Solar Wind Rips Up Martian Atmosphere
- From: baalke@xxxxxxxxxxxxx
- Date: Mon, 24 Nov 2008 13:52:57 -0800 (PST)
http://science.nasa.gov/headlines/y2008/21nov_plasmoids.htm
Solar Wind Rips Up Martian Atmosphere
NASA Science News
11.21.2008
Nov. 21, 2008: Researchers have found new evidence that the atmosphere
of Mars is being stripped away by solar wind. It's not a gently
continuous erosion, but rather a ripping process in which chunks of
Martian air detach themselves from the planet and tumble into deep
space. This surprising mechanism could help solve a longstanding
mystery
about the Red Planet.
"It helps explain why Mars has so little air," says David Brain of UC
Berkeley, who presented the findings at the 2008 Huntsville Plasma
Workshop on October 27th.
Billions of years ago, Mars
had a lot more air than it does today. (Note: Martian "air" is
primarily
carbon dioxide, not the nitrogen-oxygen mix we breathe on Earth.)
Ancient martian lake-beds and river channels tell the tale of a planet
covered by abundant water and wrapped in an atmosphere thick enough to
prevent that water from evaporating into space. Some researchers
believe
the atmosphere of Mars was once as thick as Earth's. Today, however,
all
those lakes and rivers are dry and the atmospheric pressure on Mars is
only 1% that of Earth at sea-level. A cup of water placed almost
anywhere on the Martian surface would quickly and violently boil away
-
result of the super-low air pressure.
So where did the air go? Researchers entertain several possibilities:
An
asteroid hitting Mars long ago might have blown away a portion of the
planet's atmosphere in a single violent upheaval. Or the loss might
have
been slow and gradual, the result of billions of years of relentless
"sand-blasting" by solar wind particles. Or both mechanisms could be
at
work.
Brain has uncovered a new possibility--a daily ripping process
intermediate between the great cataclysm and slow erosion models. The
evidence comes from NASA's now-retired Mars Global Surveyor (MGS)
spacecraft.
In 1998, MGS discovered that Mars has a very strange magnetic field.
Instead of a global bubble, like Earth's, the Martian field is in the
form of magnetic umbrellas that sprout out of the ground and reach
beyond the top of Mars' atmosphere. These umbrellas number in the
dozens
and they cover about 40% of the planetâs surface, mainly in the
southern
hemisphere.
For years, researchers thought the umbrellas protected the Martian
atmosphere, shielding pockets of air beneath them from erosion by the
solar wind. Surprisingly, Brain finds that the opposite can be true as
well: "The umbrellas are where coherent chunks of air are torn away."
Addressing his colleagues at the Workshop, he described how he made
the
discovery just a few months ago:
Brain was scrolling through archival data from Global Surveyor's
particles and fields sensors. "We have measurements from 25,000
orbits,"
he says. During one of those orbits, MGS passed through the top of a
magnetic umbrella. Brain noticed that the umbrella's magnetic field
had
linked up with the magnetic field in the solar wind. Physicists call
this "magnetic reconnection." What happened next is not 100% certain,
but Global Surveyor's readings are consistent with the following
scenario: "The joined fields wrapped themselves around a packet of gas
at the top of the Martian atmosphere, forming a magnetic capsule a
thousand kilometers wide with ionized air trapped inside," says Brain.
"Solar wind pressure caused the capsule to 'pinch off' and it blew
away,
taking its cargo of air with it." Brain has since found a dozen more
examples. The magnetic capsules or "plasmoids" tend to blow over the
south pole of Mars, mainly because most of the umbrellas are located
in
Mars' southern hemisphere.
Brain isn't ready to declare the mystery solved. "We're still not sure
how often the plasmoids form or how much gas each one contains." The
problem is, Mars Global Surveyor wasn't designed to study the
phenomenon. The spacecraft was only equipped to sense electrons, not
the
heavier ions which would make up the bulk of any trapped gas. "Ions
and
electrons don't always behave the same way," he cautions. Also, MGS
sampled the umbrellas at fixed altitudes and at the same local time
each
day. "We need to sample many altitudes and times of day to truly
understand these dynamic events."
In short, he told the audience, "we need more data."
Brain is pinning his hopes on a new NASA mission named MAVEN. Short
for
"Mars Atmosphere and Volatile Evolution," MAVEN is an upper atmosphere
orbiter currently approved for launch to Mars in 2013. The probe is
specifically designed to study atmospheric erosion. MAVEN will be able
to detect electrons, ions and neutral atoms; it will be able to
measure
both magnetic and electric fields; it will travel around Mars in an
elliptical orbit, piercing magnetic umbrellas at different altitudes,
angles, and times of day; and it will explore regions both near and
far
from the umbrellas, giving researchers the complete picture they need.
If magnetized chunks of air are truly being torn free, MAVEN will see
it
happening and measure the atmospheric loss rate. "Personally, I think
this mechanism is important," says Brain, "but MAVEN may yet prove me
wrong."
Meanwhile, the Mystery of the Missing Martian Air is shaping up to be
a
ripping good yarn.
.
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