There is now much evidence that Mars had a watery past, and there is more data to come come in all the time. But this evidence doesn’t necessarily give us a complete picture of Mars’ past. Was the red planet covered with watery oceans, or was most of the water trapped like ice, with erratic seasonal melting?
This week, two researchers at Caltech — Ellen Leask and Bethany Ehlmann — helped paint a clearer picture of Mars’ past by figuring out the likely behavior of the last liquid water on Mars and noting when it stopped flowing. Their secret was to detect salt deposits on the surface of Mars.
Follow the salt
There are many different salts that we have discovered on the surface of Mars, but the ones of interest here are chlorides (which likely include sodium chloride from table salt). These are particularly telling because they are the most easily soluble salts in water. So if there is water around, these chloride salts would be dissolved in it. So any deposits of these salts currently present on the surface of Mars were deposited there when the last water in that region of the planet dried up.
Fortunately, detecting chlorides from orbit is not very difficult. They have a distinct spectral signature shared only by a few other chemicals (including diamonds) that are unlikely to be found on the Martian surface in appreciable amounts. Armed with data from the Mars Reconnaissance Orbiter, the researchers mapped the presence of chloride deposits across the surface of Mars.
Analysis of these deposits provided researchers with a series of data points about the water that deposited them there.
For starters, one might expect that the last liquid water on the planet would have pooled at the bottom of the pools as they gradually dried up. But that’s not the pattern seen here. Instead, many were located in relatively narrow channels, and the height of the deposits was often higher than in nearby basins. To Leask and Ehlmann, this suggested that the water had flowed into channels but had dried up before reaching the pools they filled. The idea was supported by the fact that the drainage channels of these basins were devoid of salt deposits.
Another factor the researchers noted was that these deposits were relatively thin. While it’s difficult to get an exact depth from orbit, in some cases the team was able to estimate where impact craters destroyed the chloride deposit. These results consistently showed that the salt deposits were shallow — less than ten feet deep. While this still represents many cycles of inflowing and drying up water, Mars clearly doesn’t need the long-term water occupancy that forms thick salt deposits on Earth.
If 2 billion is “current”.
Finally, the researchers looked at the age of the salt deposits. Typically this is done by examining the number of craters in the deposit and assuming that the craters have formed regularly over the past billions of years. But most salt deposits occurred in narrow channels, so there aren’t many large surfaces to build a useful crater figure.
Instead, researchers focused primarily on dates from the underlying rock deposits, which provide a maximum age for the salt deposited on top. In one case, the team found a salt deposit on a 3.3-billion-year-old rock that was altered by an event two billion years ago. In another case, salt deposits were on top of volcanic deposits dated to 2.3 billion years ago.
This is much newer than many previous estimates of when Mars became too cold and lost too much of its atmosphere to support liquid water.
However, Leask and Ehlmann do not believe that these deposits represent a permanent presence of water. Instead, they suspect that the water appeared in these channels due to the seasonal melting of local ice deposits, and may not have even reached the nearby basins in appreciable amounts. They also note that the region that has the most salt deposits overlaps with the region where climate models predict we would see the most precipitation if Mars had a water cycle, so there’s good reason for that Assumption that there should be large ice deposits in the area.
No single analysis will provide a complete history of Mars’ watery past. But individual finds can give us a glimpse into different eras and provide pieces of the puzzle that we can eventually piece together into a larger picture.
AGU progress, 2022. DOI: 10.1029/2021AV000534 (About DOIs).
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