Galileo, the NASA space probe in
which UK scientists have played a key role, will dramatically end
its 14-year mission when it plunges into Jupiter’s dense
atmosphere on the 21st September. The spacecraft, which
has revealed a wealth of scientific data on Jupiter and its moons,
with fuel and power exhausted, will vaporize like a meteor as its
descends through the giant planet’s turbulent atmosphere (an
artist’s impression of what this might look like is available
– please see notes to editors). As well as extensive
scientific data, Galileo has provided the most visually stunning
images of Jupiter and its moons ever, especially Io and Europa. The
probe, launched in 1989 by the Space Shuttle Atlantis, also imaged
the Earth, Venus, the Moon and several asteroids during its
lifetime.

The highlights of the mission include the identification of
massive amounts of lightening activity in Jupiter’s
atmosphere, where hurricane-force winds and huge amounts of heat
from the interior whip clouds of frozen ammonia into bands that
encircle the planet, studded with giant storms, some of them larger
than the entire Earth. The planet is richer in heavy elements than
the Sun, showing that it was assembled from smaller ‘planetesimals’
rather than condensing in isolation as previously surmised.

Galileo also discovered that Jupiter’s rings are made of
small dust grains blasted off the surface of Jupiter’s four
innermost satellites by the impacts of meteoroids. The infrared
spectrometer on Galileo took the surface temperature of Io, one of
Jupiter’s moons, and discovered that many of Io’s
active volcanoes are much hotter than Earth’s, indicating a
higher magnesium or iron content for the silicate lava that erupts
from below Io’s surface than would be expected in a
terrestrial volcano.

Commenting on the mission, Prof. Ian Halliday, Chief Executive
Officer of the Particle Physics and Astronomy Research Council, the
UK’s space science agency, said, ” Galileo has been a
resounding scientific success. Our understanding of Jupiter and the
Jovian system has been increased enormously and UK scientists have
been at the forefront of some of these amazing discoveries”.

Galileo images and infrared spectra have revealed that Europa,
another Jovian moon, has a salty ocean beneath its cracked and
frozen surface, an impression that is reinforced by the lack of
craters on the moon, which show the surface to be relatively young.
Galileo has revealed that Jupiter’s largest moon, Ganymede,
has its own magnetic field and evidence has also been provided to
support the existence of a subsurface ocean on Callisto.

Scientists from Oxford University, The Open University and
Imperial College are involved in three of the eleven instruments
onboard Galileo.

Professor Tony McDonnell from the Planetary and Space Sciences
Research Institute (PSSRI) at The Open University is
Co-Investigator on the Dust Detector Subsystem which has looked at
the properties of dust particles within the Jovian system said,

“Galileo has been a truly remarkable mission. The results from
the Dust Detector Subsystem indicate that much of the dust comes
from outside the solar system indicating that it originates from
distant stars.”

Professor Fred Taylor from Oxford University, a team member for
the infrared spectrometer experiment on Galileo, said : “Galileo
has provided a fantastic database that will be a rich source of
progress in the planetary sciences for years or decades to come.
The mission has provided key information about Jupiter and its
place in the solar system.”

Dr Michele Dougherty from Imperial College is a team member on
the Magnetometer instrument that detects magnetic fields in the
spacecraft’s immediate environment. She comments, “It is
always sad when a mission comes to the end of its lifetime.
However, Galileo has exceeded expectations and provided a wealth of
scientific data resulting in the discovery of many new important
facts about the Jovian system. In addition the images produced of
Europa and Io are stunning.”

The UK scientists are working with their US and other overseas
colleagues on a massive, multi-authored book that will record the
scientific results of Galileo’s 30-year odyssey and form a fitting
epitaph for one of the most successful of man’s voyages to his
planetary neighbours so far.

Notes to Editors

Contacts
Gill Ormrod – PPARC Press Office
Tel: 01793 442012. Mobile: 0781 8013509
Email: gill.ormrod@pparc.ac.uk

Professor Tony McDonnell – PSSRI,
Open University
Tel: 01227 761352. Email: j.a.m.mcdonnell@open.ac.uk

Professor Fred Taylor – Oxford
University
Tel: 01865 272933. Email: f.taylor1@physics.ox.ac.uk

Dr Michele Dougherty – Imperial
College
Tel: 020 75947757. Email: m.dougherty@ic.ac.uk

Useful
websites
 http://galileo.jpl.nasa.gov/
 http://www.uk2planets.org.uk/
www.mediainfo.ukplanetaryforum.org
http://psri.open.ac.uk/missions/index.htm

Images
 Images,
including an artist’s impression of Galileo plunging into
Jupiter can be found at
 http://www.pparc.ac.uk/Ap/Press/Galileo_images.asp

Alternatively, please contact Gill
Ormrod in the PPARC Press Office – contact details
above.

