[meteorite-list] Venus Express Unearths New Clues to the Planet's Geological History

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 24 May 2012 09:51:31 -0700 (PDT)
Message-ID: <201205241651.q4OGpVYn003380_at_zagami.jpl.nasa.gov>

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=50378

Venus Express unearths new clues to the planet's geological history
European Space Agency
16 May 2012

ESA's Venus Express has been used to study the geology in a region near
Venus' equator. Using near-infrared observations collected by the Venus
Monitoring Camera (VMC), scientists have found evidence that the
planet's rugged highlands are scattered with geochemically more evolved
rocks, rather than the basaltic rocks of the volcanic plains. This
finding is in agreement with previous studies, which used data from the
spacecraft's Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)
to map the planet's surface in the southern hemisphere.

Investigations into the nature of Venus' surface are complicated by the
fact that the surface is concealed behind a dense covering of clouds.
Since the 1980s, radar instruments on board orbiting spacecraft have
been used to peer through these clouds to gain insight into the texture
of the surface. However, in order to understand how Venus has evolved,
geologists want to 'dig a bit deeper' and study the composition of its
rocks - information that radar imaging can't provide.

They're eager to learn if geological features revealed in radar images,
such as steep-sided domes and rugged highland terrain (called tesserae),
contain materials that are rich in silicates, such as 'felsic rocks'. On
Earth, most felsic rocks - the most common of which is granite - formed
in a water environment. This makes them particularly interesting with
regards to planetary evolution.

Since Venus Express began its observations, scientists are now starting
to unearth the planet's geology. The near-infrared channels of the VMC
and VIRTIS instruments have measured the intensity of 1
micron-wavelength radiation, which is dependent upon the surface
temperature and emissivity of the rocks. It's the latter that is
important here, as it depends on several factors, including the surface
texture and mineral composition.

In a new study, the first findings about the geology of Venus based on
VMC data have been published. The study, which was led by Alexander
Basilevsky from the Vernadsky Institute of Geochemistry and Analytical
Chemistry in Moscow, Russia, analysed the rugged highland terrain called
Chimon-mana Tessera and its surrounding volcanic plains. This region was
chosen for the VMC study because its equatorial position prevented solar
light from skewing the data; by observing the night-side of Venus and
keeping within low latitudes (40 degrees above and below the equator),
the planet eclipsed the Sun from the spacecraft.

Furthermore, the team needed to eliminate the effects that variations in
the surface temperature may have had on the intensity of 1-micron
radiation, so that any changes could be attributed to emissivity. Here,
Venus' thick atmosphere played a helping hand, as the surface
temperature has very little diurnal, seasonal or latitudinal variations;
it is almost entirely a function of surface elevation. Therefore, the
scientists compared observations of Chimon-mana Tessera with the nearby
Tuulikki volcano, which lies 10 degrees north of the equator, as both
summits are at a similar elevation (about 0.5-1 km above the plains).
"Tuulikki is a basaltic volcano and we thought it presented a good
scientific control for altitude," says Basilevsky.

However, the 'control' volcano had a surprise in store for the team: an
unexpected decrease in emissivity at its summit compared to the
surrounding plains. "It was only when we found the lower emissivity
that we looked at the volcano with maximum resolution and discovered a
steep-sided dome feature at the summit," says Basilevsky.

The decrease in emissivity found in this steep-sided dome feature is
consistent with the presence of geochemically more evolved rocks, such
as felsic rocks. Furthermore, the team also found a decrease in
emissivity of the surface material in the original target area, the
Chimon-mana Tessera. Their conclusions, which support the presence of
felsic materials on Venus, are in agreement with the earlier VIRTIS
studies. "While VIRTIS and VMC have different strengths, they have
complemented each other perfectly here by reaching a common conclusion
while studying different regions of the planet's surface," says Hakan
Svedhem, ESA Venus Express Project Scientist. The team says that their
findings aren't compromised by the fact that the volcano turned out to
be a poor control region for the effects of altitude, as the
mineralogical composition clearly plays a more important role. "The
winds are expected to be stronger at higher altitudes, blowing away the
small grains and leaving behind coarser material that exhibits greater
emissivity. Therefore, the effect of elevation, if any, should be an
increase in emissivity," explains Eugene Shalygin from the Max Planck
Institute for Solar System Research in Katlenburg-Lindau, Germany.
"However, we detected a decrease in emissivity at Chimon-mana Tessera
and the summit of the volcano, which may be caused by a change in
mineralogical composition," he continues.

The jury is still out on whether the felsic rocks on Venus, if they do
indeed exist, were created in a water environment, similar to the
process that formed most of these materials on Earth. But it is in an
enticing possibility, comments Basilevsky. "This is not the only way of
forming felsic materials, but planetary geologists, like myself, are
eager to find more similarities between Earth and Venus," he says.
However, he also points out that the Tuulikki volcano, which also showed
decreased emissivity at its summit, formed late in the geologic history
of Venus, when there couldn't have been any oceans on the planet. "If
there are felsic materials on the summit of Tuulikki, then these
particular rocks clearly formed without water," says Basilevsky.

Related publications

A.T. Basilevsky et al., "Geologic interpretation of the near-infrared
images of the surface taken at the Venus Monitoring Camera, Venus
Express", 2012, Icarus, 217, 434-450. doi: 10.106/j.icarus.2011.11.03

Notes for editors

The study reported here is based on measurements of the night-side
thermal emission from the surface of Venus taken with the Venus
Monitoring Camera (VMC).

VMC takes images in four spectral channels, covering ultraviolet to
near-infrared wavelengths; one of these, centred at 1.01 microns,
registers the night-side thermal emission from the planet's surface.

Venus Express, Europe's first mission to Earth's twin world, is
investigating the nature of our closest planetary neighbour. Launched
from the Baikonur Cosmodrome in Kazakhstan on 9 November 2005 upon a
Soyuz-Fregat launcher, it was inserted into Venus orbit on 11 April
2006, and is currently the only spacecraft in orbit around the planet.
It carries a payload comprising a combination of spectrometers,
spectro-imagers and imagers covering a wavelength range from ultraviolet
to thermal infrared, a plasma analyser and a magnetometer.

Contacts

Alexander Basilevsky
Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow, Russia
and Max-Planck-Institut f??r Sonnensystemforschung, Katlenburg-Lindau,
Germany
Email: alexander_basilevsky at brown.edu

Eugene Shalygin
Max Planck Institute for Solar System Research
Katlenburg-Lindau, Germany
Email: shalygin at mps.mpg.de

Hakan Svedhem
ESA Venus Express Project Scientist
Research and Scientific Support Department
Science and Robotic Exploration Directorate
ESA, The Netherlands
Email: hakan.svedhem at esa.int
Received on Thu 24 May 2012 12:51:31 PM PDT