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Lakes full of turbid water could have once been present on the surface of Mars.
The remains of these once murky basins can provide evidence as to what the planet was like billions of years ago and are prime candidates to search for evidence of Martian life.
A recent review article suggests that the ancient lakes should be prime targets for rovers on the red planet. These dried up basins, which date back as many as 3.7 billion years, provide evidence about the ancient atmosphere and volcanic activity of the red planet, and contain ingredients which could have supported life.
Dr Javier Cuadros is a senior researcher at the Museum and a co-author of the review. He says, 'There are approximately 500 ancient lakes that we can see on the surface of Mars, and perhaps many more we have not yet recognised from orbit.'
'These lakes were probably not all in existence at the same time, and most likely had several episodes of drying out and filling up again.'
'The sediments in the palaeolakes tell us about the ancient climate of the planet and the lakes themselves. While previous research had suggested the lakes would have been briny, we instead found that most lakes would have been filled with freshwater.'
The review was published in Nature Astronomy.
The only liquid water on Mars today is believed to exist deep underground, though water ice exists in polar ice caps and as permafrost. Over 4.1 billion years ago, however, water was much more abundant.
At this time, Earth's close neighbour is believed to have had a magnetic field which deflected solar wind away from the planet, allowing greenhouse gases to stay in the atmosphere and water to exist on the surface in liquid form.
The super-fast subatomic particles contained within the solar wind then entered Mars' atmosphere and began to strip the planet of its greenhouse gases, including water vapour. This caused the planet to cool and dry significantly, with any remaining liquid water frozen into ice.
But following this, periods of volcanic activity released more greenhouse gases into the atmosphere, raising the planet's temperature for a time and allowing liquid water to form lakes once again.
'On Earth, there are different ways in which a lake basin can form, but the lakes on Mars all generally formed in the same way,' Javier says. 'The only way in which new basins can be generated is by the impact craters left by large meteorites.'
'These craters were then fed by inlets or groundwater that filled them up over time, with some forming part of larger channel systems which can still be seen today.'
However, these lakes would have been short-lived, at least compared to those on Earth. It is estimated that they existed for anywhere from one million to as little as 100 years, partially as a result of Mars' weak gravity.
'Mars has lower gravity than the Earth, which means that there is less compaction of sediments under lakes across the planet,' Javier says. 'This makes the ground more porous, allowing water to drain into the soil more easily and causing the lake to dry up fast if water input slows.'
Lower gravity would also cause sediment to have been suspended in the water for longer, meaning Mars' lakes were probably turbid. This could have had implications for any former residents of the planet.
'Sediments in the water would limit the depth light could penetrate into these lakes,' Javier says. 'The photic zone on early Mars would have been around 10 times shallower than it is on modern Earth as a result of this, as well as the Sun being fainter at the time.'
Given the problems photosynthetic life might have faced, chemolithotrophic organisms, which use chemical reactions from mineral elements to release energy, may have been a more likely resident of the planet's murky waters.
'Chemolithotrophic organisms would have important advantages if they developed on Mars,' Javier says. 'Minerals on the surface of Mars are more water-soluble as well as richer in nutrients and energy sources than on Earth. These minerals would then have been available to microorganisms, especially if they were suspended as sediment.'
'It is also likely that hydrogen, another energy source for microbes, was made available in serpentinization reactions that are known to sustain important microbial communities deep underground on Earth.'
Some theories of how life began on Earth suggest that the earliest cells may have been chemolithotrophic and developed around undersea vents. If Martian life emerged similarly, it would provide additional support for these theories.
But life on the red planet would have faced significant threats to survival that its Earth counterparts did not, including the loss of Mars' magnetic field.
'Even if liquid water was present on the planet, we believe it may have been only for short episodes alternating with periods where it froze to ice,' Javier says. 'This makes it harder for life to evolve, or even develop in the first place.'
Researchers hope that as more probes and rovers are sent to Mars, they will continue to be able to build up a picture of Mars' lakes and continue to look for any signs of life.
'This review forms the first step of new investigations of the Martian lakes by summarising what we know so far, which allows us to take stock and discover perspectives only visible when taking a global view of a topic,' Javier adds. 'We'll be using new data from probes orbiting the planet to investigate the palaeolakes and their sediments in the near future.'