M. J. Gaffey

The dark side of Iapetus

Abstract This paper presents new photometric and spectrophotometric observations of the dark (leading) hemisphere of Saturns satellite Iapetus. Spectrophotometry from 0.3–1.0 um (May 1979) shows the dark hemisphere to be very red, similar to a few asteroids and the Moon, but with no spectral features attributable to olivine or pyroxene. Near-infrared spectrophometry in the regions 1.4–2.5 um (May 1981) and 3.0–3.8 um (February 1981) reveals water ice absorption bands, probably resulting from the polar caps intruding onto the dark hemisphere. The reflectance of Iapetus is unlike that of carbonaceous chondrites or C-type asteroids and most closely resembles the reflectance (and low albedo) of carbonaceous (organic) residue from the Murchison C2 carbonaceous chondrite. The Murchison material has the same red slope and a probable spectral feature near 0.6 um seen in Iapetus data. Three hypotheses for the formation of the dark hemisphere are discussed in light of the observational data. The favored hypothesis is that debris from Phoebe or other unknown outer satellites of Saturn impacts the dark hemisphere of Iapetus as Poynting-Robertson drag causes the debris to spiral toward Saturn. The high-velocity impacts preferentially remove ice from the satellites surface, causing enrichment of included carbonaceous material intrinsic to Iapetus. The reflectance of Phoebe itself is significantly different from that of Iapetus, suggesting that relatively little Phoebe debris lies on the dark hemisphere. There remains the possibility that the impacting debris originates from another body of composition similar to the Murchison residue and that this material is exposed on the surface of Iapetus.

Distinctive space weathering on Vesta from regolith mixing processes

The surface of the asteroid Vesta has prominent near-infrared absorption bands characteristic of a range of pyroxenes, confirming a direct link to the basaltic howardite–eucrite–diogenite class of meteorites. Processes active in the space environment produce ‘space weathering’ products that substantially weaken or mask such diagnostic absorption on airless bodies observed elsewhere, and it has long been a mystery why Vesta’s absorption bands are so strong. Analyses of soil samples from both the Moon and the asteroid Itokawa determined that nanophase metallic particles (commonly nanophase iron) accumulate on the rims of regolith grains with time, accounting for an observed optical degradation. These nanophase particles, believed to be related to solar wind and micrometeoroid bombardment processes, leave unique spectroscopic signatures that can be measured remotely but require sufficient spatial resolution to discern the geologic context and history of the surface, which has not been achieved for Vesta until now. Here we report that Vesta shows its own form of space weathering, which is quite different from that of other airless bodies visited. No evidence is detected on Vesta for accumulation of lunar-like nanophase iron on regolith particles, even though distinct material exposed at several fresh craters becomes gradually masked and fades into the background as the craters age. Instead, spectroscopic data reveal that on Vesta a locally homogenized upper regolith is generated with time through small-scale mixing of diverse surface components.

Rotational spectral variations of asteroid (8) Flora: Implications for the nature of the S-type asteroids and for the parent bodies of the ordinary chondrites

Abstract The new interpretive calibrations (olivine-pyroxene abundance, spectral signature of chondritic metal grains) have been applied to visible and near-infrared spectral reflectance data for the typical S-type asteroid, (8) Flora. Chondritic metal grains do not exhibit the steeply reddened spectral signature previously associated with a nickel-iron (NiFe) metal phase. The conditions required to produce a S-type spectral curve from a chondritic material are much more restrictive and improbable than previously supposed. Five mineralogic and patrologic parameters have been derived for the surface material of Flora from the analysis of the spectral reflectance and rotational spectral variations of this object. These include bulk mineralogy (NiFe metal, olivine, pyroxene), mafic mineral abundance (ol/py ≈ 2.8), metal nature [abundant (≈50%) and coarse-grained in the substrate], pyroxene composition (orthopyroxene > F s 50 and/or abundant clinopyroxene), and spatial mineralogic variations (olivine/pyroxene ratio and pyroxene composition do not covary in a chondritic manner). All of the diagnostic mineralogic properties of the Flora surface material indicate that this is a differentiated body. None of the derived surface properties unambiguously support a chondritic-type material and several contradict any type of undifferentiated assemblage. Flora is most probably the residual core of an intensely heated, thermally evolved, and magmatically differentiated planetesimal which was subsequently disrupted. The present surface samples layers formed at and near the core-mantle boundary in the parent body. Several lines of evidence suggest that the S-type asteroids as a class are predominantly the core fragments of disrupted, differentiated planetesimals. A main belt source of the ordinary chondrites remains elusive.

