But the climate has changed significantly over the course of the planet's 4.6-billion-year history and liquid water can not exist on the surface today, so scientists are looking underground. This keeps the lake at just below freezing temperatures, and it maintains a liquid state due to the huge weight of the ice above pressing down on it.
"What we tend to think here on Earth is that if there is liquid water, there is a good chance for life".
How much dust? It's an estimated 3 trillion kilograms displaced between the surface and atmosphere of Mars each year.
One of the 29 samples showed unusually strong reflections. So all measurements the team collected were relativistic and compared to other objects on Mars, which meant that arriving at the precise size of this subsurface water body took three years.
"The estimated temperature, which is to be debated to some degree, at the depth at which this water is occurring is said to be 205 K [90˚ F]", said Vlada Stamenković, a research scientist at NASA's Jet Propulsion Laboratory.
Though the temperature on Mars may be too cold for pure water, Orosei and fellow researchers from a number of Italian instituitions noted it was possible that the water was mixed with dissolved salts of magnesium, calcium, and sodium to form a brine, researchers said.
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Several researchers said it would be crucial to figure out whether this body of water is the only one, or part of an interconnecting body of underground aquifers - in part because a network increases the possibility it could have harboured life.
The Mars Express spacecraft discovered the body of water beneath the southern ice cap. It will also interest those studying the possibilities for life beyond Earth - though it does not yet raise the stakes in the search for biology. It therefore remains an open question whether the water is warm enough for life; perhaps Martian extremophiles are even more extreme than their cold weather terrestrial counterparts.
Scientists Roberto Orosei (L), Elena Pettinelli (C) and Enrico Flamini pose near a replica of the Cosmo Sky Med satellite before a news conference where they announce after first-time detection of liquid water on Mars by italian radar Italian radar MARSIS, on board the ESA's Mars at the Italian Space Agency headquarter in Rome, Italy July 25, 2018.
For the past 12 years, a spacecraft-mounted radar called MARSIS has been sending radio waves down to Mars, which reflect back information about the make-up of the planet below. On Earth, places like these are home to bacteria adapted to the extreme conditions of sub-glacial, briny lakes.
Indeed, scientists have found bacteria and other simple forms of life living in these kinds of extreme environments on our own planet, which use chemical reactions with salts and minerals to get the energy they need to live. On Earth those lakes are often connected by channels, forming branching riverlike networks of water that extend across vast spaces beneath the ice.
Robotic missions to the planet's surface still find surprising echoes of that bygone time, such as patches of water-ice frost forming on rocks as well as water droplets condensing like dew on a lander's leg.
Beneath this, researchers spotted something unusual 1.5km beneath the ice. And in recent years, scientists actually drilled deep beneath the Antarctic ice into one of these, the subglacial Lake Whillans, which had been cut off from the surface for millions of years.