007) | doi:10.1038/nature05438.
2Christophe Sotin, “Planetary science: Titan’s lost seas found,” Nature 445, 29-30 (4 January 2007) | doi:10.1038/445029a.
3Lunine et al., “Ethane Ocean on Titan,” Science 16 December 1983: Vol. 222. no. 4629, pp. 1229 – 1230, DOI: 10.1126/science.222.4629.1229.
4Richard A. Kerr, “FALL MEETING OF THE AMERICAN GEOPHYSICAL UNION: Snapshots From the Meeting,” Science, 5 January 2007: Vol. 315. no. 5808, p. 37, DOI: 10.1126/science.315.5808.37a.
Liquid water is a rare and beautiful substance. If you want to float your life boat, better do it here.