Remote Sensing

For Jupiter and its moons, the remote-sensing instruments will be acquiring data that may reveal the history of the Jovian system and its present composition and processes.

• SSI
  

SSI Investigations

The SSI will also be used to determine structure, motions, and radiative properties of the atmosphere of Jupiter. It will measure wind profiles by tracking how fast clouds move at various altitudes.

NIMS:
Near Infrared Mapping Spectrometer

Infrared instruments are used to detect what we think of as heat sources or temperature differences.

Spectoscopy is used to determine chemical composition.

NIMS Objectives

The geological structures will be mapped to determine their mineral distributions. Resolutions of 25 kilometers per NIMS pixel or better are planned for the Galilean satellites Europa, Ganymede, and Callisto. NIMS will make distant observations of Jupiter’s volcanic moon Io, at resolutions of 120 to 600 kilometers, to determine the moon’s surface composition and to measure temperatures of the hot spots.

Jupiter objectives: to study the atmosphere of Jupiter to determine such things as the characteristics of the Jovian cloud layers, the variations over time and space of the constituents of the atmosphere, and the temperature versus altitude profile.

NIMS will be able to monitor ammonia, water vapor, phosphine, methane, and germane and to look for previously undetected molecules. Phosphine, which is formed in the deep interior (more than 1000 kilometers deep below the clouds at temperatures near 1000 kelvin) and is rapidly destroyed at observable altitudes, is a tracer of huge upwellings of gas from deep inside the planet. NIMS will map the abundance of phosphine over a wide range of latitudes and longitudes. The goal is to understand the major deep-seated circulation patterns that power the "near-surface" meteorology (planet-girdling cloudy zones, drier belts, and localized cyclonic storm systems such as the Great Red Spot).

PPR:
Photopolarimeter /
Radiometer

The design of the instrument is based on that of an instrument flown on the Pioneer Venus spacecraft. A 10-centimeter-aperture reflecting telescope collects light, directs it to a series of filters, and, from there, measurements are performed by the detectors of the PPR.

PPR Objectives

The photometer uses seven narrow spectral bands in the visible and near-infrared wavelengths. These bands have been "tuned" to look for specific information, such as the concentration of methane and ammonia in Jupiter's atmosphere.

The radiometer provides data on the temperatures of the Jovian satellites and Jupiter’s atmosphere.

UVS/EUV:
Ultraviolet Spectrometer / Extreme Ultraviolet Spectrometer

The Cassegrain telescope of the UVS has a 250-millimeter aperture and collects light from the observation target. Both the UVS and EUV instruments use a ruled grating to disperse this light for spectral analysis. This light then passes through an exit slit into photomultiplier tubes that produce pulses or "sprays" of electrons. These electron pulses are counted, and these count numbers are the data that are sent to Earth.

UVS/EUV Objectives

The reflective properties of satellite surfaces in the ultraviolet help scientists to determine the composition and physical state of the materials that comprise the surface. One can look for ice and frost or deduce the sizes of grains.

Volcanic eruptions on Io are believed to be the source of the Io torus. Temperatures of the sulfur and oxygen ions in this plasma torus can be more than 10 times the temperatures at the surface of the Sun. These ultraviolet observations will help provide a picture of Io’s evolution and its relationship with Jupiter’s magnetic field.

Radio Science

Celestial mechanics

The celestial mechanics experiments use the radio system to sense small changes in the trajectory of the spacecraft. The spacecraft’s radio transmitter sends a signal at a well-known stable frequency. Any change in speed that the spacecraft experiences will cause the frequency of the radio signal received on Earth to change. The amount of change is dependent on the change in speed of the spacecraft, relative to Earth.

When the spacecraft passes close to Jupiter or one of the Galilean satellites, that body pulls on the spacecraft, causing its speed to change. The amount of change in speed depends not only upon the mass of the body and the distance of the spacecraft from that body but also on how that mass is internally distributed. Thus, by measuring the change in frequency of the Earth-received radio signal, the mass and internal structure of Jupiter or one of the Galilean satellites can be estimated.

The results should allow us to make a better selection of models for the interior of the satellites. This is possible because Galileo will approach the satellites much closer than did any earlier spacecraft, so that gravitational effects will be stronger and easier to observe.

Radio propagation

The spacecraft radio signal will be used to investigate Jupiter’s neutral atmosphere and ionosphere, Io’s ionosphere, and to search for ionospheres on the other Galilean satellites (Europa, Ganymede, and Callisto). This is done during radio occultation experiments, when the Galileo orbiter passes behind the planet or satellite as viewed from Earth.

The radio signal propagating from the spacecraft to Earth experiences both refraction and scattering in the atmosphere of the occulting body. (The atmosphere will bend and slow the radio signal by the process of refraction; additionally, the atmosphere will diffuse the electromagnetic waves of the signal by the process of scattering.) This causes changes in the frequency and amplitude of the signal received at a DSN tracking station on Earth. Analysis of these changes will yield information about the atmospheres and ionospheres of the Jovian system.