Sky and Telescope - November 2018 - 8

NEWS NOTES

Surface

South Polar
Layered Deposits

SOLAR SYSTEM

Water Lake Discovered on Mars

Bright radar
echoes suggesting
water

THE SECOND-OLDEST OPERATIONAL

Mars orbiter, the European Space
Agency's Mars Express, has discovered
evidence of stable, present-day liquid
water on Mars.
How is this discovery of water on
Mars different from past announcements of the same? For one, the data
come from a different instrument
examining a new location on Mars. But
most importantly, it's the ﬁrst time the
observations support a present-day body
of water that stays liquid.
The results, which appear in the
August 3rd Science, come from the
Mars Advanced Radar for Subsurface
and Ionosphere Sounding (MARSIS)
instrument. This radar sounder emits
radio waves using a pair of 20-meterlong (66-feet-long) booms that extend
on either side of Mars Express and
measures the time it takes for the radio
waves to travel to Mars, bounce, and
travel back to the spacecraft.
The long radio waves from the
instrument's 40-meter antenna can
penetrate as far as 5 kilometers (3
miles) down. Subsurface boundaries
between layers with different properties
each reﬂect the radio signal, so MARSIS
may detect multiple echoes from each
pulse. As the instrument tracks along
the ground, it builds up a 2D slice of the
subsurface called a radargram.
For many of its 15 years at Mars,
MARSIS has detailed the interior
structure of the planet's polar layered

p An example radar profile for one of 29 orbits over the 200 × 200 km study region in the south polar
region of Mars shows layers of ice and dust to a depth of about 1.5 km. The bright horizontal feature
at the top corresponds to the icy Martian surface. The brightest reflections from the base layer -
close to the center of this image - are centered around 193°E / 81°S in all intersecting orbits, outlining a 20-km-wide anomaly that Roberto Orosei and colleagues interpret as a pond of liquid water.

deposits, layers of dust and water ice
that exist at both poles. More recently,
the instrument team picked a region
of the southern deposits where the
surface looks especially bland, making
it easier to study the base of the deposits
in detail without interference from
overlying features. MARSIS scanned this
region intensely from May 29, 2012, to
December 27, 2015, collecting 29 crisscrossing radargrams.
In one particular location, where
many radargrams cross over a region
about 20 km wide, the scans kept hitting an especially bright bottom-most
boundary about 1,500 meters beneath
the surface. This unusually reﬂective
spot, which persisted over a period of
three years, suggests a material very
different from the ice above it or the
rock below it. Roberto Orosei (National
Institute of Astrophysics, Italy) and
colleagues argue that the most likely
explanation is a briny lake (or a layer of
very sludgy, liquid-water-rich sediment).
Why is it briny? The predicted temperature at the base of the southern
polar layered deposits is −68°C (−90°F),

and pure liquid water can't exist at such
low temperatures. On Earth, where
subglacial lakes in Antarctica can have
temperatures as low as -13°C, the freezing point of their water is depressed by
the presence of large amounts of salt
- much more than is present in Earth's
oceans. Mars has salts of sodium, magnesium, and calcium, not to mention a
plethora of chlorine-based perchlorates,
all of which - if concentrated enough
- could depress the freezing point
of water to as low as -75°C. So it is
physically possible that a very salty lake
persists beneath the southern polar cap.
Higher-resolution data would help
conﬁrm the lake's presence: The radar
images' resolution is about 5 km,
providing a fuzzy look at the 20-kilometer width of this putative lake. Future
missions could more precisely map its
contours - and perhaps discover detail
beyond the capabilities of MARSIS.
There's every reason to think we could
discover many more such briny spots
deep underneath the Martian south
polar cap.
■ EMILY LAKDAWALLA

The ExoMars
Trace Gas
Orbiter captured
this view of part
of Mars's south
polar ice cap on
May 13, 2018.

N OV E M B E R 2 018 * SK Y & TELESCOPE

Sky and Telescope - November 2018

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