The water-related mineral jarosite is occurs both on Earth and in sediments on Meridiani Planum, Mars. Scientists have found that by measuring the isotope ratio of argon-40 to argon-39 in the rock — a technique that can be applied to rock samples brought from Mars — they can tell how long ago it felt the touch of water and also, with a few assumptions, how warm the water was.
“Our results suggest that 4 billion-year-old jarosite will preserve its argon and, along with it, a record of the climate conditions that existed at the time it formed,” says Suzanne Baldwin (Syracuse University), one of the authors of the study, published October 15 in Earth and Planetary Science Letters.