Sky and Telescope - July 2017 - 11

MARS

Atmosphere Lost to Space

NASA'S MAVEN SPACECRAFT has

conﬁrmed that the solar wind stripped
the Red Planet of its dense, early
atmosphere, more than likely through a
process called sputtering.
Mars was not always the frozen, barren place it is now. Based on the ratio of
various gas isotopes, planetary scientists
have long suspected that the Red Planet
lost anywhere from 25% to 90% of its
initial atmosphere, with the estimates
favoring at least 50% (S&T: Sept. 2014,
p. 20). One popular explanation has
been that the solar wind stripped it away.
Now, MAVEN scientists have conﬁrmed this hypothesis. Using the spacecraft's measurements, Bruce Jakosky
(University of Colorado, Boulder) and
colleagues determined the abundances
of the noble gas argon-36 and of the
heavier isotope argon-38. The latter

SOLAR SYSTEM

M AV EN: LOCK HEED M A RTIN; SUPER NOVA: JOSEPH BRIM ACO MBE

Sun Triggers Ceres' "Air"
SCIENTISTS THINK the Sun may inadvertently create a transitory, tenuous
atmosphere around the dwarf planet
Ceres - and in an unexpected way.
Since 1991, astronomers have occasionally detected an "exosphere" (an
atmosphere so thin it's not worthy of
the name) of water molecules around
Ceres, the largest body in the asteroid
belt. Water ice makes up a considerable fraction of Ceres' makeup (S&T:
Dec. 2017, p. 16), but it was unclear
what process was sending the molecules
into space. For example, the handful of
detections did not always coincide with
the dwarf planet's closest approaches to

naturally settles lower in the Martian
air, leaving argon-36 enriched in the
upper atmosphere.
This differentiation leaves the lighter
isotope more susceptible to being torn
away by the solar wind. During sputtering, the atmosphere colludes with the
solar wind against the planet. Ultraviolet solar photons ﬁrst knock electrons from atoms and molecules in the
uppermost atmosphere, forming ions.
The magnetized solar wind then picks
up these ions, whirling them around
and ﬂinging some of them back into
the atmosphere, where they collide with
neutral atoms and molecules there -
such as argon-36 - and "sputter" them
every which way, including out of the
atmosphere entirely.
MAVEN's observations show that
today's Mars has far too little argon-36,
if it started out with a level similar to
Earth's and that of other solar system
objects. To explain the current ratio,
the planet must have lost roughly twothirds of its atmospheric argon over
its history, the team concludes in the
March 31st Science.
This estimate agrees with previous
Mars studies, including a 2013 effort by
Sushil Atreya (University of Michigan)
and others using Curiosity rover data.
The new result incorporates those measurements; MAVEN's contribution is

the evidence for how the argon isotopes
separate and how the argon is lost.
Other atmospheric constituents
would have escaped Mars with argon.
Jakosky's team estimates that, based on
the argon ratio, Mars has lost at least
0.5 bar (half the atmospheric pressure
at sea level on Earth) of its primary
atmospheric molecule, carbon dioxide. That's enough to at least partially
explain what happened to the planet's
ancient warmer, wetter climate.
■ CAMILLE M. CARLISLE

SUPERNOVAE

Supernova Erupts in Lupus

SN 2017cbv, in outskirts of NGC 5643

ON MARCH 10TH, Leonardo Tarta-

the Sun, eliminating sublimation as a
possible solution.
In 2015 NASA's Dawn spacecraft
detected both a short-lived exosphere
and a space weather "prequel": an
uptick in energetic, charged particles arriving from the Sun. Inspired,
Michaela Villarreal (University of California, Los Angeles) and colleagues dove
into archival data and discovered that
an exosphere appears after a burst of
solar wind hits Ceres. The solar wind's
high-speed particles presumably knock
water molecules out of the surface and
into space, creating an exosphere that
lasts about a week. The team reports the
result in the March 20th Astrophysical
Journal Letters.

glia (University of California, Davis)
and colleagues discovered Supernova
2017cbv in NGC 5643. This spiral
galaxy lies 55 million light-years from
Earth and sits in the far western corner
of the constellation Lupus. At discovery,
the stellar explosion was only magnitude 15, but two weeks later it had
brightened to magnitude 11.5, within
easy reach of a 6-inch telescope. Unfortunately, because NGC 5643 lies at declination -44° 08′, only observers south
of 36° north latitude could see it. Spectra indicate the explosion was a Type Ia,
the aftermath of a white dwarf's death.
The discovery is part of the D<40 Mpc
Survey, which observes galaxies within
a distance of 40 megaparsecs (Mpc),
or 130 million light-years, every night
down to a limit of about magnitude 19.
Read more at https://is.gd/sn2017cbv.

■ CAMILLE M. CARLISLE

■ BOB KING
s k y a n d t e l e s c o p e .c o m

* J U LY 2 017

https://www.is.gd/sn2017cbv http://www.skyandtelescope.com

Sky and Telescope - July 2017

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