[meteorite-list] Vesta Likely Cold and Dark Enough for Ice

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 25 Jan 2012 16:40:12 -0800 (PST)
Message-ID: <201201260040.q0Q0eCsD026195_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2012-024

Vesta Likely Cold and Dark Enough for Ice
Jet Propulsion Laboratory
January 25, 2012

Though generally thought to be quite dry, roughly half of the giant
asteroid Vesta is expected to be so cold and to receive so little
sunlight that water ice could have survived there for billions of years,
according to the first published models of Vesta's average global
temperatures and illumination by the sun.

"Near the north and south poles, the conditions appear to be favorable
for water ice to exist beneath the surface," says Timothy Stubbs of
NASA's Goddard Space Flight Center in Greenbelt, Md., and the University
of Maryland, Baltimore County. Stubbs and Yongli Wang of the Goddard
Planetary Heliophysics Institute at the University of Maryland published
the models in the January 2012 issue of the journal Icarus. The models
are based on information from telescopes including NASA's Hubble Space
Telescope.

Vesta, the second-most massive object in the asteroid belt between Mars
and Jupiter, probably does not have any significant permanently shadowed
craters where water ice could stay frozen on the surface all the time,
not even in the roughly 300-mile-diameter (480-kilometer-diameter)
crater near the south pole, the authors note. The asteroid isn't a good
candidate for permanent shadowing because it is tilted on its axis at
about 27 degrees, which is even greater than Earth's tilt of roughly 23
degrees. In contrast, the moon, which does have permanently shadowed
craters, is tilted at only about 1.5 degrees. As a result of its large
tilt, Vesta has seasons, and every part of the surface is expected to
see the sun at some point during Vesta's year.

The presence or absence of water ice on Vesta tells scientists something
about the tiny world's formation and evolution, its history of
bombardment by comets and other objects, and its interaction with the
space environment. Because similar processes are common to many other
planetary bodies, including the moon, Mercury and other asteroids,
learning more about these processes has fundamental implications for our
understanding of the solar system as a whole. This kind of water ice is
also potentially valuable as a resource for further exploration of the
solar system.

Though temperatures on Vesta fluctuate during the year, the model
predicts that the average annual temperature near Vesta's north and
south poles is less than roughly minus 200 degrees Fahrenheit (145
kelvins). That is the critical average temperature below which water ice
is thought to be able to survive in the top 10 feet or so (few meters)
of the soil, which is called regolith.

Near Vesta's equator, however, the average yearly temperature is roughly
minus 190 degrees Fahrenheit (150 kelvins), according to the new
results. Based on previous modeling, that is expected to be high enough
to prevent water from remaining within a few meters of the surface. This
band of relatively warm temperatures extends from the equator to about
27 degrees north and south in latitude.

"On average, it's colder at Vesta's poles than near its equator, so in
that sense, they are good places to sustain water ice," says Stubbs.
"But they also see sunlight for long periods of time during the summer
seasons, which isn't so good for sustaining ice. So if water ice exists
in those regions, it may be buried beneath a relatively deep layer of
dry regolith."

The modeling also indicates that relatively small surface features, such
as craters measuring around 6 miles (10 kilometers) in diameter, could
significantly affect the survival of water ice. "The bottoms of some
craters could be cold enough on average -- about 100 kelvins -- for
water to be able to survive on the surface for much of the Vestan year
[about 3.6 years on Earth]," Stubbs explains. "Although, at some point
during the summer, enough sunlight would shine in to make the water
leave the surface and either be lost or perhaps redeposit somewhere else."

So far, Earth-based observations suggest that the surface of Vesta is
quite dry. However, the Dawn spacecraft is getting a much closer view.
Dawn is investigating the role of water in the evolution of planets by
studying Vesta and Ceres, two bodies in the asteroid belt that are
considered remnant protoplanets - baby planets whose growth was
interrupted when Jupiter formed.

Dawn is looking for water using the gamma ray and neutron detector
(GRaND) spectrometer, which can identify hydrogen-rich deposits that
could be associated with water ice. The spacecraft recently entered a
low orbit that is well suited to collecting gamma ray and neutron data.

"Our perceptions of Vesta have been transformed in a few months as the
Dawn spacecraft has entered orbit and spiraled closer to its surface,"
says Lucy McFadden, a planetary scientist at NASA Goddard and a Dawn
mission co-investigator. "More importantly, our new views of Vesta tell
us about the early processes of solar system formation. If we can detect
evidence for water beneath the surface, the next question will be is it
very old or very young, and that would be exciting to ponder."

The modeling done by Stubbs and Wang, for example, relies on information
about Vesta's shape. Before Dawn, the best source of that information
was a set of images taken by NASA's Hubble Space Telescope in 1994 and
1996. But now, Dawn and its camera are getting a much closer view of Vesta.

"The Dawn mission gives researchers a rare opportunity to observe Vesta
for an extended period of time, the equivalent of about one season on
Vesta," says Stubbs. "Hopefully, we'll know in the next few months
whether the GRaND spectrometer sees evidence for water ice in Vesta's
regolith. This is an important and exciting time in planetary exploration."

Dawn's mission to Vesta and Ceres is managed by NASA's Jet Propulsion
Laboratory, Pasadena, Calif., for NASA's Science Mission Directorate in
Washington. JPL is a division of the California Institute of Technology
in Pasadena. Dawn is a project of the directorate's Discovery Program,
managed by NASA's Marshall Space Flight Center in Huntsville, Ala. UCLA
is responsible for overall Dawn mission science. Orbital Sciences Corp.
in Dulles, Va., designed and built the spacecraft. The German Aerospace
Center, the Max Planck Institute for Solar System Research, the Italian
Space Agency and the Italian National Astrophysical Institute are
international partners on the mission team. The asteroid modeling by
Stubbs and Wang is an extension of analysis originally applied to the
moon and partially funded by the NASA Lunar Science Institute.

Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jccook at jpl.nasa.gov

Elizabeth Zubritsky 301-614-5438
Goddard Space Flight Center, Greenbelt, Md.
elizabeth.a.zubritsky at nasa.gov

2012-024
Received on Wed 25 Jan 2012 07:40:12 PM PST