Sky and Telescope - July 2018 - 17

wind all link together, sometimes strongly, and we have to
understand the interactions between each of the components
in order to understand the system as a whole.

First, we needed to measure the solar properties that drive
gas's escape from the Martian atmosphere and the speciﬁc
ways the upper atmosphere's composition and structure
respond. With its nine science instruments, MAVEN measures the amount of solar ultraviolet light hitting the planet;
the solar wind speed, density, and magnetic ﬁeld; and the
solar energetic particles that are emitted from the Sun by
solar storms. On the receiving end of the physical system,
MAVEN also measures the basic state of the upper atmosphere's temperature, neutral-gas composition, and ion composition, as well as the electron properties in the ionosphere.
Recently, we've been able to add measurements of the neutral
and ionic winds in the upper atmosphere, too.
We're also determining how much gas Mars is losing
today, following the clues in the upper atmosphere most likely
to be important:
u The ions in the atmosphere that are being picked up and
stripped away by the solar wind;
u The ions that are swept up and then ﬂung back into the
atmosphere, knocking other atoms into space in a process
called sputtering;
u The properties of the ionosphere that tell us how much gas
is being removed by photochemical processes; and
u The hydrogen distribution in an extended "corona" surrounding Mars that tells us how much hydrogen is escaping to space.
Together, these measurements enable us to follow the chain
of evidence to determine the importance of each of the likely
loss processes.
MAVEN entered orbit in September 2014. With it, we've
collected measurements for longer than a full Martian year,
and we've seen Mars at all seasons. During this time, we've
seen several tens of solar storms hit Mars, including a couple
big ones, and we've also seen the intensity of the solar ultraviolet light change signiﬁcantly as the Sun has gone through
part of its 11-year cycle. We've made observations at essen-

1
0
-1
-2

10
5
0
-5

-10

-2 -1
0
1
2
Distance from Mars (RMars)

-5
0
5
10
Distance from Mars (RMars)

3×1013
Hydrogen atoms in corona
(per cm2 surface)

Distance from Mars (RMars)

tially all solar zenith angles
(the angle between the
Sun and the spacecraft as
measured from the center of
Mars), at all local solar times,
and at most latitudes. We've
observed at a wide range
of locations on the planet,
including over the regions
of strong crustal magnetic
ﬁelds and over regions with no
magnetic ﬁeld, and including all
geological provinces.
The major atoms that we've
observed being lost to space
today are hydrogen and oxygen.
(Other elements are being lost as
well, but they're harder to observe
than hydrogen and oxygen are.)
These come from H2O and CO2, broken apart by the Sun's
ultraviolet light. Hydrogen is leaving by thermal escape, which
means that the gas is hot enough that some of the hydrogen
atoms naturally move fast enough to escape Martian gravity.
The hydrogen isn't being lost at a constant rate, however -
we see a factor-of-ten variation in the escape rate throughout
the Mars year, with the greatest loss rates occurring during
the seasons when the atmosphere is the dustiest. We think
that the dust increases the atmospheric temperature, allowing the water molecules that are the source of hydrogen to
rise to higher altitudes, where they can be broken apart more
easily and where escape is possible. Recent work with data
from the Mars Reconnaissance Orbiter supports this picture.

Atomic Hydrogen

Atomic Oxygen
Distance from Mars (RMars)

L AY ERED ROCKS: N ASA / JPL / COR NELL; CONCEN TRIC RINGS: N ASA / JPL / UNIV. OF A RIZON A; H A LOS: M AV EN
TE A M / UNIV ERSIT Y OF COLOR A DO / N ASA; SE ASONS G R A PH: G REGG DINDER M A N / S&T, SOURCE: AU THOR

Missing Suspects

u CONCENTRIC RINGS
Layered sediments fill this
2-km-wide crater, which lies
inside the larger, equatorial
crater Schiaparelli. It's unclear
if the layers formed from dust
or volcanic ash settling from the
atmosphere, or as water-borne
deposits built up over time.

2×1013

1×1013

Spring

Summer
Autumn
Winter
Mars season (northern hemisphere)

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

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Sky and Telescope - July 2018

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