Mars Mission Sheds Light on Habitability of Distant Planets

How long might a rocky, Mars-like planet be habitable if it
were
orbiting a red dwarf star? It’s a complex question but
one that NASA’s Mars
Atmosphere and Volatile Evolution mission
can help answer.

“The MAVEN mission tells us that Mars lost substantial
amounts
of its atmosphere over time, changing the planet’s
habitability,”
said David Brain, a MAVEN co-investigator and a
professor at the Laboratory for
Atmospheric and Space Physics
at the University of Colorado Boulder. “We
can use Mars, a
planet that we know a lot about, as a laboratory for studying
rocky planets outside our solar system, which we don’t know
much about yet.”

At the fall
meeting of the American Geophysical Union on Dec.
13, 2017, in New Orleans,
Louisiana, Brain described how
insights from the MAVEN mission could
be applied to the
habitability of rocky planets orbiting other stars.

MAVEN carries a suite of instruments that have been measuring
Mars’
atmospheric loss since November 2014. The studies
indicate that Mars has lost
the majority of its atmosphere to
space over time through a combination of
chemical and physical
processes. The spacecraft’s instruments were chosen to
determine how much each process contributes to the total
escape.

In the past three years, the Sun has gone through periods of
higher
and lower solar activity, and Mars also has experienced
solar storms, solar
flares and coronal mass ejections. These
varying conditions have given MAVEN
the opportunity to observe
Mars’ atmospheric escape getting cranked up and
dialed down.

Brain and his colleagues started to think about applying these
insights to a hypothetical Mars-like planet in orbit around
some type of
M-star, or red dwarf, the most common class of
stars in our galaxy.

The researchers did some preliminary calculations based on the
MAVEN data. As with Mars, they assumed that this planet might
be positioned at
the edge of the habitable zone of its star.
But because a red dwarf is dimmer
overall than our Sun, a
planet in the habitable zone would have to orbit much
closer
to its star than Mercury is to the Sun.

The brightness of a red dwarf at extreme ultraviolet (UV)
wavelengths combined with the close orbit would mean that the
hypothetical
planet would get hit with about 5 to 10 times
more UV radiation than the real
Mars does. That cranks up the
amount of energy available to fuel the processes
responsible
for atmospheric escape. Based on what MAVEN has learned, Brain
and
colleagues estimated how the individual escape processes
would respond to
having the UV cranked up.

Their calculations indicate that the planet’s atmosphere could
lose 3 to 5 times as many charged particles, a process called
ion escape. About
5 to 10 times more neutral particles could
be lost through a process called
photochemical escape, which
happens when UV radiation breaks apart molecules in
the upper
atmosphere.

Because more charged particles would be created, there also
would
be more sputtering, another form of atmospheric loss.
Sputtering happens when
energetic particles are accelerated
into the atmosphere and knock molecules
around, kicking some
of them out into space and sending others crashing into
their
neighbors, the way a cue ball does in a game of pool.

Finally, the hypothetical planet might experience about the
same
amount of thermal escape, also called Jeans escape.
Thermal escape occurs only
for lighter molecules, such as
hydrogen. Mars loses its hydrogen by thermal
escape at the top
of the atmosphere. On the exo-Mars, thermal escape would
increase only if the increase in UV radiation were to push more
hydrogen to the
top of the atmosphere.

Altogether, the estimates suggest that orbiting at the edge of
the
habitable zone of a quiet M-class star, instead of our
Sun, could shorten the
habitable period for the planet by a
factor of about 5 to 20. For an M-star
whose activity is amped
up like that of a Tasmanian devil, the habitable period
could
be cut by a factor of about 1,000 — reducing it to a mere
blink of an
eye in geological terms. The solar storms alone
could zap the planet with
radiation bursts thousands of times
more intense than the normal activity from
our Sun.

However, Brain and his colleagues have considered a
particularly
challenging situation for habitability by placing
Mars around an M-class star.
A different planet might have
some mitigating factors — for example, active
geological
processes that replenish the atmosphere to a degree, a
magnetic
field to shield the atmosphere from stripping by the
stellar wind, or a larger
size that gives more gravity to hold
on to the atmosphere.

“Habitability is one of the biggest topics in astronomy, and
these estimates demonstrate one way to leverage what we know
about Mars and the
Sun to help determine the factors that
control whether planets in other systems
might be suitable for
life,” said Bruce Jakosky, MAVEN’s principal
investigator at
the University of Colorado Boulder.

MAVEN’s principal investigator is based at the University of
Colorado’s Laboratory for Atmospheric and Space Physics,
Boulder. The
university provided two science instruments and
leads science operations, as
well as education and public
outreach, for the mission. NASA’s Goddard Space
Flight Center
in Greenbelt, Maryland, manages the MAVEN project and provided
two science instruments for the mission. NASA’s Jet Propulsion
Laboratory, a
division of Caltech in Pasadena, California,
manages the Mars Exploration
Program for NASA’s Science
Mission Directorate, Washington.

For
more information about MAVEN, visit:

https://www.nasa.gov/maven

News Media Contact

Laurie Cantillo / Dwayne Brown
NASA Headquarters, Washington
202-358-1077 / 202-358-1726
laura.l.cantillo@nasa.gov / dwayne.c.brown@nasa.gov

Written by Elizabeth Zubritsky
NASA’s Goddard Space Flight Center, Greenbelt, Md.

2017-319