When it comes to understanding how
terrestrial planets evolve around other
stars, Mars may provide a good stand-in.
Early Mars once boasted an atmosphere
and even oceans, but as the active sun
evolved, it began to strip the Red Planet of
its air, a process that has also been
spotted in action outside the solar
system. NASA’s MAVEN mission,
currently orbiting Mars, may help
scientists to understand the atmosphere
of other rocky worlds undergoing that
process.
“We’re at the brink of starting to
understand the habitability of terrestrial
planets,” said Shannon Curry, a research
scientist at the University of California,
Berkeley. Part of the MAVEN team , Curry
spoke at the 228th American
Astronomical Society meeting in San
Diego in June about the spacecraft’s
observations of the Martian atmosphere.
“Mars right now is our ground truth for
planets of this size,” Curry said. [ Mars
Atmosphere Being Stripped By Solar Wind
(Video)
]
‘Stupid solar cycle’
Early Mars was warm and wet, with
flowing rivers and enormous oceans. But
the planet is smaller than Earth, and too
small to hold onto its protective
atmosphere. Over time, the low gravity,
combined with radiation from the sun,
stripped away the air on Mars , removing
liquid water from the surface and leaving
behind the dead red world seen today.
MAVEN can help peer into the planet’s
past by examining how much atmosphere
it’s losing today. The satellite launched in
2013 and entered orbit around Mars
within a year, where it began measuring
what’s happening in the planet’s air. The
probe observed how solar flares,
radiation and large bursts known as
coronal mass ejections (CMEs) affect the
planet’s atmosphere.
“The solar environment we’re talking
about is specific to our sun today, but all
of these are physical processes that occur
on planetary bodies throughout the
universe,” Curry said.
By understanding how the sun’s activity
today affects the Martian atmosphere,
scientists can better estimate how well
exoplanets can hold onto their own.
Of course, such observations require a
little help from the sun — help which has
been rather lacking recently. According to
Curry, the current solar cycle is the lowest
it has been since the ice age, not
producing many CMEs or solar storms.
Flares especially have been few and far
between. While discussing flare-related
loss, Curry said, “We’ve been getting
nothing because of the stupid solar cycle.
“We’re really wishing for big storms at
Mars and haven’t really gotten any of
them,” she added.
Blowing out the atmosphere
The low solar activity doesn’t mean
MAVEN hasn’t gathered any information.
The spacecraft has still been able to
observe the shape of the atmosphere as it
stretches behind the planet. The tear-
drop-shaped atmosphere is similar to
how a comet sheds material, but not
identical: In addition to a trailing tail that
stretches more than twice the width of the
planet, Mars also spouts plumes from its
poles.
“It’s not simply a stream behind the
planet,” Curry said. “It’s actually crafting
these different plume features.”
By probing the atmosphere in far- and
mid-ultraviolet, MAVEN created maps of
escaping carbon, hydrogen and oxygen,
all of which flee the atmosphere at
different rates. Curry said the water in the
upper atmosphere is contributing to the
escaping hydrogen.
Mars isn’t the only planet known to have
an escaping atmosphere. For another
example, Curry pointed to GJ 436b , a
Neptune-size world with a comet-like trail
of escaping hydrogen behind it. The
planet isn’t close enough to its star to
completely lose its atmosphere, but with a
tail stretching 9 million miles (14 million
kilometers) behind it, the exoplanet could
provide a clue into how rocky super-
Earths form. Understanding how the sun
helps strip the Martian atmosphere could
provide further insight into the process.
MAVEN’s measurements have allowed
scientists to determine that, if the sun
were as quiet through its history as it is
now, the Red Planet would have lost
about 63,000 gigagtons of atmosphere, a
number that corresponds to a surface
layer of water 15 feet (4.5 meters) deep.
The quiet solar cycle leads to a
deceptively small atmosphere, but it
provides a floor for estimations. [How
NASA’s MAVEN Mars Orbiter Works
(Infographic)
]
“MAVEN’s observations during a quiet
solar cycle are incredibly helpful as they
place a lower limit on the atmospheric
escape,” Collins told Space.com. “Our
measurements now serve as baseline
numbers, in effect.”
The sun hasn’t been still through
MAVEN’s whole mission. In March 2015,
an enormous CME caught the edge of the
Red Planet, resulting in what Curry called
“an explosion of plasma.” MAVEN
examined the real-time escape rates of
the planet’s atmosphere, including an
uptick in the process known as
sputtering.
Sputtering occurs when charged particles
ejected from the sun fall into the
atmosphere and collide with neutral
particles (that have no charge). The
collisions knock some of the neutral
particles out of the planet’s grasp,
sending them hurtling through space.
Curry said that although sputtering is the
smallest escape process under current
conditions, it is thought have had the
strongest effect on destroying the Martian
atmosphere when the sun was younger
and more active.
When the sun is quiet, particles from the
solar wind knock out a few atmospheric
particles at a gradual pace. But
explosions such as the March CME
dramatically increase how much of the
atmosphere blows off, as much as 10 to
20 times more than during quiet periods.
Curry compared the difference to a child
entering a pool or a ball pit.
“If you carefully step in, not much is
going to come out,” she said. “But if you
get a running start from 100 feet away,
all of the balls or water comes out of the
pool.”
Earlier in its lifetime, the sun was more
active than it is today, so sputtering was
probably a more important process for
removing the atmosphere, Curry said.
Understanding that process will help
scientists figure out what they see on
other worlds.
“Using Mars as a ground truth for
understanding atmospheric escape from
exoplanets will be really important,
especially when we are able to observe
the makeup and density of exoplanets and
their atmospheres, or even
magnetospheres someday,” Curry said.
While Venus has a sweltering atmosphere
and Earth a comfortable one, Mars’ thin
atmosphere makes it rather unique, as
does its smaller mass. The Red Planet
also released less gas from its surface in
its early history, which Curry said would
result in a very different atmospheric
evolution. Combining observations from
all those planets can help to understand
the variety of worlds that exist beyond the
solar system.
“We can take our specific wealth of
knowledge about Mars’ size, density,
atmospheric composition and history,
which we know about through imaging
and rovers, to put together a timeline of
when there was water and a thick
atmosphere, and how those inventories
were depleted,” Curry said.


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