History of Mars’ habitability preserved in ancient dunes
Scientists who study the possibility of life on Mars want to know how habitable the planet might have been millions or billions of years ago. Was Mars ever able to support life as we know it, at least microbial? The evidence from landers, rovers and orbiters over the past few decades has continued to indicate that Mars was indeed once more habitable than it is now. But then conditions changed; the water on the surface dried up and the atmosphere became thinner and drier. Late last month, an international team of researchers reported on a new study documenting the changing habitability of Mars. These scientists examined ancient sand dune fields preserved in rocks in Gale Crater, where the Curiosity rover has been exploring an ancient lakebed since 2012.
Curiosity had already confirmed that Gale Crater used to be a lake or series of lakes a few billion years ago. Now, it has also found evidence for an ancient dune field – called the Stimson formation – that is still preserved as a layer of rocks that lies on top of the older lake bottom rock layers.
This change in rock layers provides clues as to how the climate changed and how the environment shifted from a habitable one to the uninhabitable arid desert we see today.
It also helps scientists better understand various surface and atmospheric processes that were active at the time, such as the direction of the blowing sand that formed the dunes. The researchers were even able to figure out the shape, size and migration direction of the largest dunes.
One discovery is that there were once dunes nestled right up against the central mountain in Gale Crater, called Mount Sharp. They had formed on a wind-eroded surface at a 5 degree angle. Those dunes were what are known as compound dunes; each large dune had its own set of smaller “satellite” dunes that migrated in different directions from the main dunes. From the paper:
Analysis of the sedimentary structures generated by the complex interaction of these two scales of dune indicates that the large dunes migrated north, and that the smaller superimposed dunes migrated across the faces of the large dunes toward the northeast.
Dunes are of course common Earth, and they are on Mars as well. Mars has vast dune fields today, not just the ancient preserved ones from billions of years ago. Steve Banham, lead author of the new study, discussed how such dunes form and how they can be preserved:
As the wind blows, it transports sand grains of a certain size, and organizes them into piles of sand we recognize as sand dunes. These landforms are common on Earth in sandy deserts, such as the Sahara, the Namibian dune field, and the Arabian deserts. The strength of the wind and its uniformity of direction control the shape and size of the dune, and evidence of this can be preserved in the rock record.
If there is an excess of sediment transported into a region, dunes can climb as they migrate and partially bury adjacent dunes. These buried layers contain a feature called ‘cross-bedding,’ which can give an indication of the size of the dunes and the direction which they were migrating. By investigating these cross beds, we were able to determine these strata were deposited by specific dunes that form when competing winds transport sediment in two different directions.
It’s amazing that from looking at Martian rocks we can determine that two competing winds drove these large dunes across the plains of Gale Crater three and a half billion years ago. This is some of the first evidence we have of variable wind directions, be they seasonal or otherwise.
The dune fields are thought to have formed after the lake in Gale Crater dried out. The bottom of the crater, and lower flanks of the mountain, are composed of ancient lakebed sediments. Higher up on Mount Sharp are non-sedimentary sandstone rock layers. Most of Curiosity’s mission so far has been spent examining the sedimentary layers, containing mudstones and clays, for evidence of past habitability. Banham added:
More than 3.5 billion years ago this lake dried out, and the lake bottom sediments were exhumed and eroded to form the mountain at the center of the crater, the present-day Mount Sharp. The flanks of the mountain are where we have found evidence that an ancient dune field formed after the lake, indicating an extremely arid climate.
While analysis of the preserved dune fields helps to answer questions about the changing habitability of Mars, it also appears to indicate that the habitability potential lessened when the dunes were formed, after the lake dried up. When the dunes formed, there was less water available for any microbes, and the landscape was starting to change to the dry desert we see today. The dunes would also not be ideal for preserving traces of any past life, either. From the paper:
The presence of large, wind-driven dunes indicates that the region was extremely arid, and that – at the time the Stimson dune field existed – the interior of Gale Crater was devoid of surface water, unlike the setting recorded by the older, underlying lake sediments of the Murray formation.
The vast expanse of the dune field wouldn’t have been a particularly hospitable place for microbes to live, and the record left behind would rarely preserve evidence of life, if there was any.
This desert sand represents a snapshot of time within Gale Crater, and we know that the dune field was preceded by lakes, yet we don’t know what overlies the desert sandstones further up Mount Sharp. It could be more layers deposited in arid conditions, or it could be deposits associated with more humid climates. We will have to wait and see.
Although geologists have been reading rocks on Earth for 200 years, it’s only in the last decade or so that we’ve been able to read Martian rocks with the same level of detail as we do on Earth.
Curiosity is now continuing to drive further up the flanks of Mount Sharp, and will study rock layers higher up to document any changes in ancient wind patterns, Banham said:
We’re interested to see how the dunes reflect the wider climate of Mars, its changing seasons, and longer-term changes in wind direction. Ultimately, this all relates to the major driving question: to discover whether life ever arose on Mars.
Mars’ atmosphere is substantially thinner today then it was back then, but the planet still has active dune fields. All of the rovers and landers have seen dunes, as well as smaller ripples, up close. Orbiters have photographed them all over the planet, including at the poles. The dunes come in a variety of shapes and sizes, and often closely resemble dunes and dune fields on Earth. Just as it is often described to be, Mars truly is a desert world.
Bottom line: The changing habitability of Mars has been preserved in ancient dune fields in Gale Crater according to a new study from researchers at Imperial College London.