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The surface of Mars, by all the measurements we’ve taken, is currently an inhospitable wasteland. Only dust devils roam its arid surface; the only water is permanent ice. Yet the evidence that water once flowed and collected on the planet’s surface continues to mount.
The presence of this liquid water means that Mars could have supported life as we know it … but a burning question remains: how the heck was Mars hot enough for it, in the early days of the Solar System, when the young Sun was it colder? and more subdued?
New research has found an answer: Geothermal heat could have risen from deep within the planet, in which case the best place for life to flourish would have been underground.
“Even if greenhouse gases such as carbon dioxide and water vapor are pumped into the early Martian atmosphere in computer simulations, climate models still struggle to keep Mars warm and humid in the long term,” said the scientist. Lujendra Ojha Planetarium at Rutgers University in New Brunswick.
“My co-authors and I propose that the paradox of the weak young Sun can be reconciled, at least in part, if Mars had high geothermal heat in the past.”
The weak young Sun paradox is the contradiction between the presence of liquid water in the early Solar System and the weakness of the Sun. According to our knowledge of stellar evolution, in the 1 billion years after its formation 4.6 billion years ago, the heat and light from the Sun would only have been about 70 percent of its current production.
Even today, Mars is a cold place. It is 1.5 times the distance from Earth to the Sun, and only receives about 43 percent of the solar flux that Earth receives. Therefore, its average temperature is much lower than that of Earth: -63 degrees Celsius (-81 degrees Fahrenheit). Of course, that’s just the average; the temperature rises above the melting point of water, about 30 degrees Celsius (although, because the atmospheric pressure on Mars is currently so low, the ice sublimates instead of melting).
During Noah’s period on Mars, between 4.1 and 3.7 billion years ago, it is believed that water was abundant on the planet’s surface; however, climate models struggle to reach temperatures above -0.15 degrees Celsius.
The possibility that the planet has warmed from within, keeping the groundwater liquid in the long term, is not a new notion. Hydrothermal minerals excavated deep underground from comet impacts, Noah-era clays, and evidence of groundwater diagenesis at various sites support internal warming models.
Here on Earth, we see the effects of geothermal warming beneath the ice sheets at high latitudes. The radioactive decay of elements such as uranium, potassium and thorium in the planet’s crust generates heat that spreads to the surface; not much, but when there is a thick layer of ice preventing heat from escaping, enough heat can be trapped to melt some of that ice, creating subglacial lakes.
So, Ojha and his team investigated the possibility that this could have happened on Mars during Noah. They modeled the thermophysical evolution of ice and calculated how much heat it would take to produce meltwater and subglacial lakes on a cold, icy Mars.
They then compared this with various data sets from Mars to determine if this would have been feasible on Mars 4 billion years ago. And they found that the conditions for melting groundwater would have been ubiquitous at the time, with volcanism and meteorite impacts possibly providing additional heat.
It’s still possible that the surface of Mars was warm and humid for a while, but that climate, the researchers said, would not have been stable in the long term. Mars lost its magnetic field quite early in its history, sometime around Noah, and once the magnetic field was gone, the thick, Earth-like atmosphere would not have lasted much longer.
Only at great depths, kept liquid by geothermal heating, could the water have been stable in the long term, the researcher said. If there were life on the surface, it could have followed the water in.
“At such depths, life could have been sustained by hydrothermal (warming) activity and rock-water reactions,” Ojha said. “Thus, the subsurface may represent the longest living environment on Mars.”
Other research using sonar suggests that liquid water is still present underground on Mars today, although the reason why it doesn’t melt could be quite different. Scientists believe that underground lakes on Mars could be extremely salty, as salinity lowers the freezing point of water.
And scientists have found evidence of mud volcanism on Mars, where moist subsurface sediments are pushed up and out of underground pressure. The water, of course, would sublimate once it reached the surface. But each piece of evidence points to a very different type of Mars once the crust is broken.
Three new missions to Mars were launched in July this year, arriving in February 2021. We may be very close to getting many more responses.
The research has been published in Scientific advances.
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