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The purpose of the atmosphere structure experiment on the Galileo Probe is to measure the physical properties of the atmosphere of Jupiter as functions of altitude during Probe entry and descent. Properties measured are the temperature and pressure of the atmosphere, and deceleration of the Probe, which is a measure of the atmospheric density. These measurements will permit the Probe altitude to be determined, relative to the level where pressure is 100 kPa (1 atmosphere), and from this, the velocity of descent. Unsteady decelerations in descent will characterize turbulence in the atmosphere, and a smooth oscillatory unsteadiness in the descent velocity will indicate gravity wave structures and their altitudes.
The atmosphere has previously been examined by remote sensing, but never below the level where pressure is 100 kPa, nor above the 100-Pa (1-millibar) level except for an occultation of the star Regulus, which gave two single-level temperatures high in the thermosphere. If all goes as planned, the Probe will report the structure of the atmosphere in fine detail from 750 km above to 150 km (90 miles) below the 100-kPa level, a total altitude range of 900 km (540 miles) extending from a threshold in the exosphere to the end of the Probe mission at pressures that are planned to exceed 1 MPa (10 atmospheres) and, with maximum success, may reach 2 MPa (20 atmospheres).
Key features of interest are the stability of the atmosphere and the levels at which the cloud layers occur. The stability will indicate whether the atmosphere is overturning, or is stagnanatly layered, as in a temperature inversion. The atmosphere is expected to be overturning vigorously, transporting heat up from Jupiter's deep interior, or possibly as a result of local thunderstorms. The nature of the convection is an important question, since it may shed light on the nature of Jupiter's internal circulation, for which there are diverse models. The cloud layer levels have, until now, been indirectly inferred. Separate layers of ammonia ice crystals and water droplets or mist are expected, along with a deeper layer of ammonium hydrosulfide, for which there is no direct evidence. The recent observations of the Comet Shoemaker-Levy impact gave evidence of abundant sulfur, which supports the possibility of occurrence of this exotic cloud. The cloud layers will have thermal signatures, such as changes in lapse rate (rate of change of temperature with altitude), which the experiment will sense in passing through them.
The experiment will also play an important role in the effort to determine the horizontal wind profile from Doppler shifts in the radio transmissions of data to the Orbiter.
Essentially, the atmospheric structure experiment is three sensors (measuring pressure, temperature, and acceleration) together with an electronics package to control the experiment sequences and prepare the data, after they are acquired, for transmission to the Galileo Orbiter and eventually to Earth. The experiment sequences, which define the exact time and number of independent observations that are taken by the sensors, are carefully established so that the fine structure of the atmosphere can be deduced. For example, once the Probe is well into the atmosphere, during key descent phase, independent pressure and temperature observations are taken every two seconds. During the same period the key accelerometer channel located along the primary body axis of the Probe is sampled every 16 seconds.
The principal investigator for the atmosphere structure experiment is Al Seiff of Ames Research Center, a veteran of many deep-space missions. Al feels that the poorly characterized and possibly hazardous environment of Jupiter is his biggest potential problem. He worries that the temperature sensors and electronics may be adversely affected by the passage of the Probe through the clouds, during which time the entire Probe may be charged to tens of kilovolts. As for the instrument itself, which was built by the Martin Marietta Corporation in Denver, Al does not foresee any fundamental problems.
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