In Search of Ancient Seas

There’s no question that Mars had water in the past. The evidence is everywhere, from eroded shorelines and river bottoms to crystals that could only have formed in the presence of water. The big question for planetary scientists (among many) is how long ago that water flowed (and precipitated).

Scientists at the Planetary Science Institute are presenting what they consider to be very strong evidence that flowing water and precipitation helped shape the Martian surface around the Valles Marineris until at least 3 billion years ago — during a period in Mars’s geologic history known as the Hesperian Era.

It’s long been suspected that there was running water on Mars during its first billion years of existence, up to the end of the Noachian era. But, the big question was, did the watery past extend later into Mars history?  To answer that, a team of PSI researchers led by Catherine Weitz, studied images from the Mars Reconnaissance Orbiter’s HiRISE instrument and focused in on light-toned layer deposits seen on the plains near the Valles Marineris. It appeared to the team that the light-colored deposits are very different from the ones inside the giant canyon nearby.  To a geologist, this suggests that the processes that created the deposits outside the Valles Marineris were different from the ones inside the canyon.

The team also found valley systems in the area they studied that were probably created by running water in two other areas of light-toned layered deposits adjacent to Valles Marineris. All of these phenomena point toward what geologists call fluvial processes — those associated with running water.

Now, there are other processes such as volcanism or wind deposition (called aeolian processes), that can create light-toned deposits like the ones the PSI team has been studying. However, the material is so distinct from other lighter-colored deposits and is so closely associated with valleys and channels, that it’s not difficult to see where fluvial processes could be responsible. It’s like looking at the remains of an ancient riverbed on Earth and using the deposits to tell the story of past flooding and precipitation activity. To see it on Mars in areas where we know the relative ages of the surface regions and know what rocks and minerals exist there is fascinating and gives us a much clearer picture of the action of water over a longer period in this planet’s history.