Mars was home to liquid water for more time than scientists initially believed. This means that life, if it ever did develop on Mars, existed also existed more recently.
The fan-shaped sedimentary landforms found on Mars, which were the result of liquid water currents, extend Mars’ wet history by hundreds of millions of years.
“We’ve known for decades that Mars had rivers and lakes around 3.5 billion years ago, but in the past few years there has been a growing body of evidence that substantial amounts of liquid water continued to erode the Martian surface for hundreds of millions of years,” said Morgan, lead author on “The global distribution and morphologic characteristics of fan-shaped sedimentary landforms on Mars” published in Icarus.
Deltas, alluvial fans, and other water-formed landforms are the most unambiguous markers of past climates. Researchers explored patterns in their distribution and morphological properties by conducting a global survey of these features.
The researchers discovered how alluvial fans are located at lower elevations than the more ancient valley networks.
It is evident from this characteristic that stable liquid water was restricted to Mars’ lower, warmer regions as it cooled and dried.
There was liquid water on Mars for a long time, between about 3.6 billion and at least 2.5 billion years ago. The surface of Mars was not wet for this entire time, however.
Researchers speculate that liquid water may have existed episodically on Mars due to the planet’s movements – such as axial tilt, orbital eccentricity, precession – or volcanic activity.
In a second paper published in Science Advances, scientists used this fan-shaped database to examine changes in Mars’ climate over time. The change in water flow distribution on Mars during the time was also charted in the Science Advances article, “Changing spatial distribution of water flow charts major change in Mars’ greenhouse effect”.
In order to better understand how Mars’ climate evolved during these two eras of its early history, climate models were used to compare the distribution of valley networks, which are dated to the earlier era, and alluvial fans, which are dated to the later era.
Despite Mars’ global average temperature cooling from 25 degrees Fahrenheit to 5 degrees Fahrenheit over time, liquid water remained stable in some areas of the planet.
When sediments are deposited at the base of mountains, fan-shaped alluvial deposits are left behind by rivers. Most of the cities of Los Angeles, Las Vegas, and Phoenix are built on top of alluvial fans, which are prominent features in the southwest.
The surfaces of the red planet and the moon Titan are also known to have alluvial fans. Martian alluvial fans reach lengths of up to 40 kilometers (25 miles), and their formation was enabled by massive amounts of flowing liquid water.
One of the most interesting things about Martian fans is that many of them emerged much later than the valley networks, considered the strongest structural evidence for surface water on early Mars.
The valley networks date from around 3.6 billion years ago, while alluvial fans date from 2.5 billion to 3 billion years ago.
Even though that is still very ancient – Earth 2.5 billion years ago did not have multicellular life – it greatly lengthens the time during which Mars could have stable liquid water and thus be habitable.
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