Voyager 2 at Neptune: Imaging Science Results

Abstract

Voyager 2 images of Neptune reveal a windy planet characterized by bright clouds of methane ice suspended in an exceptionally clear atmosphere above a lower deck of hydrogen sulfide or ammonia ices. Neptune's atmosphere is dominated by a large anticyclonic storm system that has been named the Great Dark Spot (GDS). About the same size as Earth in extent, the GDS bears both many similarities and some differences to the Great Red Spot of Jupiter. Neptune's zonal wind profile is remarkably similar to that of Uranus. Neptune has three major rings at radii of 42,000, 53,000, and 63,000 kilometers. The outer ring contains three higher density arc-like segments that were apparently responsible for most of the ground-based occultation events observed during the current decade. Like the rings of Uranus, the Neptune rings are composed of very dark material; unlike that of Uranus, the Neptune system is very dusty. Six new regular satellites were found, with dark surfaces and radii ranging from 200 to 25 kilometers. All lie inside the orbit of Triton and the inner four are located within the ring system. Triton is seen to be a differentiated body, with a radius of 1350 kilometers and a density of 2.1 grams per cubic centimeter; it exhibits clear evidence of early episodes of surface melting. A now rigid crust of what is probably water ice is overlain with a brilliant coating of nitrogen frost, slightly darkened and reddened with organic polymer material. Streaks of organic polymer suggest seasonal winds strong enough to move particles of micrometer size or larger, once they become airborne. At least two active plumes were seen, carrying dark material 8 kilometers above the surface before being transported downstream by high level winds. The plumes may be driven by solar heating and the subsequent violent vaporization of subsurface nitrogen.

Neptune’s ring arcs: VLT/NACO near-infrared observations and a model to explain their stability

“…Assuming a density of 1 g cm −3 , this corresponds to a radius ∼5.2 km for m 1 and between 1.1 and 1.4 km for the other co-orbital bodies. This is compatible with the Voyager data that exclude undetected satellites of radius larger than 6 km (Smith et al 1989;Porco et al 1995). The initial geometric semi-major axis is a = 62 932.7 km, that is, 0.25 km outside the 42:43 CIR (resonant argument Fig.…”

“…The ring mass determined to shift the CER to the arcs' position, 0.002 Galatea's mass, assumes an eccentricity of 10 −6 for Galatea and would correspond to a small satellite of 10 km in radius (for a density of 1 g cm −3 ). The mass required in this model cannot be contained in a single body since Voyager data excluded undetected satellites of radius larger than 6 km (Smith et al 1989;Porco et al 1995). The exact origin of the small residual orbital eccentricity of Galatea, consistent with a forcing by Adams ring's small mass, has to be determined too.…”

“…The brightness longitudinal profiles are given in equivalent width, that is, the width of a perfect Lambert diffuser that would reflect sunlight at the distance of Neptune. Following earlier studies of Neptune's rings (Smith et al 1989;Porco et al 1995), the equivalent width is defined by E(λ) = μ I(λ)/F(λ)dr, where I(λ) is the observed flux reflected from the arcs, πF(λ) is the incident solar flux, λ is the wavelength, and μ is the cosine of the emission angle with respect to the ring-plane normal.…”

“…Four arcs (Courage, Liberté, Egalité, Fraternité) confined in a 40 degrees azimuthal range are embedded in the much fainter continuous Adams ring around Neptune (Smith et al 1989). These incomplete rings have been stable since their discovery in 1984 (Hubbard et al 1986), while they should be destroyed in a few months through differential Keplerian motion.…”

Does Europa have a subsurface ocean? Evaluation of the geological evidence

“…We note that a tantalizing dim "haze" seems to exist above Europa's limb in one high-phase El9 Galileo image (s484888253); however, our analysis shows that this is a false double-exposure rather than a real phenomenon, probably resulting from incomplete closing of the camera shutter blades in combination with an imaging mode that does not include a preexposure erasure of the CCD array. Stereo observations offer another way to detect active venting, and this is the way Triton's plumes were discovered [Smith et al, 1989]. No such evidence has been seen from stereo views of Europa, but a systematic search has not yet been completed.…”