This
illustration depicts charged particles from a solar storm stripping away
charged particles of Mars' atmosphere, one of the processes of Martian
atmosphere loss studied by NASA's MAVEN mission, beginning in 2014.
Unlike Earth, Mars lacks a global magnetic field that could deflect
charged particles emanating from the Sun. Image credit: NASA/GSFC. › Full image and caption
To receive the same amount of starlight as Mars receives from our Sun, a
planet orbiting an M-type red dwarf would have to be positioned much
closer to its star than Mercury is to the Sun. Image credit: NASA/GSFC. › Full image and caption
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.
