Sky and Telescope - July 2018 - 18
Red Planet Resolution
ION LOSS Mars loses many
of its oxygen ions through a
polar plume, shown in this
illustration of loss averaged
over a Martian year. Red
indicates the most loss, blue
the least. The Sun is at left.
The major way Mars loses oxygen today is by what's called
photochemical loss. In this process, an ionized oxygen molecule
(O2+) collides with an electron and recombines to form a
neutral molecule. This recombination releases enough energy
to break the molecule into two separate oxygen atoms and
give them enough energy that, if one is moving upwards, it
will escape into space (if it doesn't hit anything else ﬁrst).
Adding these losses up, we've discovered that Mars is losing gas to space at a rate of about 2 to 3 kg/second. That's 10
million billionths (10 −16) of the total atmosphere lost each
second. That may not sound like much, but it's enough of a
trickle that, over the more than 4 billion years of Martian
history, it could have removed enough oxygen and hydrogen
to create a global layer of water a few meters thick. Equivalently, the oxygen that has been lost would have made enough
CO2 to produce an atmosphere ten times thicker than the
present one of 6 millibars atmospheric pressure.
u HISTORY OF MARS Planetary scientists
divide the Red Planet's history into three
periods, the Noachian, Hesperian, and
Amazonian (sometimes with the prequel
Pre-Noachian). Mars likely lost its global
magnetic field around 4.1 billion years ago.
Assuming that's when the solar wind began
stripping off the atmosphere, then Mars lost
the bulk of its atmosphere by 3.5 billion years
ago. This period overlaps with the planet's
transformation from widespread hydrous and
volcanic activity to a global freeze-out.
J U LY 2 0 1 8 * S K Y & T E L E S C O P E
We don't think that the loss rate has been constant in
time, however. The Sun's ultraviolet radiation and its wind of
energetic particles were both more intense early in our solar
system's history and would have driven greater gas loss. Now
that we know the speciﬁc processes involved, though, we can
calculate how much loss would have occurred earlier in Martian history if these same processes were at work.
Using these extrapolations, it appears that more than a
half bar of CO2 could have been lost to space. This is roughly
100 times the amount of CO2 that is in today's atmosphere,
and it is enough to have produced signiﬁcant greenhouse
warming. Given that the young Sun might have unleashed
solar storms more often than it does today, even more CO2
could have been stripped away.
This result might sound a bit hand-wavy, but we can
determine the total loss more directly, too, thanks to argon.
Argon's heavier isotope naturally settles lower in the Martian
Global magnetic ﬁeld disappears (approximate)
Atmosphere loss (bulk)
-4.5 billion years
Tharsis Volcanic Plateau