Mineralogical-petrological characterization of near-earth asteroids

Studies of near-Earth asteroids are aimed at determining their dynamical and structural history. The mineralogy and petrology of 17 near-Earth asteroids are characterized using reflectance spectroscopy with ground-based telescopes as one method to address their major issues. Implications for the origin and evolution are discussed in a separate paper. Assuming the surfaces are composed of cosmically abundant materials, the presence of certain mineralogical species can be determined from diagnostic absorption features and spectral characteristics which have been studied under known laboratory conditions and understood in terms of crystal field theory. With one possible exception, the surface composition of near-Earth asteroids consists of common rock-forming minerals such as olivine, pyroxene, and phyllosilicates. Opaque components are present but cannot be mineralogically identified with existing experimental data. The spectrum of 2201 Oljato cannot be interpreted in terms of common rock-forming minerals. This spectrum was examined for cometary features because its high orbital eccentricity suggests a possible relation to comets. No common cometary features are identified in its spectrum. The predominance of mafic silicate absorption features in spectra of near-Earth asteroids compared to the majority of main-belt asteroids may be a primary compositional feature or may be the signature of relatively fresher asteroid material.

The composition and origin of the Iapetus dark material

Abstract Telescopic observations, laboratory simulations, and photogeological studies have been conducted to investigate the composition of the dark material on Iapetus and the reasons for its peculiar distribution. The results of our earlier studies ( D.P. Cruikshank, J.F. Bell, M.J. Gaffey, R.H. Brown, R. Howell, C. Beermen, and M. Rognstad, 1983 , Icarus 53 90–104) are confirmed and extended. Improved telescopic spectra of leading and trailing hemispheres were obtained over the range 0.3–2.6 μm. A mixing model was used to correct the leading-side spectral data for the presence of regions of bright terrain at both poles. The resulting true dark-unit spectrum is much redder in the visible and near-IR than previous uncorrected spectra, but gradually flattens near 2.0 μm. This unusual spectrum is reproduced with an intimate mixture of simulated meteoritic organic polymers (10%) and hydrated silicates (90%) which corresponds to an extension of the compositional trends in known carbonaceous meteorites toward lower formation temperatures. Structures in the dark/bright transition zone visible in the Voyager 2 images are inconsistent with an eruptive origin for the dark material; this zone is probably the region of grazing impacts of dust spiraling in from Phoebe. The dark material is probably a native component of the original icy surface concentrated in a thin, devolatilized regolith produced by the Phoebe-dust bombardment. Similar “ultracarbonaceous” material dominates the surfaces of the D-type asteroids and may be the elusive nonice component in the surfaces of other icy satellites. An Iapetus-like bombardment regime may be the cause of the large hemispheric asymmetry in regolith properties on Callisto.

Near-Earth Asteroids: Possible Sources from Reflectance Spectroscopy

Spectra of near-Earth asteroids were compared to spectra of selected asteroids, planets, and satellites to determine possible source regions. The diversity of reflectance spectra of the near-Earth asteroids implies different mineralogical compositions and hence more than one source region. The presence of near-Earth asteroid spectral signatures similar to those of certain main-belt asteroids supports models that derive some of these asteroids from the 5:2 Kirkwood gap and the Flora family by gravitational perturbations. Planetary and satellite surfaces are different in composition than the near-Earth asteroids, which is in agreement with theoretical arguments that such bodies should not be sources. Some near-Earth asteroids supply portions of Earths meteorite flux, but other sources must also contribute.

The Tucson revised index of asteroid data

Abstract Attention is called to the availability of the TRIAD computer file, a compilation of all reliable physical parameters for minor planets.