Sky and Telescope - April 2017 - 18

Mars Rover Update

ing scientists, because there's been less time for solar and
cosmic radiation to break down any interesting molecules
contained within the upper few centimeters of rock accessible
to Curiosity's drill.
SAM also hunted for organics in the doggie-bagged Cumberland samples, and an analysis of one of them yielded the
detection of chlorobenzene - a ring of six carbon atoms
bonded to ﬁve hydrogens and one chlorine. Chlorobenzene
likely formed when perchlorate ions present in the rock
reacted with some hydrocarbon molecules. Perchlorate is a
powerful oxidizer that was detected in modern Martian soil
by the Phoenix lander. In water, perchlorate will attack complex organic molecules, breaking them into smaller pieces.
Thus, the mission had succeeded in detecting organic
material locked within the rocks when they ﬁrst formed, billions of years ago. If life had existed then, evidence of it could
be preserved in Martian rocks - a promising ﬁnd for future
paleobiology missions.
p ERODED ROCK Wind has worn down Gale's terrain, as seen in these
rocks from Naukluft Plateau (left top and bottom) and Upheaval Dome
(right, near Pahrump Hills).

0.8 ppb in southern summer. After exploring many possible
correlations, the SAM team now thinks that the seasonal
variation in this tiny amount of methane isn't from microbes;
instead, it's tied to seasonally varying levels of solar ultraviolet radiation breaking up organic material on the ground.
Curiosity measures the intensity of sunlight with a deckmounted UV sensor that's part of its REMS weather suite. A
small amount of carbon-rich material rains year-round onto
the ruddy terrain as micrometeorites. When Mars is closest
to the Sun (a period coinciding with southern summer), the
stronger-than-average sunlight breaks down organics into
much smaller molecules, including methane, causing the
seasonal methane cycle.

Wheel Problems
The drive toward Mount Sharp took a toll on the rover: Mission engineers discovered serious damage to the wheels. The
wind had eroded the hard rock of the crater ﬂoor into spiny
prominences that stuck up like sharks' teeth; embedded in
the ground, they neither shifted out of the way nor collapsed
under the rover's weight. Instead, they speared the wheels like
so many can openers.
The wheel damage put the brakes on the mission's rapid
driving progress.
While the engineers drove more carefully and studied the
problem, they steered the rover into sandy valleys among
ridges of layered rock, heading for an interesting, striatedlooking outcrop they named the Kimberley formation. Sticking
to valleys rather than the plateau tops meant a twistier path

As the rover drove, SAM's science team continued to perform
different kinds of lab analyses on Cumberland material that
had been "doggie bagged" inside some of the instrument's
sample cups. By studying the amount of argon produced by
potassium-40's radioactive decay, the researchers determined
an age for the rocks of 4.2 billion years (give or take 350 million). Since these were sediments, this age represents that of
the original rocks, once part of the highlands and crater rim,
that were ground down and later deposited on Gale's ﬂoor.
At the same time, SAM measured another isotope of
argon as well as those of helium and neon, produced when
cosmic rays bombarded near-surface atoms in the rock.
The amounts of these noble-gas isotopes showed that the
rocks had only been exposed at the surface for 80 ± 30 million years. Together, the results mean that the rocks within
Gale are old but - at least at Cumberland - have only been
exposed recently, presumably by the activity of sandblasting
winds that have eroded Gale's rocks into sometimes fantastic
shapes. Such short exposure ages are good for organics-hunt18

A P R I L 2 0 1 7 * SK Y & TELESCOPE

p HOLEY ROLLER Despite their careful design, Curiosity's wheels
became increasingly punctured by pointy, immobile rocks the rover drove
over. (The regular-shaped rectangular holes, on the other hand, are intentional: Scientists use the marks they leave in the rover's tracks to verify
the distance the rover has traveled.)

A LL IM AG ES: N ASA / JPL- CA LTECH / MSSS

Isotopic Ages

Sky and Telescope - April 2017

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