An artist's concept of NASA's Galileo spacecraft in orbit around Jupiter. Photo: NASA/JPL

Many observations are returned this week as the Galileo spacecraft continues to play back science data acquired during its May flyby of Ganymede, Jupiter's largest moon. Playback is interrupted on Thursday so the spacecraft can perform standard maintenance on its propulsion systems.

Parts of 24 observations are returned from the Solid-State Imaging camera (SSI), Near-Infrared Mapping Spectrometer (NIMS), Photopolarimeter Radiometer (PPR), and Fields and Particles instruments. The Fields and Particles instruments are the Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instrument.

In addition to playback, Galileo's Dust Detector is monitoring the dust environment surrounding the spacecraft. Periodic readouts of the instrument's memory during mid- and late July showed thousands of impacts occurring on some days. Real-time data collection will allow scientists to get better information on the size, speed, and origin of these micron- and submicron-sized particles.

First on the playback schedule, SSI returns portions of two context observations that were centered at the same locations on Ganymede as some high-resolution images that were returned previously. These two observations are part of a campaign of five high-resolution and five context observations that will allow scientists to get a better idea of how different features and terrains came into existence on Ganymede's surface. In addition, combination of the high-resolution and context observations for each region will allow stereo images of the regions to be produced.

Throughout the week, the Fields and Particles instruments return portions of two observations. One is a 60-minute high-resolution recording of the plasma, dust, and electric and magnetic fields surrounding Ganymede, which is the only planetary moon known to have its own magnetic field. Portions of a month-long low-resolution survey of Jupiter's magnetosphere are the second set of data returned this week. The lengthy survey not only provides context for the high-resolution recording but also provides scientists with information on both the inner and outer regions of Jupiter's magnetosphere and the transition out into the solar wind.

NIMS and SSI continue next with the return of more observations of Ganymede. NIMS returns a scan performed just off of Ganymede's limb, which is designed to give scientists more information on Ganymede's tenuous atmosphere. SSI then returns an image of enigmatic smooth dark terrain, which is characterized by a wispy appearance. NIMS returns to the playback schedule with a spectral scan of the Perrine region on Ganymede, which will provide information on the distribution and composition of ice and non-ice materials.

PPR joins the fray with the return of observations of Ganymede and Europa. First, PPR returns information describing the daytime thermal properties of Ganymede's surface. Next, and throughout the week, PPR returns nine polarimetry observations of Europa. The polarimetry measurements will allow scientists to learn about surface texture and small-scale surface properties. Each of the nine observations is taken at a different solar phase angle.

Next, NIMS returns two more observations. The first consists of a high-resolution spectral map of Ganymede's entire disk. The second observation is a distant scan of Europa while the icy moon is in Jupiter's shadow. A very low signal is expected, but detection of an elevated signal would suggest the presence of anomalously warm regions of the surface. Such regions could be caused either by unusual surface materials or by the presence of recent ice-volcanic activity.

The remaining five observations are returned by PPR, with a shift in focus to Jupiter's atmosphere. Two observations return polarimetry measurements of the atmosphere, which will provide scientists with information on the structure and temperature of its upper levels. Next, PPR returns a thermal map of recently-merged white ovals in Jupiter's atmosphere. White ovals are storms that occur between two adjacent zonal jet streams, and last for decades. Two of them have merged within the past few months to create a single storm. Finally, PPR returns two scans of Jupiter's limb. These scans are designed to detect upwellings in Jupiter's atmosphere.