Why do some science instruments detect the gas on the Red Planet while
others don't?
Reports of methane detections at Mars have captivated scientists and
non-scientists alike. On Earth, a significant amount of methane is produced
by microbes that help most livestock digest plants. This digestion process
ends with livestock exhaling or burping the gas into the air.
While there are no cattle, sheep, or goats on Mars, finding methane there is
exciting because it may imply that microbes were, or are, living on the Red
Planet. Methane could have nothing to do with microbes or any other biology,
however; geological processes that involve the interaction of rocks, water,
and heat can also produce it.
Before identifying the sources of methane on Mars, scientists must settle a
question that's been gnawing at them: Why do some instruments detect the gas
while others don't? NASA's Curiosity rover, for instance, has repeatedly
detected methane right above the surface of Gale Crater. But ESA's (the
European Space Agency) ExoMars Trace Gas Orbiter hasn't detected any methane
higher in the Martian atmosphere.
"When the Trace Gas Orbiter came on board in 2016, I was fully expecting the
orbiter team to report that there's a small amount of methane everywhere on
Mars," said Chris Webster, lead of the Tunable Laser Spectrometer (TLS)
instrument in the Sample Analysis at Mars (SAM) chemistry lab aboard the
Curiosity rover.
The TLS has measured less than one-half part per billion in volume of
methane on average in Gale Crater. That's equivalent to about a pinch of
salt diluted in an Olympic-size swimming pool. These measurements have been
punctuated by baffling spikes of up to 20 parts per billion in volume.
"But when the European team announced that it saw no methane, I was
definitely shocked," said Webster, who's based at NASA's Jet Propulsion
Laboratory in Southern California.
The European orbiter was designed to be the gold standard for measuring
methane and other gases over the whole planet. At the same time, Curiosity's
TLS is so precise, it will be used for early warning fire detection on the
International Space Station and for tracking oxygen levels in astronaut
suits. It's also been licensed for use at power plants, on oil pipelines,
and in fighter aircraft, where pilots can monitor the oxygen and carbon
dioxide levels in their face masks.
Still, Webster and the SAM team were jolted by the European orbiter findings
and immediately set out to scrutinize the TLS measurements on Mars.
Some experts suggested that the rover itself was releasing the gas. "So we
looked at correlations with the pointing of the rover, the ground, the
crushing of rocks, the wheel degradation—you name it," Webster said. "I
cannot overstate the effort the team has put into looking at every little
detail to make sure those measurements are correct, and they are."
Webster and his team reported their results today in the Astronomy &
Astrophysics journal.
As the SAM team worked to confirm its methane detections, another member of
Curiosity's science team, planetary scientist John E. Moores from York
University in Toronto, published an intriguing prediction in 2019. "I took
what some of my colleagues are calling a very Canadian view of this, in the
sense that I asked the question: 'What if Curiosity and the Trace Gas
Orbiter are both right?'" Moores said.
Moores, as well as other Curiosity team members studying wind patterns in
Gale Crater, hypothesized that the discrepancy between methane measurements
comes down to the time of day they're taken. Because it needs a lot of
power, TLS operates mostly at night when no other Curiosity instruments are
working. The Martian atmosphere is calm at night, Moores noted, so the
methane seeping from the ground builds up near the surface where Curiosity
can detect it.
The Trace Gas Orbiter, on the other hand, requires sunlight to pinpoint
methane about 3 miles, or 5 kilometers, above the surface. "Any atmosphere
near a planet's surface goes through a cycle during the day," Moores said.
Heat from the Sun churns the atmosphere as warm air rises and cool air
sinks. Thus, the methane that is confined near the surface at night is mixed
into the broader atmosphere during the day, which dilutes it to undetectable
levels. "So I realized no instrument, especially an orbiting one, would see
anything," Moores said.
Immediately, the Curiosity team decided to test Moores' prediction by
collecting the first high-precision daytime measurements. TLS measured
methane consecutively over the course of one Martian day, bracketing one
nighttime measurement with two daytime ones. With each experiment, SAM
sucked in Martian air for two hours, continuously removing the carbon
dioxide, which makes up 95% of the planet's atmosphere. This left a
concentrated sample of methane that TLS could easily measure by passing an
infrared laser beam through it many times, one that's tuned to use a precise
wavelength of light that is absorbed by methane.
"John predicted that methane should effectively go down to zero during the
day, and our two daytime measurements confirmed that," said Paul Mahaffy,
the principal investigator of SAM, who's based at NASA's Goddard Space
Flight Center in Greenbelt, Maryland. TLS' nighttime measurement fit neatly
within the average the team had already established. "So that's one way of
putting to bed this big discrepancy," Mahaffy said.
While this study suggests that methane concentrations rise and fall
throughout the day at the surface of Gale Crater, scientists have yet to
solve the global methane puzzle at Mars. Methane is a stable molecule that
is expected to last on Mars for about 300 years before getting torn apart by
solar radiation. If methane is constantly seeping from all similar craters,
which scientists suspect is likely given that Gale doesn't seem to be
geologically unique, enough of it should have accumulated in the atmosphere
for the Trace Gas Orbiter to detect. Scientists suspect that something is
destroying methane in less than 300 years.
Experiments are underway to test whether very low-level electric discharges
induced by dust in the Martian atmosphere could destroy methane, or whether
abundant oxygen at the Martian surface quickly destroys methane before it
can reach the upper atmosphere.
"We need to determine whether there's a faster destruction mechanism than
normal to fully reconcile the data sets from the rover and the orbiter,"
Webster said.
Reference:
Christopher R. Webster et al, Day-night differences in Mars methane suggest
nighttime containment at Gale crater, Astronomy & Astrophysics (2021).
DOI:
10.1051/0004-6361/202040030
Tags:
Space & Astrophysics