Science Briefs

Bobbing for Buoyancy in Jupiter's Deep Atmosphere

Photo of Jupiter and Europa
Fig. 1: Jupiter and its icy moon Europa, as imaged during the Cassini flyby in December 2000.

Spacecraft observations of other planets afford an opportunity to test our understanding of the dynamic behavior of atmospheres and oceans for exotically different scales, temperatures and other physical conditions than ever encountered on the Earth. Jupiter, in particular, with its complex arrangement of counter-flowing, 100 meter per second jet streams, poses an outstanding challenge to the theory of geophysical fluid motions organized by rotation and the stable buoyant layering of temperature and density.

Although Jupiter's upper-level flow is well marked over latitude and longitude by the rapid movement of its cloud-top features, the associated cover of ammonia and other condensing compounds has so far prevented remote sensing of its temperature-density structure at deeper levels where the winds originate, more than 100 kilometers below. The gas giant planet's lack of a lower solid surface precludes even a reliable estimate of the true depth of its currents. In this respect the puzzle of Jupiter's wind motions is similar to the challenge of understanding the "abyssal circulation" of the Earth's ocean, as surveyed for many years aboard numerous sea-going vessels.

In December 1995, the Galileo atmospheric probe provided the first direct measurements of Jupiter's wind, temperature, and atmospheric composition at sub-cloud levels. The probe's delivery to Jupiter aboard the Galileo orbiter required that it be targeted for a region 6 degrees to the north of the equator, nearly 30 degrees to the north of the Great Red Spot and within the white cloudy zone shown toward the top of Figure 1. Yet, partly as a result of the anomalous, local "hot-spot" environment encountered at the north-equatorial entry site, as well as the performance limits of its instruments at the deepest levels recorded, the buoyant layering of Jupiter's wind system remains a controversial issue.

Static stability graph; see caption and text
Fig. 2: The static stability of Jupiter's deep wind layer, as inferred from the gravity wave analysis of Galileo probe Doppler tracking. (A large version is available)

Recently, I worked with Galileo Principal Investigator David Atkinson (University of Idaho) to analyze residual fluctuations in the Galileo probe Doppler signal as an atmospheric "seismogram" for the deep temperature structure of Jupiter's wind layer. After removing the effects of the east/west winds, the downward descent of the probe and the swing under its parachute, the residual fluctuations in the Doppler signal show a downward bunching of apparently regular oscillations indicating the presence of a vertically propagating wave, with a rapid upward increase in its variable vertical velocity exceeding 1 meter per second (see Figure 2). As we show in a recent paper, the application of the theory of atmospheric waves to the probe's Doppler data implies a weak but stronger-downward layering of temperature and density near the 20 bar level.

This result significantly corroborates an independent but preliminary estimate of a stable layered temperature structure for the same region from data collected by the probe's Atmospheric Structure Instrument. It is also consistent with an apparent downward increase in the water abundance and therefore the mean molecular weight inferred from the probe's mass spectrometer instrument. The associated horizontal scale of the inferred buoyant layering is quite comparable to the dynamical requirements of Allison's (2000) similarity theory for Jupiter's wind currents, developed as a conceptual extension of Henry Stommel's inertial model for the Atlantic Gulf Stream.

Although the geometrical thickness of Jupiter's atmospheric current system is at least a hundred times larger than that in the Earth's oceans, it is interesting to note that its modeled pressure-depth is roughly comparable to that of the ocean's thermocline, near the half-kilometer depth of the 500 decibar level.

Reference

Allison, M. 2000. A similarity model for the windy Jovian thermocline. Planet. Space Sci. 48, 753-774.

Allison, M., and D.H. Atkinson 2001. Galileo probe Doppler measurements as the wave-dynamical signature of weakly stable, downward-increasing stratification in Jupiter's deep wind layer. Geophys. Res. Lett. 28, 2747-2750.

Contact

Please address all inquiries about this research to Dr. Michael Allison.