Former planet
may have a tail
Pluto appears to trail a wispy
cloud of carbon monoxide
By Ron Cowen
Just like Mickey’s dog, the former planet
Pluto has a tail — or at least a hint of one.
Scientists have detected a wisp of
carbon monoxide in Pluto’s thin upper
atmosphere, extending a quarter of the
way to its largest moon, Charon, or about
3,400 kilometers above Pluto’s surface.
For more Atom & Cosmos stories,
visit www.sciencenews.org
To the astronomers who detected it with
the James Clerk Maxwell Telescope
on Hawaii’s Mauna Kea, the cloud
appears to have properties resembling
a comet’s gas tail.
“Whether Pluto’s atmosphere forms
a tail is just a suggestion on our part,”
says Jane Greaves of the University of
St. Andrews in Scotland.
The gas is too thin to image directly. But
a tiny shift in the wavelength of microwaves emitted by the carbon monoxide
hints that the gas is receding from Earth
and the sun, with ions and some neutral
atoms in the gas swept up by the solar
wind — just as a comet’s gas tail would be.
Dry ice suggests recently wet Mars
Frozen carbon dioxide could thicken Red Planet’s atmosphere
By Ron Cowen
A newfound reservoir of dry ice on Mars
suggests that the planet’s surface has been
wetter in the relatively recent past, though
not necessarily warmer than today.
The new study adds to evidence that
Mars once had a carbon dioxide atmosphere thick enough to keep liquid
water on the surface from evaporating.
It’s unclear whether the planet would
have been hospitable for life, however,
672
560
448
336
224
112
0
Thickness (m)
because temperatures on Mars may
actually have been slightly colder during times when the atmosphere had a
greater amount of carbon dioxide.
Roger Phillips of the Southwest
Research Institute in Boulder, Colo.,
and his colleagues base their findings
on radar studies by the Mars Reconnaissance Orbiter of the layered deposits at
Mars’ south polar cap. Earlier studies
had indicated that a veneer of frozen carbon dioxide sits atop part of the cap with
a thin layer of water ice beneath it. But a
detailed analysis of radar reflected from
different layers of the cap reveals that
beneath the frozen water lies a volume of
carbon dioxide ice 30 times greater than
previously estimated, the team reports
online April 21 in Science.
This unexpected reservoir of dry ice is
intriguing, Phillips says, because about
every 100,000 years Mars is known to
dramatically tilt its spin axis. During
these periods of high polar tilt, enough
The thickness of a newfound reservoir
of frozen carbon dioxide at Mars’ south
polar cap varies from a few meters
(blue) to more than 500 meters (red).
Planetary scientist Mike Brown of
Caltech says that a future array of radio
telescopes called ALMA will be able
to detect and precisely measure the
carbon monoxide emission.
Greaves and her colleagues report in
the May Monthly Notices of the Royal
Astronomical Society: Letters that the
emission is brighter than a tentative
detection made by another group 11 years
ago, indicating that the amount of gas
in Pluto’s upper atmosphere has
increased since then.
“Pluto’s atmosphere continues to
change and continues to be different
from what we expect,” says Brown.
sunlight falls on the poles to vaporize
the frozen carbon dioxide and release it
into the atmosphere, roughly doubling
the atmospheric pressure on the Red
Planet. With a denser atmosphere, liquid
water could persist on the surface rather
than evaporating, and might account for
some of the features on Mars that appear
to have been carved by water, such as
channels and gullies, Phillips says.
Although the newly found reservoir
could nearly double the mass of carbon
dioxide in Mars’ atmosphere, the resulting climate alterations would be “
modest” and would not generate a warmer,
wetter Mars, notes Peter Thomas of
Cornell University in a commentary also
published online April 21 in Science.
Phillips concurs and notes that during
times of high tilt, more carbon dioxide
frost would settle on the planet’s surface.
The reflectivity of the frost, along with
other effects, would offset any greenhouse warming from the extra gas in the
atmosphere and would tend to maintain
the chilly temperatures now typical.
Warmer conditions would require
a much thicker carbon dioxide atmosphere supplied by an additional source
of the compound, such as carbonates in
Martian rocks, says Thomas. The abundance of carbonates in the rocks is still
under exploration. s
JPL-CALTECH, NASA, UNIV. OF ROME, SOU THWES T RESEARCH INSTITUTE
www.sciencenews.org
