Purported reservoir is buried beneath 1. 5 kilometers of polar ice
ATOM & COSMOS
Lake of liquid water detected on Mars
BY LISA GROSSMAN
A Mars orbiter has detected a wide lake
of liquid water hidden below the planet’s southern ice sheets. There have been
much-debated hints of tiny, ephemeral
amounts of water on Mars before. But if
confirmed, the lake is the first discovery
of a long-lasting cache of the liquid.
“This is potentially a really big deal,”
says planetary scientist Briony Horgan
of Purdue University in West Lafayette,
Ind. “It’s another type of habitat in
which life could be living on Mars today.”
The lake is about 20 kilometers across,
planetary scientist Roberto Orosei of
the National Institute of Astrophysics
in Bologna, Italy, and colleagues report
online July 25 in Science. But the water
is buried beneath 1. 5 kilometers of ice.
Orosei and colleagues spotted the lake
using the European Space Agency’s Mars
Express. The spacecraft aimed radar
at the planet to probe beneath the surface. As radio waves passed through the
ice, they bounced off different materials
embedded in the ice. The brightness of
the reflection tells scientists about the
material doing the reflecting; liquid water
makes a brighter echo than ice or rock.
Combining 29 radar observations
taken from May 2012 to December
2015, Mars Express revealed a bright
spot surrounded by much less reflective
areas in the ice layers near Mars’ south
pole. Orosei and colleagues considered
other explanations, such as radio waves
bouncing off a hypothetical layer of car-
bon dioxide ice at the top of the sheet,
but those options either wouldn’t pro-
duce the same radar signal or were too
contrived to be likely.
That left one option: a lake of liquid
water. Subglacial lakes on Earth have
been discovered in the same way.
The lake is probably not pure water.
Temperatures at the bottom of the ice are
about –68° Celsius, and pure water would
freeze there, even under the
pressure of the ice. But salt
in the water could lower the
freezing point. Sodium, magnesium and calcium salts
have been found elsewhere
on Mars (SN: 4/11/09, p. 12).
The pool could also be more
mud than water, but that
could still be habitable, Horgan says.
Previously, scientists have discovered
extensive solid water ice sheets under the
Martian dirt (SN Online: 1/11/18). There
were also hints that liquid water flowed
down cliff walls on Mars (SN: 10/31/15,
p. 17), but those may instead be dry avalanches. The Phoenix lander saw what
looked like frozen water droplets near
the north pole in 2008, but that water
may have been melted by the lander itself.
“If this [lake] is confirmed, it’s a sub-
stantial change in our understand-
ing of the present-day habitability of
Mars,” says Lisa Pratt, NASA’s planetary
Though the lake’s depth is unclear,
the volume dwarfs any previous signs of
liquid water on Mars, Orosei says. The
lake has to be at least several tens of centimeters deep for Mars Express to have
noticed it. That means it could contain
at least 10 billion liters of water. “When
we’ve talked about water in other places,
it’s in dribs and drabs,” Horgan says.
Mars Express began orbiting the Red
Planet in 2003. It took researchers more
than a decade to collect enough data to
convince themselves the lake was real.
For the mission’s first several years,
limitations in the spacecraft’s computer
forced the team to average hundreds of
radar pulses together before sending the
data back to Earth. That strategy some-
times canceled out the lake’s reflections,
Orosei says. On some orbits, the bright
spot was visible; on others, it wasn’t.
In the 2010s, the scientists switched
to a technique that let the team store the
data and send it back more slowly. Then
in 2015, months before the end of the
observing campaign, the experiment’s
principal investigator, Giovanni Picardi of
Sapienza University of Rome, died unex-
pectedly. “It was incredibly sad,” Orosei
says. “We had all the data,
but we had no leadership.
The team was in disarray.”
Discovering the lake is “a
testament to perseverance,”
says planetary scientist
Isaac Smith of the Planetary
Science Institute, who is
based in Lakewood, Colo.
But there is room for doubt, Smith
says. NASA’s Mars Reconnaissance
Orbiter has seen no sign of the lake, even
in CT scan–like 3-D views of the poles. It
could be that the NASA probe’s radar is
scattering off the ice in a different way,
or that the wavelengths it uses don’t
penetrate as deeply. Smith and other
members of the Mars Reconnaissance
team will look again and try to create a
3-D view from the Mars Express data.
“I expect there will be debate,” Smith
says. “This paper is well earned. But we
should do some more follow-up.” s
type of habitat
in which life
could be living
on Mars today.”
NRadar observations (rainbow colors) have revealed what may be a 20-kilometer-wide lake (blue triangle) hidden beneath ice near Mars’
south pole, not far from
the most visible part of
the southern ice cap.