Venus caught in the wind from the Sun (Forwarded)

ESA News
http://www.esa.int

28 November 2007

Caught in the wind from the Sun

Venus Express has exposed the true extent to which the Sun strips away the
atmosphere of Venus. This process could be an important contribution to the
way the planet has evolved to become so different from the Earth.

The Sun has probably been stripping away the Venus' atmosphere throughout
the planet's four-thousand million-year history. Unlike Earth, Venus does
not possess an intrinsic magnetic field to protect its atmosphere from the
solar wind, a constant stream of electrically charged particles emitted by
the Sun. Instead, the solar wind interacts directly with the upper
atmosphere of Venus.

"The solar wind interaction is important because it defines the conditions
at the boundary of the atmosphere with space and it is a very active
boundary," says Stanislav Barabash at the Swedish Institute of Space
Physics, Kiruna, Sweden.

In particular, the interaction causes Venus's atmosphere to lose its gases
in the form of ionized particles. The Analyzer of Space Plasmas and
Energetic Atoms (ASPERA) on Venus Express has been studying this interaction
and has revealed, for the first time, the composition of the escaping
particles. They are predominantly hydrogen, oxygen and helium ions.

The first two components of the escaping ions are highly important because
they are the chemical constituents of water. Water molecules are thought to
be the principal contributors of hydrogen in the upper atmosphere of Venus.
The solar ultraviolet radiation breaks water into electrically charged
'atoms', turning it into what scientists call a plasma, and this plasma is
then accelerated into space. Thus, two hydrogen atoms should be escaping for
every one of oxygen.

The highly elliptical polar orbit of Venus Express, with its closest
approach to the planet of 250350 km and its furthest reach of 66,000 km, is
perfect for studying the escaping planetary ions. This is because it covers
the region of the solar wind interaction near the planet and the region
directly downstream of the planet, as well as for reference the unperturbed
solar wind far away from the planet.

Prior to Venus Express, scientists knew that particles were probably
escaping from Venus's atmosphere but they could only guess the composition
of those particles.

ASPERA finally established the composition of the escaping plasma and
measured that the escape of hydrogen to oxygen is, indeed, in the same ratio
as water: two hydrogens for every oxygen.

This may be the critical observation in understanding why Venus is so dry
today. Perhaps the solar wind has gradually stripped the planet of water.
The way to estimate this is to observe the process for as long as possible,
watching for changes in the escape rate, brought about by different amounts
of solar activity.

Solar activity controls almost every aspect of the Venus plasma environment.
Although Venus has no intrinsic magnetic field, the interplanetary magnetic
field carried by the solar wind piles up above the planet's upper
atmosphere, creating a weak magnetic envelop around Venus. This prevents the
solar wind from entering the atmosphere.

The magnetometer instrument (MAG) on Venus Express has shown that this field
is wrapped around the day-side of the planet and continues to hug the shape
of the planet on the night-side. The configuration of the night-time
magnetic field seems likely to promote the acceleration of plasma, resulting
in further loss of the atmosphere.

"For the first time, we could perform the in-situ observations of the
near-Venus plasma environment at solar minimum. We see a very different
picture of solar wind interaction with Venus than the well-known
observations at solar maximum." says Tielong Zhang, Space Research Institute
of Austrian Academy of Sciences, Austria.

During Venus Express's extended mission, solar activity is expected to rise
from its current minimum levels, giving planetary scientists the conditions
they need in order to study this exotic process further and understand just
how important it has been for Venus.

Notes for editors:

The findings appear in the 29 November issue of the scientific journal
Nature, in the papers: 'Venus loses its water through the plasma wake', by
S.Barabash et al., and 'Little or no solar wind enters Venus' atmosphere at
solar minimum', by T.Zang et al.

For more information:

Stanislav Barabash, ASPERA Principal Investigator
Swedish Institute of Space Physics, Kiruna, Sweden
Email: stas @ irf.se

Tielong Zhang, MAG Principal Investigator
Space Research Institute of Austrian Academy of Sciences, Austria
Email: Tielong.Zhang @ oeaw.ac.at

Hakan Svedhem, ESA's Venus Express Project Scientist
Email: Hakan.Svedhem @ esa.int

[NOTE: Images supporting this release are available at
http://www.esa.int/SPECIALS/Venus_Ex...G373R8F_1.html ]