Suppose you were told that it was possible to determine the surface temperature and presence of water on some planet other than Earth – at at time more than 4 billion years in the past? Sounds a little far-fetched, no? In fact, the deduction actually seems quite plausible, if you consider the reasoning.
The specific finding is that about 4 billion years ago, there was a spot on Mars that was wet and enjoying essentially shirt-sleeve weather – actually a bit warmer than a typical summer day on the beach at San Francisco.
How could anyone figure that out? Well, in the first place, a piece of Mars landed on Earth about 13,000 years ago in the form of a meteorite known as ALH84001. Standard methods of isotopic analysis established that minerals in the rock crystallized about 4 billion years ago.
But that’s only the beginning of the story. Carbonates in the minerals (which must have formed somewhat before the minerals themselves crystallized) contained rare isotopes of carbon and oxygen (carbon-13 and oxygen-18). The exact ratio of these two isotopes depends on the temperature at which the carbonates originally formed. That temperature must have been around 18° C. And the only way that carbonates could have formed at that temperature was by precipitation from liquid water. Voilà.
If this conclusion holds up, it will be the first time that we have good evidence the surface of Mars was once considerably warmer than it is now.
Researchers at the California Institute of Technology (Caltech) have directly determined the surface temperature of early Mars for the first time, providing evidence that’s consistent with a warmer and wetter Martian past.
By analyzing carbonate minerals in a four-billion-year-old meteorite that originated near the surface of Mars, the scientists determined that the minerals formed at about 18 degrees Celsius (64 degrees Fahrenheit). “The thing that’s really cool is that 18 degrees is not particularly cold nor particularly hot,” says Woody Fischer, assistant professor of geobiology and coauthor of the paper.
The article abstract is a good, reasonably understandable summary of the research:
Despite evidence for liquid water at the surface of Mars during the Noachian epoch, the temperature of early aqueous environments has been impossible to establish, raising questions of whether the surface of Mars was ever warmer than today. We address this problem by determining the precipitation temperature of secondary carbonate minerals preserved in the oldest known sample of Mars’ crust—the approximately 4.1 billion-year-old meteorite Allan Hills 84001 (ALH84001). The formation environment of these carbonates, which are constrained to be slightly younger than the crystallization age of the rock (i.e., 3.9 to 4.0 billion years), has been poorly understood, hindering insight into the hydrologic and carbon cycles of earliest Mars. Using “clumped” isotope thermometry we find that the carbonates in ALH84001 precipitated at a temperature of approximately 18 °C, with water and carbon dioxide derived from the ancient Martian atmosphere. Furthermore, covarying carbonate carbon and oxygen isotope ratios are constrained to have formed at constant, low temperatures, pointing to deposition from a gradually evaporating, subsurface water body—likely a shallow aquifer (meters to tens of meters below the surface). Despite the mild temperatures, the apparently ephemeral nature of water in this environment leaves open the question of its habitability.