Oxford Scientists Uncover Strange Workings Of Jupiter's Great Red Spot

Royal Astronomical Society Press Notices

Date: 27 March 1998

Ref. PN 98/12 (NAM9)

Issued by:

Dr Jacqueline Mitton
RAS Public Relations Officer
Phone: Cambridge ((0)1223) 564914
FAX: Cambridge ((0)1223) 572892
E-mail: jmitton@ast.cam.ac.uk

and

Peter Bond
Space Science Advisor
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E-mail: 100604.1111@compuserve.com

OXFORD SCIENTISTS UNCOVER THE STRANGE WORKINGS OF JUPITER'S GREAT RED SPOT -- AND OTHER SECRETS OF THE JOVIAN ATMOSPHERE

For well over two years, scientists from the department of Atmospheric, Oceanic & Planetary Physics at Oxford University have been studying weather and cloud formations on a world more than 600 million km away -- the planet Jupiter. The latest results of their research will be presented at the UK National Astronomy Meeting on Tuesday 31st March by Professor Fred Taylor, co-investigator for the NIMS instrument on the Galileo spacecraft, which is currently orbiting Jupiter. These results include the revelation of remarkable structure in Jupiter's Great Red Spot.

The giant planet Jupiter is famous for its colourful, swirling clouds. The most notable feature among this ever-changing turbulence is the Great Red Spot, a huge storm system that could swallow up three Earths and is known to have existed for at least three centuries.

One of the instruments on board the Galileo orbiter being used to study the Jovian atmosphere and the Great Red Spot is the Near-Infrared Mapping Spectrometer (NIMS). The capability of NIMS to obtain spatial and spectral information simultaneously is ideal for investigating the composition, vertical layering, optical thickness, and fine structure of Jupiter's mysterious cloud layers. The scientists hope that continued observations with NIMS will help to explain a number of the following mysteries:

Although theories abound, it is still not known what gives rise to the bright colorations of the Jovian clouds -- for example, the red pigment in the Great Red Spot or the various yellows and browns.
The nature of the circulation which gives rise to the east-west, belt-zone cloud structure is controversial.
What creates and sustains the various giant weather systems (of which the Great Red Spot is just one example of a whole family of different types of giant eddies).
The analysis of the data is still at a relatively early stage, but several preliminary results will be presented at NAM.
Winds, Storms and the Great Red Spot
Jupiter has high winds, and a large number of very large, very long-lived storm systems can be seen on the planet at any one time. The most famous of these is the Great Red Spot (GRS), which is revealed as having a most remarkable structure in the new data. Most astronomers believed it was a deep mass of cloud. Instead, it has a spiral arm structure of clouds, with gaps between which enable NIMS to see through the GRS into the deep, relatively clear atmosphere below. Futhermore, the cloud structure is higher in the centre by more than 10 km and tilted towards one side, something like a crooked spiral staircase. What seems to be happening is that wet air from the deep atmosphere is rising rapidly in a relatively narrow region in the centre of the GRS, and then spraying out above the tops of the ammonia clouds while rotating, rather like a giant garden sprinkler. In some ways this is similar to what happens in a terrestrial hurricane, but the Jovian storm is much bigger than the entire Earth.
The Nature of Jupiter's Cloud Layers
As expected, the main cloud layer on Jupiter is made up of frozen ammonia crystals, and lies at a pressure level of around half a bar (1 bar is the mean pressure at the surface of the Earth). Although anticipated to resemble terrestrial cirrus clouds, the Jovian, ammonia-ice version is made of particles around a hundred times smaller than those in water-ice clouds on Earth.
The ammonia clouds are overlain by a thick haze at much higher levels in Jupiter's atmosphere. This appears to be a photochemical smog made up of liquid hydrocarbon droplets. A similar layer blankets Saturn's moon Titan and prevents us from seeing Titan's surface. Although thinner than Titan's, the Jovian haze is unexpectedly substantial, and varies with time and place across the planet.
There is a thicker cloud layer below both the haze and the ammonia cloud. This may be the theoretically-predicted hydrogen sulphide (as NH4SH) cloud at around the one-and-a-half bar level (one and a half times the sea level air pressure on Earth), or a combination of that and an even deeper water cloud. New data is being acquired to try to resolve this point.
The Composition of Jupiter's Atmosphere
Jupiter's atmosphere is mainly hydrogen, with about 15% helium and a number of minor constituents, the most important of which are measured and mapped by NIMS. Weather on Earth centres around the condensation and evaporation of water. On Jupiter three species, ammonia, phosphine, and water vapour, can condense, making for a remarkably complicated climate. The new data have shown that water, in particular, is very variable. This helps explain the very low water abundance measured by the Galileo probe when it plunged into Jupiters clouds in December 1995. It happened, by chance, to enter a particularly dry region.
Notes
The Oxford researchers are part of an international science team for the Near Infrared Mapping Spectrometer on the Galileo orbiter (Principal Investigator is Dr. Robert W. Carlson of the Jet Propulsion Laboratory in Pasadena, California).
Galileo is a $1.5 billion NASA mission to explore the Jupiter system at close quarters over a long period. The orbiter has been returning data on the planet and its four largest moons since 7 December, 1995. A probe was also released into Jupiter's atmosphere which returned unique information on the structure and composition of the planets cloud layers. Although the primary mission is now over, the orbiter and the NIMS experiment are in good health and an extended mission is under way. This is focusing on detailed studies of the icy satellite Europa, which is thought to have a sub-surface ocean.
Images
NIMS images showing full hemisphere views of Jupiter are available on the Web site at: http://www.atm.ox.ac.uk/user/irwin/
Contact
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     Professor Fred W. Taylor,
     Head of Atmospheric, Oceanic and Planetary Physics,
     University of Oxford.
     Telephone: +44 (1865) 272903 Fax: +44 (1865) 272924
     E-mail: F.Taylor@physics.oxford.ac.uk

Press room at the National Astronomy Meeting, University of St Andrews

     (8.30 - 18.00 Tue 31 March to Thur 2 April; 9.00 - 12.00 Fri 3 April):
     Phone: 01334-462168 and 01334-462169
     Fax: 01334-463130
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