Sky & Telescope - November 2021 - 9

LEFT: ESA / DRL / FU BERLIN; RIGHT: NASA
MARS
" Liquid Water " Spots Below Martian Surface Might Be Clay
RESEARCHERS HAVE IDENTIFIED
dozens of radar-bright spots within the
layers of ice and sediment deposits at
Mars's south pole. These discoveries follow
the 2018 announcement of a large
radar-bright area beneath those deposits
that researchers said might indicate a
subglacial lake (S&T: Nov. 2018, p. 8).
However, the additional finds, as well as
further experiments and analysis, suggest
that the spots might be ancient clay
deposits instead of liquid water.
In a study published in the July Geophysical
Research Letters, Aditya Khuller
(Arizona State University) and Jeffrey
Plaut (Jet Propulsion Laboratory) used
the Mars Advanced Radar for Subsurface
and Ionosphere Sounding (MARSIS)
instrument aboard the European
Space Agency's Mars Express orbiter to
chart the subsurface boundary where
the polar deposits end and the Martian
interior begins. Their investigation
revealed dozens of new smaller radarbright
regions at this interface, which
appear to be widespread beneath the
south pole's ice-and-dust deposits.
If these bright areas are indeed lakes,
they must be very salty. Even though the
subsurface temperatures on Mars drop
below the freezing point of pure water,
dissolved salts can keep water liquid at
lower temperatures.
However, a new study from a team
led by Carver Bierson (Arizona State
University), also in the July Geophysical
Research Letters, suggests that supersalty
water isn't the only option. His
pLeft: An artist's illustration shows the iceand-dust
deposits at Mars's south pole.
Right: This image shows radar measurements
of smectite clays, conducted under Mars-like
conditions in the lab. The results reproduce the
radar signatures detected by MARSIS.
team shows that highly conductive
materials, like saline ice and hydrated
clays, can strongly reflect radar.
Another study in the August issue
of the same journal, led by Isaac Smith
(York University, Canada), supports
the case for clays. Smith's team investigated
a type of clay called smectites,
which are common on the Red Planet's
surface and found near the edge of the
IN BRIEF
Resolving the Mars Methane Mystery
New measurements from NASA's Curiosity rover show that methane concentrations near
the Martian surface vary on a daily cycle, a finding that could help reconcile conflicting
data. Curiosity first sniffed the gas on June 15, 2013, and has since found background
methane levels between 0.2 and 0.7 parts per billion in volume (ppbv). But in 2019,
the European Space Agency's ExoMars Trace Gas Orbiter failed to find methane after
several months of operation. ExoMars examines sunlight that has traveled through the
atmosphere's upper layers and should be able to detect particle concentrations as low
as 0.05 ppbv. Then John Moores (York University, Canada) realized that the discrepancy
could boil down to the time of day: Curiosity takes methane measurements at night,
when the atmosphere is cool and calm, while ExoMars looks at the atmosphere at sunset
after a day of Sun-driven mixing. The Curiosity team tested this idea by bracketing
a nighttime measurement, which yielded 0.5 ppbv, with two daytime ones, which didn't
pick up any methane at all. The data take a step toward reconciling the Curiosity and
ExoMars results, but work remains to understand why methane doesn't build up in the
atmosphere over time. Christopher Webster (Jet Propuulsion Laboratory), Moores, and
colleagues reported the results in the June Astronomy & Astrophysics.
¢JAVIER BARBUZANO
Read more at https://is.gd/Marsmethane.
sk yand tele scope .o r g * NOVEMBER 2021
9
south polar cap. The scientists studied
smectites in a laboratory under frigid
conditions that mimic the Martian
subsurface. They found that hydrated
smectites make a brittle paste when frozen,
one that could reproduce the radar
signatures detected by MARSIS.
Liquid water might have hydrated
smectites more than 100 million years
ago, before layers of ice and dust preserved
the clays. But Bierson says that
salty ice and other clays are still radarreflective
contenders.
¢LAUREN SGRO
Read additional details from these studies
at https://is.gd/Marsclay.

https://www.is.gd/Marsclay https://www.is.gd/Marsmethane http://skyandtelescope.org

Sky & Telescope - November 2021

Table of Contents for the Digital Edition of Sky & Telescope - November 2021