Sky and Telescope - March 2016 - 10

News Notes

MARS I Red Planet Losing Gas to Solar Wind's Onslaught
Plume

Solar wind

NASA / GREG SHIRAH

Windsock

Bow shock

Measurements by NASA's MAVEN orbiter have detected atmospheric ions leaving Mars at a high
rate, driven off by the planet's interaction with the solar wind. Colors indicate the ions' speeds,
with red the fastest. Although the atoms leaving through the polar plume move faster, most of the
atmosphere (about 75%) escapes through the windsock-shaped tail.

NASA's MAVEN orbiter has painted a
detailed picture of how the solar wind
robs Mars of its atmosphere.
Today's Mars is surrounded by only
a whiff of gas, but scientists think its
ancient atmosphere was thicker. A
primary theory for where that gas went
is that the solar wind blew it away. The
solar wind is a river of charged particles
that flows out from the Sun, carrying
with it the solar magnetic field. All the
planets plow through this flow as they
orbit our star. Because Mars lost its global
magnetic field more than 4 billion years
ago, the solar wind smacks directly into
its upper atmosphere at several hundred
kilometers per second.
NASA sent the MAVEN spacecraft to
the Red Planet to monitor this interaction

(S&T: Sept. 2014, p. 20). After analyzing
the first seven months of mission data,
principal investigator Bruce Jakosky (University of Colorado, Boulder) and his team
have built a cohesive picture of where gas
in the upper atmosphere is going. They
found that it leaves the planet three ways:
in a polar plume, as a downwind tail, and
from an extended cloud of gas surrounding the entire planet.
The Mohawk-esque plume is a stream
of ions driven from the planet's dayside.
These ions are caught up in an electric
field that's created by the solar magnetic field as it blows into Mars's upper
atmosphere. Because linked magnetic
and electric fields are perpendicular to
each other, the plume is directed upward
from the pole, instead of following the

solar wind's flow around the planet (see
the simulation at left). About 25% of the
atmosphere lost leaves via this route.
The second, and predominant, exit
ramp is the windsock-shaped tail streaming behind Mars's nightside. The tail
forms as the solar wind hits the upper
atmosphere and is deflected around the
planet, just like air bends into a bow
shock around a supersonic jet. About 75%
of the atmosphere lost leaves this way.
A smattering of gas also escapes from
the top of the atmosphere as neutral
atoms, creating a tenuous corona far from
the planet's dayside. Once these atoms
become ionized, the solar wind drives
them back toward Mars and into the
interplanetary depths beyond.
Adding up all these losses, the scientists find that, globally, Mars is losing at
least a few trillion trillion (1024) atoms
per second. That translates into about
100 grams, or a quarter pound. "I can't
help but imagine hamburgers flying out
of Mars, once per second," jokes mission
co-investigator David Brain (University of
Colorado, Boulder).
But Mars can lose its gas a lot faster.
When a coronal mass ejection slammed
into the planet in March 2015, for
example, MAVEN found that escape
rates spiked by a factor of 10 to 20. That's
important, because scientists think that
the young Sun was far more active than
the middle-aged star we live with now. A
lot of things can influence how well the
solar wind strips the planet's atmosphere
away, however, and the team hasn't yet
figured out what escape rates would have
been in the distant past. Mars potentially
lost as much atmosphere as Earth has
now, so the rate might have been 100 to
1,000 times greater in that early period,
Jakosky speculates. The team details
these and other results in several papers
in the November 6th Science and a special
issue of Geophysical Research Letters.
■ CAMILLE M. CARLISLE

Read more about MAVEN's results and watch a video of the solar wind stripping Mars's atmosphere away at http://is.gd/mavenions.
R

10

March 2016 sky & telescope


http://www.is.gd/mavenions

Sky and Telescope - March 2016

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Sky and Telescope - March 2016 - A
Sky and Telescope - March 2016 - B
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