Further images can be found on the NASA website listed
above.

Webcast
 The September 21st end of
mission event will be web cast live at http://www.jpl.nasa.gov/webcast/galileo/

Costs
 The overall
mission costs are approximately $1.5 billion. PPARC contributed
£1 million to the mission costs, through the production of
instruments.

Background facts on Jupiter
 Jupiter reigns supreme
among our nine planets, containing two-thirds of the planetary mass
of the solar system. In composition it resembles a small star. Its
interior pressure may reach 100 million times the pressure on
Earth’s surface. Jupiter’s magnetic field is immense, even in
proportion to the size of the planet, stretching millions of miles
into the solar system. Electrical activity in Jupiter is so strong
that it pours billions of watts into Earth’s own magnetic field
every day.

Jupiter is endowed with 16 moons, a ring system, and an
immense, complex atmosphere. Its atmosphere bristles with lightning
and swirls with huge storm systems, including the Great Red Spot, a
storm that has persisted for at least 100 years, perhaps as long as
300 years. Some scientists theorize that beneath the atmosphere
there is no solid mass at the center of Jupiter, but that the
planet’s unique temperature and pressure conditions sustain a core
whose density is more like liquid or slush.
The reddish color of the “Great Red Spot” is a puzzle to
scientists, but several chemicals, including phosphorus, have been
proposed as a reason. In fact, the colors and mechanisms driving
the appearance of the entire atmosphere are still not well
understood. These mysteries cannot be solved by taking pictures.
Direct measurements from within the atmosphere are necessary –
measurements like those made by the Galileo Probe.

Mission Highlights

Jupiter’s Storms and Rings
Using data from the Galileo Probe’s plunge into the top cloud
layers of Jupiter, Galileo has discovered that Jupiter has
thunderstorms many times larger than Earth’s. These storms result
from the vertical circulation of water in the top layers, leaving
large areas where air descends and becomes dry like the Sahara
desert, and other areas where water rises to form the
thunderstorms. Galileo has also found that Jupiter’s rings are made
of small dust grains blasted off the surface of Jupiter’s four
innermost satellites by the impacts of meteoroids.

Hot Active Volcanoes on Io
Now considered the solar system’s most active body, Io’s volcanoes
were first discovered by Voyager 1 in 1979 and result from 100
meter (328 ft) tides in its solid surface. By taking Io’s
temperature with Galileo’s instruments, scientists now know that
some of Io’s volcanoes are hotter than Earth’s. From this,
scientists surmise that lava made of silicate material rich in
magnesium erupts from below Io’s surface.

A Possible Ocean on Europa
Possessing more water than the total amount found on earth, Europa
appears to have had a salty ocean beneath its icy cracked and
frozen surface. Galileo images show ice “rafts” the size of cities
that appear to have broken off and drifted apart, a frozen “puddle”
smooths over older cracks, warmer material bubbles up from below to
blister the surface, evaporative-type salts are exposed. A
remarkable lack of craters show the surface to be relatively young.
Europa has a thin oxygen atmosphere and an ionosphere.

Ganymede’s Own Magnetic Field
Internal tidal friction again causes surprising effects on the
solar system’s largest moon. Galileo revealed that Ganymede has its
own magnetic field. Perhaps from a slightly different orbit in its
past, enough heat from tidal friction caused the separation of
material inside Ganymede and this stirring of a molten core or iron
sulfide is believed to generate Ganymede’s magnetic field.

Does An Ocean Hide Beneath Callisto’s Surface?
There is evidence to support the existence of a subsurface ocean on
Callisto. The ocean would have to be deep enough inside the moon
that it does not affect the heavily cratered surface on top.
Instead the ocean could be showing itself indirectly, through the
magnetic field it generates. This could come from electric flow in
a salty ocean generated by Jupiter’s strong magnetic field passing
through it.

The Particle Physics and Astronomy Research Council (PPARC) is the
UK’s strategic science investment agency. It funds research,
education and public understanding in four broad areas of science –
particle physics, astronomy, cosmology and space science.

PPARC is government funded and provides research
grants and studentships to scientists in British universities,
gives researchers access to world-class facilities and funds the UK
membership of international bodies such as the European
Organisation for Nuclear Research, CERN, the European Space Agency
and the European Southern Observatory. It also contributes money
for the UK telescopes overseas on La Palma, Hawaii, Australia and
in Chile, the UK Astronomy Technology Centre at the Royal
Observatory, Edinburgh and the MERLIN/VLBI National Facility.

Issued by the Particle Physics and Astronomy Research Council on
16th September 2003.