12 ASTRONOMY • SEPTEMBER 2019
QUANTUM GRAVITY
Geological evidence sug-
gests water once freely flowed
across Mars’ surface. Now, the Mars
Reconnaissance Orbiter’s Shallow Radar
(SHARAD) has found new clues about
the Red Planet’s past climate in layers
of water ice and sand buried beneath
the planet’s northern ice cap. The newly
discovered ice layers also make up the
planet’s third-largest reservoir of water,
after the ice caps themselves.
The find was published May 22 in
Geophysical Research Letters; a second
paper, published the same day in the
same journal, confirmed the reservoirindependently, using gravity data. The
researchers used SHARAD to peer
1.5 miles (2.4 kilometers) beneath Mars’
north polar cap and study a site called
the cavi unit. Radar data revealed alter-
nating layers of sand and ice in the cavi
unit, which the team believes preserves
a record of Mars’ past ice caps.
Astronomers know that Mars, like
Earth, wobbles on its axis over tens of
thousands of years. As its angle of tilt
changes, the climate changes as well,
growing warmer and cooler over time.
The researchers think the ice in the
cavi unit was laid down during colder,glacial periods in Mars’ past. Previously,
planetary scientists assumed that ice
would melt away during warmer periods.
But SHARAD revealed the ice was instead
covered by sand, which insulated it from
the Sun’s rays and prevented it from
melting or evaporating into space in Mars’
thin atmosphere.
The discovery could shed light on
Mars’ past habitability. By understanding
how much water was globally available
on Mars in the past, the researchers say,
they can develop a better idea of whether
there was enough liquid water at the
equator to support life. — K.H., A.K.Polar ice offers a peek at Mars’ past
APOLLO-ERA QUAKES HINT
THE MOON IS STILL ACTIVE
A new analysis of quakes on the Moon
between 1969 and 1977 reveals that our
satellite is likely still tectonically active.
The results, published May 13 in
Nature Geoscience, are based on data
from detectors laid down by Apollo
astronauts half a century ago. Now,
a team of scientists led by Thomas
Watters of the Smithsonian Institution
in Washington, D.C., has tracked the
epicenter of 28 small moonquakes. They
found that eight occurred within about
20 miles (30 kilometers) of fault scarps,
which are cliffs that form when the
Moon’s surface shears away from itself
as the crust slowly contracts. Based on
these findings, it appears that lunarfaults, like Earth’s fault lines, cause
quakes when pieces of the surface rub
against each other.
Additionally, six of the eight quakes
occurred during times of the month
when the tidal stresses between the
Moon and Earth were at their greatest.
Such stresses would make faults more
likely to slip and thus cause a quake.
“It’s quite likely that the faults are
still active today,” said study co-author
Nicholas Schmerr of the University
of Maryland in a press release. “You
don’t often get to see active tectonics
anywhere but Earth, so it’s very excit-
ing to think these faults may still be
producing moonquakes.” — K.H., A.K.LUNAR LINES. Fault scarps, like the one seen
here, are visible across the Moon’s surface.
New research suggests these faults may still be
tectonically active. NASA/GSFC/ARIZONA STATE UNIVERSITYLAYER CAKE.
Exposed
alternating layers
of ice (white)
and sand (blue) at
Mars’ north pole
reveal the planet’s
climate history
in this false-color
composite image.
NASA/JPL/UNIVERSITY
OF ARIZONA