[show abstract][hide abstract] ABSTRACT: The dusty jovian ring system must be replenished continuously from embedded source bodies. The New Horizons spacecraft has performed a comprehensive search for kilometer-sized moons within the system, which might have revealed the larger members of this population. No new moons were found, however, indicating a sharp cutoff in the population of jovian bodies smaller than 8-kilometer-radius Adrastea. However, the search revealed two families of clumps in the main ring: one close pair and one cluster of three to five. All orbit within a brighter ringlet just interior to Adrastea. Their properties are very different from those of the few other clumpy rings known; the origin and nonrandom distribution of these features remain unexplained, but resonant confinement by Metis may play a role.
[show abstract][hide abstract] ABSTRACT: New Horizons obtained 400 ring images of the Jovian ring system using the Long Range Reconnaissance Imager (LORRI). This camera has a broad bandpass spanning wavelengths lambda = 0.35 to 0.85 µm. The ring was imaged at phase angles 7°-159°. In addition, one sequence of near-IR spectra (lambda = 1.25 to 2.5 µm) was obtained by the Linear Etalon Imaging Spectral Array (LEISA) for compositional studies. Two ring rotation movies during Jupiter approach were used to search for small moons embedded within the system. These bodies might serve as source bodies for the prevalent ring dust. No moons were detected down to a threshold of 500 m radius, suggesting a sharp cutoff in the population of inner Jovian moons below 8-km Adrastea. Although this search focused on the main Jovian ring, any 1-km moons from orbital radius r = 100,000 km to beyond the orbit of Amalthea (r = 181,000 km) should have been detected multiple times. More surprisingly, the ring revealed two clusters of tiny clumps, one pair and one set of three. These are definitively not moons because they have longitudinal extents of a few tenths of a degree. Separations between clumps are 2 to 4° but are not uniform. These clump families both orbit within a brightness peak just interior to the orbit of Adrastea, at r = 128,740 km. Their origin is unknown. They are not visible at high phase angles, indicating that they are composed primarily of larger "parent'' bodies, not dust. They are definitely not related to a clump detected in Cassini images of the Jovian ring from December 2000, indicating that at least some ring clumps are transient. The large quadrant asymmetries reported in earlier images from Voyager and Galileo are completely absent in the new data.
[show abstract][hide abstract] ABSTRACT: New Horizons passed Jupiter on February 28, 2007, en route to its Pluto
encounter in 2015. During the flyby, the spacecraft made extensive
observations of Jovian system. We will discuss preliminary conclusions
from the 300 ring images obtained by the Long Range Reconnaissance
Imager (LORRI). This camera has a broad bandpass spanning wavelengths
λ = 0.35 to 0.85 μm. The ring was imaged at phase angles
α = 7∘ to 159°. In addition, one sequence of near-IR
spectra (λ = 1.25 to 2.5 μm) was obtained by the Linear Etalon
Imaging Spectral Array (LEISA).
Primary science goals were as follows. (1) A comprenhensive search for
embedded moons as small as 1 km, to better characterize the sources of
the prevalent dust. This was accomplished via two complete rotation
movies during approach. (2) Acquisition of a radially-resolved phase
curve. Because μ-sized dust forward-scatters but the parent bodies
backscatter, changes in the radial profile with α will allow us to
decouple the locations and properties of each population. (3) Better
characterization of the mysterious asymmetries, "patchiness" and
"ripples" noted in some Voyager and Galileo images. This entailed
duplicated observations and "movies" of the system. (4) Studies of the
vertical distribution of dust in the inner "halo" and outer "gossamer"
rings. These observations occurred during the high-phase viewing
opportunities late in the encounter, including edge-on observations when
New Horizons crossed the ring plane at α = 140°.
To date, only a fraction of the data set has been downlinked.
Nevertheless, the images obtained so far are of extremely high quality.
Metis and Adrastea are extremely bright in the approach phase movies,
suggesting that our goal of detecting much smaller embedded moons is
achievable. However, some of the long-exposure halo and gossamer ring
images may have been lost due to saturation.