New Horizons will observe the four primary moons of Jupiter — Io, Europa, Ganymede and Callisto — along with a proposed search for inner satellites. Jovian system satellite observations fall into three categories:

1) Io monitoring, which includes creating an Io composition map, and Io plume inventory and making hot spot, atmosphere, eclipse and phase curve observations;
2) Europa, Ganymede and Callisto atmosphere, composition map, eclipse and phase curve observations;
3) Irregular satellite observations (if any happen to be near the spacecraft's trajectory at the time of the encounter)

Io Observations

Io monitoring at regular longitudinal increments will enhance current science data about this key active Jupiter satellite. There are four main goals:

1) Top priority is to complete a 2007 global map of Io at full resolution (using the Long Range Reconnaissance Imager [LORRI] instrument) and using any available color options (Ralph instrument) to investigate the long-term stability of Io resurfacing by volcanic deposits or frost. This new map will provide our only high-resolution snapshot of Io between Galileo's last global map of 2000-2001 and future Jupiter missions. Observation of new volcanic deposits of different styles will provide a longer baseline from which to constrain resurfacing rates and modes. Comparison with the Galileo map (and that of Voyager, 28 years earlier) requires that the best possible resolution imaging be obtained at each longitude (15-degree longitudinal coverage is optimal). Extensive longitudinal coverage is also required to produce a photometrically and geometrically undistorted surface map.

2) An important measurement is the determination of temperatures and distribution of high-temperature volcanism on Io, to constrain magma composition and eruption mechanisms. This will be accomplished using multi-wavelength nightside imaging of volcanic thermal emission using the MVIC and LEISA components of the Ralph camera. Global coverage should be possible by careful selection of sub-spacecraft longitudes and phase angles.

3) The shape of Io (which can be described as a triaxial ellipsoid) is plausibly distorted regionally by internal dynamics, which may be manifest as long-wavelength topographic undulations. These have not been fully confirmed by Voyager or Galileo due to data anomalies. Mapping with LORRI at 15-degree longitudinal coverage will also allow for the mapping of Ionian global topography using limb profiling techniques. New Horizons will provide improved data over the Galileo results, which were compromised by limited coverage, compression artifacts and CCD radiation noise.

4) The distribution and lifetime of volcanic plumes on Io is of great interest, as plumes provide a means for replenishing and redistributing surface and atmospheric volatiles, are a probe of eruption styles, and are likely to be a major supplier of dust to the Jovian magnetosphere. Plumes seen by Voyager and Galileo (and Hubble Space Telescope and Cassini, to a limited extent) tend to be concentrated at low latitudes and fall into two categories: large, short-lived "Pele type" plumes, up to 400 kilometers high; and small, long-lived "Prometheus type" plumes, typically 100 kilometers high. However, scientists do not know if these patterns are typical of Io's long-term behavior, and Galileo attempts to inventory Io's plumes were compromised by low data rates, and data compression artifacts.

New Horizons can potentially provide both the most complete inventory of volcanic plumes since Voyager, and extend the time-base of plume lifetime data a decade beyond Galileo's intensive observations.

Additional goals for Io observations include:

• Generate a compositional map of the sunlit portions of Io
• Make time-resolved measurements of reflected brightness at different wavelengths and phase angles
• Observe integral phase curves of Io
• Measure the auroral emissions produced by the bombardment of magnetospheric particles into the atmosphere of Io (see the magnetosphere section).

Europa, Ganymede and Callisto Observations

Observation goals for the primary icy Galilean satellites include:

• Imaging infrared spectroscopy (Ralph instrument, LEISA component) of Europa, Ganymede and Callisto at several longitudes for compositional mapping, searches for minority species, and testing the novel analysis techniques (for example, temperature mapping) to be applied to Pluto system observations.
   
• Panchromatic highest resolution imaging (LORRI) of Europa, Ganymede and Callisto at several longitudes.
   
• Re-examination of Europa for possible change detection. The photometric complexities of Europa's surface require similar viewing conditions between observations to increase confidence in results.
   
• Terminator imaging to improve discrimination of topography in areas otherwise seen only at high-Sun illumination by Voyager and Galileo. (The New Horizons flyby will not provide surface resolution improvement for any of the icy satellites over that obtained by either Voyager or Galileo, however.) This will provide an opportunity to map the distribution of the remarkable and poorly understood large depressions in Europa's icy shell, which were not systematically surveyed by Galileo.

LEISA observations of the Galilean satellites are especially valuable, since its spectral resolution is so much higher than any previous imaging spectrometer on a spacecraft visiting Jupiter.

