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The series of alternating bright dots moving from top left to bottom right is a channel filled with salt deposits.-
So far, there’s a lot of evidence that Mars had a watery past, and there’s more to come Come any time. But this evidence does not necessarily give us a complete picture of Mars’ past. It is a red planet covered with a watery oceanor most of the water Trapped like icewith irregular seasonal thaws?
This week, two researchers at Caltech – Elaine Liske and Bethany Ellman – helped provide a clearer picture of Mars’ past by discovering the possible behavior of the last liquid water on Mars and determining when it stopped flowing. Their secret is to track salt deposits on the Martian surface.
follow the salt
There are many different salts we find on Mars, but the important one here is chloride (which may include sodium chloride in table salt). These are very useful because they are the most water-soluble salts. So if there’s water around, these chloride salts will dissolve in it. Any of these salt deposits currently sit on the surface of Mars, and are then placed there as the last of the water in that region of the planet dries up.
Fortunately, detecting chlorides from orbit does not pose much of a challenge. They have a distinct spectral fingerprint that only two other chemicals (including diamond) have that are unlikely to be found in significant quantities on Mars. So, armed with data from the Mars Reconnaissance Orbiter, the researchers mapped out the presence of chloride deposits across the Martian surface.
This analysis of the sediment provided the researchers with a number of data points about the water that placed it there.
For starters, you might expect that the last liquid water on the planet could collect at the bottom of the pool as they gradually dry up. But this is not the pattern we see here. In contrast, many are in relatively narrow channels, and sediment elevations are often higher than in adjacent basins. For Leask and Ehlmann, this indicates that water has flowed into the channels but dried up before reaching the basins they filled. This idea is supported by the fact that the outlet from this basin contains no salt deposits.
Another factor the researchers identified is that these sediments are relatively weak. While it is difficult to get exact depths from orbit, in some cases, the team was able to estimate where the crater from the impact interfered with chloride deposition. These results consistently show that the salt deposit is shallow – less than three meters. Although this still causes many broken water cycles and droughts, it is clear that Mars does not need the long term water dwelling that leads to the formation of thick salt deposits on Earth.
When 2 billion “modern”
Finally, the researchers looked at the age of the salt deposits. Usually, this is done by examining the number of craters in the sediment and assuming that drilling has been done regularly over the last few billions. But most of the salt deposition occurs in narrow channels, so there isn’t much of a large surface on which to build a useful hole count.
Instead, the researchers focused mostly on the date of bedrock deposition, which provides an upper bound on the age of the salt deposited on it. In one case, the team found salt deposits sitting on top of a 3.3 billion year old rock, which had been altered by a two billion year old event. In other cases, salt deposits are on top of volcanic sediments that are 2.3 billion years old.
This is much more recent than many previously thought when Mars was very cold and lost much of its atmosphere to allow liquid water.
However, Leask and Ehlmann do not believe that these deposits represent the permanent presence of water. Instead, they suggest that water arose in this channel due to the seasonal melting of local ice deposits and may not have entered the neighboring basin in significant quantities. They also note that the region with the most salt deposits overlaps with where climate models predict we will see the most rainfall when Mars has a water cycle, so there is good reason to believe there should be significant ice deposits in the region.
No single analysis will give a complete history of Mars’ watery past. But individual results can give us a glimpse into a different era, providing pieces of the puzzle that we can eventually collect to form a bigger picture.
AGU ancestor2022. DOI: 10.1029/2021AV000534 (About DOI).