Galileo's Instruments Perform Observations of Ganymede, Europa, Jupiter, and the Jovian Magnetosphere

A full day lies ahead for Galileo as its instruments perform observations of Ganymede, Europa, Jupiter and the Jovian magnetosphere. The spacecraft flies over the surface of Ganymede at 3:10 a.m PDT today [see Note 1] at an altitude of 808 kilometers (502 miles) and a speed of 11.3 kilometers per second (7.0 miles per second, or 25,200 miles per hour). Galileo also makes its closest approach to Europa, Jupiter and Io at 2:29 p.m., 9:53 p.m., and 11:40 p.m. PDT, at ranges of 595,000 kilometers (370,000 miles), 479,000 kilometers (298,000 miles), and 380,000 kilometers (236,000 miles), respectively.

During the Ganymede flyby, the spacecraft will pass behind this largest of the Galilean moons as seen from the Earth and Sun. The solar occultation is uneventful as Galileo does not use solar power to operate. However, Galileo's transmissions to Earth will pass through Ganymede's tenuous atmosphere (as they did yesterday with Jupiter's atmosphere) as the spacecraft moves behind the icy moon, until they are completely blocked from reaching Earth. About 30 minutes later, the spacecraft will emerge from behind Ganymede and communications will be restored. As with yesterday's Jupiter occultation, during this passage, Galileo's radio signal is weakened and refracted by the tenuous atmosphere. The changes in the signal are again measured by radio scientists to learn more about the structure and electron density of Ganymede's tenuous atmosphere.

The Fields and Particles instruments also take advantage of the flyby to perform a 60-minute high-resolution recording of the plasma, dust, and electric and magnetic fields surrounding Ganymede. Ganymede is the only planetary moon known to have its own internally-generated magnetic field, and thus, its own magnetosphere. During the recording, Galileo hopes to actually penetrate magnetic field lines that both originate and close on Ganymede's surface. This will allow scientists to obtain a far more complete understanding of how the magnetic fields and magnetospheres of both Ganymede and Jupiter interact with one another.

The first remote sensing observation of the day is performed by the Photopolarimeter Radiometer (PPR) as it takes high-resolution thermal measurements of Ganymede's surface. Remote sensing observations occur both during the 60-minute Fields and Particles recording, and afterwards as the spacecraft moves away from Ganymede. Next, the Plasma Wave instrument (PWS) performs an observation dedicated to the detection of chorus emissions within Ganymede's magnetosphere. A chorus signal is seen in the electromagnetic fields measured by PWS when plasma is being accelerated by a particularly efficient type of wave-particle interaction. By detecting and analyzing chorus emissions, scientists hope to understand more about how Ganymede's unique magnetosphere operates.

The PWS observation is followed by a series of five observations of Ganymede performed by the Solid-State Imaging camera (SSI). The observations are designed to provide scientists with information regarding questions of how different features and terrains come to exist on Ganymede's surface. The regions examined in these mosaics are believed to have been created by processes internal to Ganymede. However, are the processes volcanic, tectonic, or from some other mechanism? This observation set may help answer the question. The first mosaic of images captures smooth bright terrain and possibly sheltered grooved terrain. The second looks at a transition region between bright and dark terrain. Yet another mosaic contains pristine dark terrain, believed to be the oldest type of terrain on Ganymede. The fourth observation captures another region of smooth bright terrain containing bands with a smooth, 'plank-like' appearance. Finally, the last mosaic of images captures a caldera-like feature.

The Near-Infrared Mapping Spectrometer (NIMS) takes the next observation of Ganymede. In it, NIMS obtains a spectral scan of a dark crater, surrounding ice, and background dark regions. The scan will allow scientists to determine if there are any differences in the composition of these different types of terrains. Then, SSI returns to the observation schedule with five mosaics centered at the locations of the high-resolution mosaics taken earlier, but covering a much wider area of the surface. These images will provide the geologic context for the high-resolution samples. In addition, the motion of the spacecraft along its flight path will allow stereo images to be produced by combining data from the two sets of images.

NIMS takes another look at Ganymede by performing a scan just off of the moon's limb. The observation should allow scientists to gain more knowledge on the characteristics of Ganymede's tenuous atmosphere. SSI then takes another image of Ganymede, this time of enigmatic smooth dark terrain with a wispy appearance to it. Then, NIMS performs a spectral scan of the Perrine region of Ganymede's surface. Again, the scan will provide scientists with much desired information about the composition of the region. PPR is next on the observation plan. In an observation of Ganymede's dayside, PPR gathers information on the thermal properties of the surface in the presence of daylight.

PPR continues in the observation spotlight by shifting attention from Ganymede and initiating a series of polarimetry observations of Europa. Polarimetry measurements allow scientists to learn about surface texture and small-scale surface properties. For the remainder of the day, PPR makes nine observations of Europa at different solar phase angles. Interspersed with these observations are two observations performed by NIMS. The first is a return to Ganymede and is designed to provide a high-resolution spectral map of Ganymede's entire disk. This global map can then be used for global scale comparisons of Ganymede to the other Galilean satellites. The second observation is a distant view of Europa while the moon is in Jupiter's shadow. A very low signal is expected, but detection of an elevated signal would suggest the presence of anomolously warm regions of the surface, due to either unusual surface materials, or the presence of recent ice-volcanic activity on Europa.

The last two observations of the day turn their attention to Jupiter's atmosphere. Both performed by PPR, they are designed to capture polarimetry measurements of the atmosphere, which will provide scientists with information on the structure and temperature of its upper levels.

The excitement is not over. Come back tomorrow for more discoveries with Galileo!

Note 1. Pacific Daylight Time (PDT) is 7 hours behind Greenwich Mean Time (GMT). The time when an event occurs at the spacecraft is known as Spacecraft Event Time (SCET). The time at which radio signals reach Earth indicating that an event has occurred is known as Earth Received Time (ERT). Currently, it takes Galileo's radio signals 50 minutes to travel between the spacecraft and Earth.