David H. Atkinson

Thermal structure of Jupiter's atmosphere near the edge of a 5‐μm hot spot in the north equatorial belt

Thermal structure of the atmosphere of Jupiter was measured from 1029 km above to 133 km below the 1-bar level during entry and descent of the Galileo probe. The data confirm the hot exosphere observed by Voyager (∼900 K at 1 nanobar). The deep atmosphere, which reached 429 K at 22 bars, was close to dry adiabatic from 6 to 16 bars within an uncertainty ∼0.1 K/km. The upper atmosphere was dominated by gravity waves from the tropopause to the exosphere. Shorter waves were fully absorbed below 300 km, while longer wave amplitudes first grew, then were damped at the higher altitudes. A remarkably deep isothermal layer was found in the stratosphere from 90 to 290 km with T ∼ 160 K. Just above the tropopause at 260 mbar, there was a second isothermal region ∼25 km deep with T ∼ 112 K. Between 10 and 1000 mbar, the data substantially agree with Voyager radio occultations. The Voyager 1 equatorial occultation was similar in detail to the present sounding through the tropopause region. The Voyager IRIS average thermal structure in the north equatorial belt (NEB) approximates a smoothed fit to the present data between 0.03 and 400 mbar. Differences are partly a result of large differences in vertical resolution but may also reflect differences between a hot spot and the average NEB. At 15 4 bars, probe descent velocities derived from the data are consistently unsteady, suggesting the presence of large-scale turbulence or gravity waves. However, there was no evidence of turbulent temperature fluctuations >0.12 K. A conspicuous pause in the rate of decrease of descent velocity between 1.1 and 1.35 bars, where a disturbance was also detected by the two radio Doppler experiments, implies strong vertical flow in the cloud seen by the probe nephelometer. At p < 0.6 bar, measured temperatures were ∼3 K warmer than the dry adiabat, possible evidence of radiative warming. This could be associated with a tenuous cloud detected by the probe nephelometer above the 0.51 bar level. For an ammonia cloud to form at this level, the required abundance is ∼0.20 × solar.

An overview of the descent and landing of the Huygens probe on Titan

Titan, Saturns largest moon, is the only Solar System planetary body other than Earth with a thick nitrogen atmosphere. The Voyager spacecraft confirmed that methane was the second-most abundant atmospheric constituent in Titans atmosphere, and revealed a rich organic chemistry, but its cameras could not see through the thick organic haze. After a seven-year interplanetary journey on board the Cassini orbiter, the Huygens probe was released on 25 December 2004. It reached the upper layer of Titans atmosphere on 14 January and landed softly after a parachute descent of almost 2.5 hours. Here we report an overview of the Huygens mission, which enabled studies of the atmosphere and surface, including in situ sampling of the organic chemistry, and revealed an Earth-like landscape. The probe descended over the boundary between a bright icy terrain eroded by fluvial activity—probably due to methane—and a darker area that looked like a river- or lake-bed. Post-landing images showed centimetre-sized surface details.

The vertical profile of winds on Titan

One of Titans most intriguing attributes is its copious but featureless atmosphere. The Voyager 1 fly-by and occultation in 1980 provided the first radial survey of Titans atmospheric pressure and temperature and evidence for the presence of strong zonal winds. It was realized that the motion of an atmospheric probe could be used to study the winds, which led to the inclusion of the Doppler Wind Experiment on the Huygens probe. Here we report a high resolution vertical profile of Titans winds, with an estimated accuracy of better than 1 m s-1. The zonal winds were prograde during most of the atmospheric descent, providing in situ confirmation of superrotation on Titan. A layer with surprisingly slow wind, where the velocity decreased to near zero, was detected at altitudes between 60 and 100 km. Generally weak winds (∼1 m s-1) were seen in the lowest 5 km of descent.

The Galileo Probe Doppler Wind Experiment: Measurement of the deep zonal winds on Jupiter

During its descent into the upper atmosphere of Jupiter, the Galileo probe transmitted data to the orbiter for 57.5 min. Accurate measurements of the probe radio frequency, driven by an ultrastable oscillator, allowed an accurate time history of the probe motions to be reconstructed. Removal from the probe radio frequency profile of known Doppler contributions, including the orbiter trajectory, the probe descent velocity, and the rotation of Jupiter, left a measurable frequency residual due to Jupiters zonal winds, and microdynamical motion of the probe from spin, swing under the parachute, atmospheric turbulence, and aerodynamic buffeting. From the assumption of the dominance of the zonal horizontal winds, the frequency residuals were inverted and resulted in the first in situ measurements of the vertical profile of Jupiters deep zonal winds. A number of error sources with the capability of corrupting the frequency measurements or the interpretation of the frequency residuals were considered using reasonable assumptions and calibrations from prelaunch and in-flight testing. It is found that beneath the cloud tops (about 700 mbar) the winds are prograde and rise rapidly to 170 m/s at 4 bars. Beyond 4 bars to the depth at which the link with the probe was lost, nearly 21 bars, the winds remain constant and strong. Corrections for the high temperatures encountered by the probe have recently been completed and provide no evidence of diminishing or strengthening of the zonal wind profile in the deeper regions explored by the Galileo probe.

Galileo Doppler Measurements of the Deep Zonal Winds at Jupiter

Changes in the speed of the Galileo probe caused by zonal winds created a small but measurable Doppler effect in the probe relay carrier frequency. Analysis of the probe relay link frequency allows direct measurements of the speed of Jupiters zonal winds beneath the cloud tops. The deep winds were prograde and strong, reaching a sustained 190 to 200 meters per second at an altitude marked by a pressure of 24 bars. The depth and strength of the zonal winds severely constrain dynamic modeling of the deeper layers and begin to rule out many shallow weather theories.

