New Horizons could perform five types of observations during the Jupiter flyby:

1) Ring phase curve observations
2) High-resolution imaging of the Jovian ring
3) Search for inner satellites
4) Ring plane crossing observations
5) Studies of quadrant asymmetries

Ring Phase Curve

Two open questions about the Jovian ring system are: Where are the parent bodies in the system? How many are there?

The ring is composed primarily of micron-sized dust, but tiny grains are quickly lost from the system and must be replenished regularly from some embedded source. The two moons Adrastea and Metis certainly play a role, but other parent bodies that are meters to kilometers in size are probably also present. The key to answering these issues is in observations taken from a direction when the Sun is directly overhead, that is, with images taken at low phase angles.

The only direct evidence for additional parent bodies comes from a few low-phase, high-resolution ring images obtained by the Galileo spacecraft. These images show very different radial structure in the ring than the high-phase (dust-dominated) images show, suggesting that the parent bodies have a different radial distribution from the dust. The parent bodies appear to be concentrated primarily between Adrastea and Metis, the two embedded moons that may serve as "shepherds" for this population.

Several spacecraft and instruments have obtained phase curves of the main Jovian ring, but these results are radially unresolved. The unique science that New Horizons can obtain is the first radially resolved phase curve for the ring. This will enable us, for the first time, to distinguish unambiguously the locations of the dust (forward-scattering) and parent (backscattering) populations.

High-Resolution Imaging of the Jovian Ring

Dynamical interactions between particles create the remarkable fine structures seen in Saturn's rings. Presumably, similar processes happen among the parent bodies in Jupiter's rings, but the system's detailed radial structure has not been well observed.

The finest resolution ever obtained on the Jovian ring was 6 kilometers per pixel, in a small set of Galileo Solid State Imaging (SSI) pictures from the G28 encounter. However, these images have very low signal-to-noise ratio and are corrupted by numerous charged particle hits, the result of being so deep inside Jupiter's magnetosphere. Furthermore, only the tip of the rings was imaged at this resolution. Because of these limitations, structure in the Jovian ring on scales below about 100 kilometers has not been detected.

Search for Inner Satellites

The smallest known inner satellite of Jupiter is Adrastea, with a radius of 8 kilometers. It orbits near the outer tip of the Jovian ring. A radius of 8 kilometers is also the detection limit from the Cassini flyby. Recent Hubble Space Telescope images would be able to detect somewhat smaller moons (about 5 kilometers) but coverage is very incomplete. Searching for other moons will provide important information about the gap in size range between the ring moons and the largest embedded parent bodies. "Shepherding" by smaller embedded moons might also explain some of the additional radial structures hinted at in the finest-resolution Galileo images.

A comprehensive search for small inner moons is a unique capability of New Horizons.

Ring Plane Crossing

Several interesting observations can be carried out only when the Jovian ring is nearly or exactly edge-on to the observer:

1) The ring's vertical thickness can be measured directly. This number is currently limited to less than 30 kilometers in backscatter but only less than 100 kilometers at high phase. An accurate value might reveal the difference between the thickness of dust particles, which can be perturbed by Jupiter's inclined magnetic field, and the parent bodies that remain equatorial.

2) A few Galileo images showed a "halo bloom," in which the thickness increases with decreasing radius across the main ring. However, this phenomenon was only seen at one phase angle, so it is unclear what range of particle sizes is really affected.

3) Edge-on viewing will provide the best sensitivity to the halo and Gossamer rings, which have not been well detected except at the highest phase angles.

4) Some models predict vertical asymmetries for the main ring and halo, in which the dust is displaced vertically due to perturbations from Jupiter's magnetic field. However, no available data is suitable to test this prediction.

5) Within a few tenths of a degree of the ring plane, Galileo revealed a set of vertical "ripples," or bending waves in the main ring. No known resonances fall in the region, so the features remain completely unexplained. The features appear to be fairly regular, with wavelengths of about 1,000 kilometers. New observations could provide more details about their locations, amplitude and rotational dependencies.

Longitudinal Asymmetry

Every observation of the main ring to date has shown brightness variations with longitude. These often take the form of a quadrant asymmetry, where one arm of the ring is brighter than the other. However, there are inconsistencies about which arm is brighter. This phenomenon has no accepted explanation; one possibility is that non-spherical grains receive a preferential orientation due to Jupiter's magnetic field.