UK SCIENTISTS STUDY THE LARGEST PLANET
Scientists from a number of UK universities are playing their part in the Galileo mission to unravel the secrets of the largest planet, Jupiter. Preliminary results from the early part of the mission are published in the journal Science on 18 October and include reports from three teams involving UK participants.
The Near Infrared Mapping Spectrometer (NIMS) Jupiter is famous for its colourful, swirling clouds. The most notable feature among this ever-changing turbulence is the Great Red Spot, a giant storm system which could swallow up three Earths and which is known to have existed through at least three centuries. One of the instruments on board the Galileo Orbiter which is studying the Jovian atmosphere and the Great Red Spot is the Near Infrared Mapping Spectrometer (NIMS). Professor Fred Taylor from the Department of Atmospheric, Oceanic & Planetary Physics at Oxford University is one of the co- investigators for this instrument.
With the support of the Particle Physics and Astronomy Research Council (PPARC) and the former Science and Engineering Research Council (SERC), Professor Taylor and his colleagues helped design and develop NIMS, the first of a new class of imaging spectrometer capable of acquiring three dimensional pictures of planetary atmospheres and measuring the temperature, composition and cloud properties. The Galileo spacecraft entered Jupiter orbit in December 1995 and the Near Infrared Mapping Spectrometer performed its first spectral studies of Jupiter and its satellites in June 1996. The first spectra were targeted on a 'hot spot' on Jupiter, where the deep atmosphere can be probed.
The scientists reported 'two striking finds': less water vapour than expected on the basis of existing ideas about Jupiter's atmosphere and the absence of a predicted dense water cloud. The amount of water vapour had been predicted from theories of solar system formation, which assign Jupiter the same mix of chemical elements as found in the Sun.
As expected, ammonia clouds have been found, although amounts are highly variable. Other minor constituents present in the atmosphere are germane (GeH4), phosphine (PH3) and deuterated methane (CH3D). A variety of weather systems and at least four levels of cloud have also been found and work is in progress to characterise these and investigate their composition and behaviour. Studies of localized hot spots in the upper cloud cover show low ammonia and water vapour content coinciding with regions where heat is escaping from the warm lower atmosphere.
Spectral images of the Great Red Spot show that it consists of spiralling ribbons of red cloud with gaps of relatively clear 'air' between. The main vortex is much smaller than the spot itself. Surrounding the Great Red Spot is a ring of white clouds which lie 3 to 7 km lower, and outside that a turbulent region with giant plumes of cloud and high winds. 'It looks rather like a giant top hat which is slightly tilted,' said Professor Taylor.
The Dust Experiment
Among the other aspects of the Jovian environment being studied by Galileo are the streams of dust particles surrounding the planet and its powerful magnetic field. Co- investigator on the dust experiment is Professor Tony McDonnell of the University of Kent. The experiment detects impacts from dust particles which hit a target and vaporise. Not only does this provide information on the mass of the incoming particle but an approximate orbit can be derived from the particle's estimated direction of travel and velocity.
About 100 dust impacts per day were recorded prior to Galileo's arrival at Jupiter in December 1995. These impacts were caused by tiny (submicrometer) particles that were barely large enough to be detected. After the closest approach to Io on 7 December, no more impacts of this type were recorded, and it seems likely that volcanic eruptions on Io are the source of the particles. The dust instrument continues to record hits from larger, micrometer-sized grains, some of which seem to be interstellar dust which has arrived from outside the solar system.
One of the most intriguing scientific puzzles is the link between the four large moons and Jupiter's magnetic field. The magnetic field is measured by the spacecraft's magnetometer every time that Galileo flies close to one of these satellites. Early results suggested that the volcanic moon Io has an iron core which is generating a magnetic field. Now the source of the decrease of Jupiter's magnetic field intensity when Galileo passed by Io is not so clear. Based on their results from the Galileo plasma wave experiment, some scientists contend that most of the Jovian magnetic signature is caused by an atmospheric plasma wave. Among those trying to unravel the mystery is Professor David Southwood of Imperial College, London, one of the designers of the magnetometer on Galileo.
Recent results show that Ganymede also seems to possess a magnetic field. 'It is much easier to envisage for Io than Ganymede,' said Professor Southwood, 'because it has an internal source of heat. Io's interior is stretched by the gravitational pull of Jupiter and the other moons so it has a stable heat source. Io also has a core that seems to be made of iron. Ganymede's core is rocky, so how does it have a magnetic field?'
Results from the Galileo mission continue to be returned. Further flybys of the large moons are scheduled to take place for at least another year.
ContactsProfessor Fred W. Taylor, University of Oxford, Department of Atmospheric, Oceanic & Planetary Physics, Clarendon Laboratory, Parks Road, Oxford OX1 3PU. Phone: (0)1865-272903; Fax (0)1865-272924
Professor J. A. M. (Tony) McDonnell, Unit for Space Sciences, University of Kent, Canterbury, CT2 7NR. Phone: (0)1227-459616. Fax: (0)1227-762616.
Information on the Galileo mission can also be found on the World Wide Web at: http://www.jpl.nasa.gov/galileo/