Sky and Telescope - July 2018 - 19

air than the lighter one, creating
a predictable ratio of argon-36 to
Mars has lost
argon-38 in the upper atmosphere.
about 98%
This ratio will be different at the
of its original
surface, where things are well
atmosphere -
mixed. The argon ratio at the ground
it's both been
has been measured most accurately
lost to space
by the SAM instrument on the Mars
and locked up in
Curiosity rover. Combining Curiossurface minerals
ity and MAVEN measurements with
and ice.
the argon ratio seen on other solar
system objects tells us that Mars has lost to space about twothirds of the argon that had ever been in its atmosphere.
Argon also reveals how it was lost. Because argon is a
noble gas, it can't be removed by chemical processes such as
interacting with the surface and forming mineral deposits
there, as carbon dioxide forms carbonates. Only the physical
sputtering mechanism will work. If Mars lost a lot of argon
via sputtering, it probably lost similar amounts of carbon
dioxide that way, too. Other processes, such as photochemical loss, would have removed even more of the CO2, giving us
conﬁdence that our extrapolations from current oxygen and
hydrogen loss aren't totally off the mark.
Following the trail of evidence from MAVEN and what
we know about the Sun, the majority of this loss would
have occurred early in the planet's history, when the solar
ultraviolet light and the solar wind were most intense. In
the very earliest epochs, Mars's global magnetic ﬁeld likely
protected the atmosphere from stripping - the ﬁeld would
have kept the solar wind standing off at a greater distance,
as happens on Earth today. Based on magnetic records in the
planet's surface, we think the global magnetic ﬁeld turned off
roughly 4 billion years ago. That switch would have allowed
the solar wind to hit the upper atmosphere directly, triggering signiﬁcant atmospheric loss to space. The bulk of Mars's
atmosphere then would have been lost within a few hundred
million years. As the solar radiation and wind calmed down
later in history, loss would have slowed to a gradual leak, but
the damage would have been done. The loss we see occurring
today would be the tail end of that slow leak.
This timing of the Sun's stripping of the atmosphere will
sound familiar: It matches the timing of the climate change

IONS: N ASA'S SCIEN TIFIC VISUA LIZ ATION ST UDIO / M AV EN SCIENCE TE A M; AUROR AS: N ASA GSFC /
U NIV ERSIT Y OF COLOR A D O; TIMELINE: TERRI DUBÉ / S&T

ALL BUT GONE

p GLOBAL AURORA Unlike on Earth, Mars has planet-wide auroral
events, and the one in September 2017 was 25 times brighter than any
aurora previously seen on Mars. An image before the event (left) shows
just noise; the right-hand image shows the event at its peak.

inferred from the geologic features left by water. That's not by
our design; it's the result of totally different lines of evidence
converging on the same picture. Scientists suspected that the
bulk of the Martian atmosphere has been lost to space, but
MAVEN has conﬁrmed it. This means that escape to space
was the major process responsible for changing the Martian
climate from its early, potentially warm and wet environment
to the cold, dry planet that we see today.

Not Quite a Closed Case
Where do we go from here? Our results are based on one
Mars year so far. But not every year is the same on Mars. The
lower atmosphere's water cycle and the behavior of dust there
can vary signiﬁcantly from one year to another and can affect
the supply of gas to the upper atmosphere. MAVEN hasn't
observed the effects of a global dust storm yet. The planet's
dust storm season begins this summer, though, so perhaps
we'll get lucky.
In addition, the Sun goes through an 11-year cycle of
behavior, which changes the properties of the solar wind and
our star's ultraviolet output. MAVEN arrived just after solar
maximum in the current cycle and was able to see how a
moderately active Sun affects Mars's atmosphere, but it was
a very weak solar maximum, with only a few strong solar
storms hitting the planet. We aren't entirely sure yet what the

MARTIAN ATMOSPHERIC LOSS

-3.0

Present-day
loss rate

Oxygen lost after
4.2 billion years at
present rate

Actual oxygen loss
after 4.2 billion
years (lower limit)

1.6-11×1026
atoms/sec

2.5 meters

16 meters

Expressed as equivalent
global H2O layer

Expressed as global
H2O layer

0.078 bar

0.46 bar

Expressed as
equivalent CO2 loss (in
atmospheric pressure)

Expressed as
CO2 loss

-2.5

Amazonian

to
Today

Hydrogen

Episodic Volcanism
Glaciers
Polar Layered Terrains

~6×1025
atoms/sec
Oxygen

s k y a n d t e l e s c o p e . c o m * J U LY 2 0 1 8

http://www.skyandtelescope.com

Sky and Telescope - July 2018

Table of Contents for the Digital Edition of Sky and Telescope - July 2018