One of the most intriguing mysteries to emerge from Galileo Near-Infrared Mapping Spectrometer (NIMS) exploration of Europa involves the composition of "non-ice" material(s) on Europa's surface. This surface component could sample Europa's interior ocean, offering a probe of that astrobiologically interesting environment. Or, it could result from radiolysis in the harsh charged-particle environment of the Jovian magnetosphere. Some combination of the two seems most likely, since non-ice material is concentrated on Europa's trailing hemisphere, which receives the highest magnetospheric radiation doses, and is also concentrated along linea and chaos regions, where oceanic materials are expected to have reached the surface most recently (according to some theoretical models). More than one type of non-ice material may be involved.

NIMS had inadequate spectral resolution to identify the composition of the non-ice material, nor was it able to distinguish between non-ice materials broadly distributed over the trailing hemisphere and the non-ice materials associated with linea. LEISA can do a lot better, in terms of spectral resolution, within its wavelength range. When New Horizons is close enough to exceed the spatial resolution achievable with adaptive optics from Earth-based observatories (about 300 kilometers per pixel), it will offer unique capabilities for identifying the non-ice materials, exploring their compositional variability, and mapping their spatial distribution.

Non-ice materials are also present on Ganymede's trailing hemisphere, and New Horizons will make observations similar to those described above for Europa.

Additionally, a large swath of Ganymede's surface remains completely unexplored by any infrared mapping spectrometer. This region extends westward from about 330 to 90 degrees longitude, corresponding roughly to the sub-Jovian hemisphere. Since these longitudes will be oriented toward New Horizons during part of the close approach phase, the team can do the first-ever compositional mapping of the region.

When New Horizons is close enough to do better than about 300 kilometer/pixel resolution with LEISA, it has an opportunity to produce data with better spatial resolution that can be achieved from Earth-based or Earth-orbiting observatories and better spectral resolution than was achieved by Galileo NIMS. These unique capabilities offer the possibility of new discoveries, such as finding isolated regions with peculiar compositions or associating specific compositional characteristics with geological units. Increased understanding of surface processes, such as volatile transport, grain growth, and radiolysis will benefit from the ability to study compositions and textures as functions of latitude, longitude and geological age.

Callisto, and possibly Ganymede, serve as important test cases for the technique of temperature mapping by means of the shapes of temperature-sensitive near-infrared absorption bands. The New Horizons team plans to use this technique at Pluto and Charon. A major difficulty with it is that temperature effects on the albedo spectra must be disentangled from comparable textural and geometric factors. Can the team really do this? Callisto is an excellent test case to address this question. If it encounters problems, finding them in 2007 will enable it to identify what additional modeling capabilities must be developed or laboratory studies must be done prior to the Pluto encounter. Callisto is the best Jovian system test-bed for the technique for several reasons:

• It rotates slowest, so its diurnal temperature variation is largest.
• Its rotation rate is comparable to the rate at which New Horizons passes by, so New Horizons will be able to look at one region at different local times of day.

It has shallower water (H₂O) absorption bands, more like what the team can expect to see on Charon, and possibly on Pluto and its smaller moons, and on KBOs. On Europa, the H₂O bands it would be using at Charon are mostly saturated, so they are not really suitable.

By observing one region of Callisto at different times of day, scientists will see part of the diurnal thermal cycle, and at the same time, get enough data to separate effects due to texture, varying viewing and illumination geometry, and possible vertical compositional or textural gradients — all of which can potentially throw off the process of extracting temperatures.

New Horizons' visible and ultraviolet wavelength instruments offer valuable scientific opportunities for the Galilean satellites as well.

New Horizons can add to our understanding of the topography of Europa, despite the relatively low spatial resolution compared to most Galileo images. In addition to the ubiquitous ridges and chaos regions, Europa's surface is crossed by a series of mysterious low-relief (about 500 meters deep), large scale (about 50 kilometers wide) troughs and depressions, which are superposed on the finer-scale topography and may form a global network of roughly circular features. The large size of these features means they can be seen in low-spatial-resolution images, but only very close to the terminator. Thus, a global inventory was not possible from Galileo images, which had limited longitudinal coverage. New Horizons can obtain extensive longitudinal near-terminator coverage with the Long Range Reconnaissance Imager (LORRI), and can thus significantly improve our knowledge of these features.

Global LORRI images of Ganymede and Callisto will complement Galileo image coverage, and in Ganymede's case be used to search for deviations from the satellite's expected triaxial shape. Such may confirm the Galileo radio tracking discovery of a mass anomaly in Ganymede's northern polar region.

Europa's very tenuous oxygen atmosphere can also be studied by New Horizons, via disk-integrated Alice observations of OI emission lines at 1304 and 1356 Angstroms produced by electron impact dissociation of molecular oxygen. These lines have been studied from Hubble and Cassini, but much remains to be learned, due to the limited spatial and temporal sampling of existing data. Observations at different longitudes dominated by either ice or non-ice surface components will shed light on the interaction between the atmosphere and the surface, and observations at different Jupiter magnetic longitudes will illuminate the excitation processes that render the atmosphere visible.