Deep winds on Jupiter as measured by the Galileo probe

The Doppler Wind Experiment on the Galileo probe provided the first in situ data on wind speeds in Jupiters atmosphere. Initial analysis of the results indicated that wind speeds increase with depth, rather than decaying to zero below the cloud tops or remaining relatively constant as had previously been assumed. But this earlier analysis was subject to several potential sources of error, as highlighted by the fact that wind speeds measured at the cloud tops did not seem to match those inferred from tracking clouds in images obtained by the Voyager spacecraft. Here we report new analyses of the probe data that use a corrected treatment of the timing errors, adopt the measured (rather than predicted) descent trajectory, and incorporate a new calibration of the instrumentation that takes into account the unexpectedly high temperatures encountered by the probe. We determine wind speeds at the cloud tops (700-mbar level) in the range 80–100 m s−1, in agreement with the results of cloud tracking; the speed increases dramatically between 1 and 4 bar, and then remains nearly constant at ∼170 m s−1down to the 21-bar level. The increase in wind speed implies a latitudinal density gradient of 0.5% per degree in the 1–2 bar altitude range, but whether these winds are driven by internal heat or absorbed sunlight remains uncertain.

THE HUYGENS DOPPLER WIND EXPERIMENT Titan Winds Derived from Probe Radio Frequency Measurements

A Doppler Wind Experiment (DWE) will be performed during the Titan atmospheric descent of the ESA Huygens Probe. The direction and strength of Titans zonal winds will be determined with an accuracy better than 1 m s−1 from the start of mission at an altitude of ∼160 km down to the surface. The Probes wind-induced horizontal motion will be derived from the residual Doppler shift of its S-band radio link to the Cassini Orbiter, corrected for all known orbit and propagation effects. It is also planned to record the frequency of the Probe signal using large ground-based antennas, thereby providing an additional component of the horizontal drift. In addition to the winds, DWE will obtain valuable information on the rotation, parachute swing and atmospheric buffeting of the Huygens Probe, as well as its position and attitude after Titan touchdown. The DWE measurement strategy relies on experimenter-supplied Ultra-Stable Oscillators to generate the transmitted signal from the Probe and to extract the frequency of the received signal on the Orbiter. Results of the first in-flight checkout, as well as the DWE Doppler calibrations conducted with simulated Huygens signals uplinked from ground (Probe Relay Tests), are described. Ongoing efforts to measure and model Titans winds using various Earth-based techniques are briefly reviewed.

The Huygens Doppler Wind Experiment

A Doppler Wind Experiment (DWE) will be performed during the Titan atmospheric descent of the ESA Huygens Probe. The direction and strength of Titan’s zonal winds will be determined with an accuracy better than 1 m s−1 from the start of mission at an altitude of ~ 160 km down to the surface. The Probe’s wind-induced horizontal motion will be derived from the residual Doppler shift of its S-band radio link to the Cassini Orbiter, corrected for all known orbit and propagation effects. It is also planned to record the frequency of the Probe signal using large ground-based antennas, thereby providing an additional component of the horizontal drift. In addition to the winds, DWE will obtain valuable information on the rotation, parachute swing and atmospheric buffeting of the Huygens Probe, as well as its position and attitude after Titan touchdown. The DWE measurement strategy relies on experimenter-supplied Ultra-Stable Oscillators to generate the transmitted signal from the Probe and to extract the frequency of the received signal on the Orbiter. Results of the first in-flight checkout, as well as the DWE Doppler calibrations conducted with simulated Huygens signals uplinked from ground (Probe Relay Tests), are described. Ongoing efforts to measure and model Titan’s winds using various Earth-based techniques are briefly reviewed.

Wind speeds measured in the deep jovian atmosphere by the Galileo probe accelerometers

The atmosphere of Jupiter has a complex circulation which, until recently, has been observable only at the cloud tops,; the mechanisms driving the winds, and the nature of the interior circulation, remained unknown. Recent analyses of the radio signal from the Galileo probe, obtained during its descent into the jovian atmosphere, have suggested a vigorous interior circulation below the 4-bar level. Here we report an independent measurement of the winds below the cloud tops, making use of the data obtained by the two accelerometers on the descending probe. We find evidence for two distinct wind regimes, in general agreement with the Doppler radio measurements: a region of wind shear between 1 and 4 bar, where the wind speed increases dramatically with depth; and then a region of constant high-velocity winds down to at least the 17-bar level.

Galileo Probe Doppler residuals as the wave-dynamical signature of weakly stable, downward-increasing stratification in Jupiter's deep wind layer

Doppler radio tracking of the Galileo probe-to- orbiter relay, previously analyzed for its in situ measure of Jupiters zonal wind at the equatorial entry site, also shows a record of significant residual fluctuations apparently indicative of varying vertical motions. Regular oscillations over pressure depth in the residual Doppler measurements of roughly 1-8 Hz (increasing upward), as filtered over a 134 sec window, are most plausibly interpreted as gravity waves, and imply a weak, but downward increasing static stability within the 5 - 20 bar region of Jupiters atmosphere. A matched extension to deeper levels of an independent inertial stability constraint from the measured vertical wind shear at 1 - 4 bars is roughly consistent with a static stability of ~ 0.5 K/km near the 20 bar level, as independently detected by the probe Atmospheric Structure Instrument.