Australia Telescope helps to probe Jupiter


The Galileo probe entering
Jupiter's atmosphere


On 8 December (Australian time) the Australia Telescope took part in a most fruitful experiment in planetary science, as a probe from the Galileo spacecraft plunged into Jupiter to make the first ever in situ measurements of one of the outer planets. Jupiter has an unusual wind pattern of alternating east-west jetstreams. With telescopes we can measure only the speed of the planet's cloud tops; we cannot determine what drives the winds. To shed some light on this the Galileo probe was used to measure how wind speeds on Jupiter vary with atmospheric depth.



Radio signals from the probe were beamed to the main Galileo spacecraft, directly overhead, but were also detected by both the USA's Very Large Array (VLA) telescope and the Australia Telescope Compact Array. The Doppler shifts of the signals received on Earth directly from the probe have been used to refine estimates of the speeds of Jupiter's winds based on the signals sent to the main Galileo craft.


The orbiter was almost directly above the probe, and so the probe-orbiter measurements were subject to two main sources of error: uncertainty in the probe's longitude and errors in calculating its rate of vertical fall. But the probe-Earth link was nearly aligned with the east-west winds propelling the probe; the probe-Earth Doppler measurements were thus insensitive to uncertainty in the rate of fall, and hence can be used to determine the wind speed more accurately. The probe-Earth measurements can also be used to refine the estimates of the probe's longitude.


The signal to be captured on Earth was incredibly weak &shyp; too weak to be detected by the telescopes in real time. (Only 1% of the probe's signal power went towards the Earth, which in any case was
4 000 times further from the probe than the orbiter was!) To get the probe signal's frequency, the incoming signal was recorded at each telescope, multiplied by a reconstructed model of the probe signal, and then coherently integrated over time: the frequency of the model signal was adjusted until the correlation amplitude peaked.


The probe experiment found that the speeds of Jupiter's winds are higher than predicted (deep in the atmosphere, about 650 km h-1). More importantly, they are nearly constant below the cloud levels. This suggests that the winds are not driven by heat from sunlight or from the condensation of water vapour, mechanisms which drive winds on Earth, but instead are powered by heat welling up from the planet's depths.


R. Preston, W. Folkner, J. Border (JPL); W. Wilson, M. Oestreich, M. Kesteven (ATNF);
K. Wellington (CSIRO Division of Radiophysics); J. Navarro (NRAO)


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