http://www.skyandtelescope.com.au 9MAVEN: LOCKHEED MARTIN; SUPERNOVA: JOSEPH BRIMACOMBE; RAVIOLI MOON: NASA / JPL-CALTECH / SPACE SCIENCE INSTITUTE
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 wasn’t always the frozen, barren
place it is now. Based on the ratio of
various gas isotopes, scientists have
long suspected that the Red Planet lost
anywhere from 25% to 90% of its initial
atmosphere, with the estimates favouring
at least 50%. One 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-
and of the heavier isotope argon-38.
The latter naturally settles lower in the
Martian air, leaving argon-36 enriched
in the upper atmosphere.
This leaves the lighter isotope
more susceptible to being torn away
by the solar wind. Ultraviolet solar
photons first knock electrons from
atoms and molecules in the uppermost
atmosphere, forming ions. The
magnetised solar wind then picks up
these ions, whirling them around and
flinging some of them back into theMars’ atmosphere
lost into space
through ‘sputtering’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 two-
thirds of its atmospheric argon over its
history, the team concludes.
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. CARLISLESUPERNOVA ERUPTS IN LUPUS
ON MARCH 10TH, Leonardo
Tartaglia (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 15-cm telescope.
Fortunately, because NGC 5643 lies
at declination –44° 08′, observers in
the South Hemisphere were well-
placed to 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<
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.
■ BOB KINGSN 2017cbv, in outskirts of NGC 5643Pan: Saturn’s ravioli-shaped moon
This close-up shows the bizarre shape of Saturn’s moon Pan. The Cassini spacecraft took the image during
a flyby on March 7th, when it came within about 24,000 km of the moonlet. A mere 35 km across, Pan is
nestled in the Encke Gap within Saturn’s A ring, where it kicks up spiral density waves in the ring. Scientists
suspect the flange of ice around Pan’s equatorial bulge is ring material the moon has swept up and
collected as it cruises through the Encke Gap. The skirt of ice towers several kilometres above the surface.
■ DAVID DICKINSON