"Follow the water," astrobiologists say, and the search for extraterrestrial life will inevitably prove fruitful.
Water is almost universally recognized as essential for life, therefore many scientists believe life will be found wherever researchers find abundant liquid water. Motivation for such research stems from the insurmountable problems for a naturalistic origin of life on Earth.
From an astronomical perspective, Mars is an obvious place to look for liquid water. The planet is physically similar to Earth, and has signatures of water in various places and geological features similar to those on Earth that form by water. Evidence from the past year seems to support the idea of ubiquitous water on Mars.1,2 However, another growing body of evidence argues that liquid water existed on the surface of Mars only for brief periods of time.3,4
One powerful find shows that the surface temperature on Mars has been above 0oC for no more than a few million years over the last 4 billion years. Ironically, the meteorite that engendered claims of fossilized Martian life is the same meteorite that establishes the perpetual frostiness of the Martian surface (and the top few kilometers of the Martian crust). Martian meteorite ALH84001 was dated at 4 billion years old using both argon and uranium/lead radioisotopic measurements. Argon is a noble gas so its abundance is sensitive to the amount of heating the meteorite has experienced. However, the argon date for the meteorite matches the uranium/lead date, so scientists determined that its temperature was below the freezing point of water for all but a maximum of 1 million years--far too brief to expect life under naturalistic scenarios.
A second discovery reveals increasing deposits of olivine (a mineral-like silicate containing iron and magnesium) in recent and ancient geological features on Mars. Olivine weathers quickly in the presence of liquid water. Therefore, abundant olivine deposits would not be expected if long-standing bodies of water existed on the Martian surface during the last 4 billion years. (However, liquid water pockets a few kilometers deep in the crust can interact with olivine, starting nonbiological processes which explain the atmospheric methane that some scientists have used to argue for Martian bacteria.5)
It appears that the "follow the water" strategy remains frozen by scientific advance. The growing body of evidence that long-standing liquid water never existed on Mars strengthens the case that Earth is the only life-supporting body in the solar system, and likely the universe.
- David C. Catling, "Twin Studies on Mars," Nature 436 (2005): 42-43.
- "Researchers Detect Methane on Mars," http://www.spacedaily.com/news/mars-life-04v.html, accessed November 30, 2005.
- David L. Shuster and Benjamin P. Weiss, "Martian Surface Paleotemperatures from Thermochronology of Meteorites," Science 309 (2005): 594-600.
- P. R. Christensen et al., "Evidence for Magmatic Evolution and Diversity on Mars from Infrared Observations," Nature 436 (2005): 504-09.
- Christopher Oze and Mukul Sharma, "Have Olivine, Will Gas: Serpentinization and the Abiogenic Production of Methane on Mars," Geophysical Research Letters 32 (2005): L10203.