Stephen M. Larson

Structure and particle properties of Saturn's E Ring

We have systematically reanalyzed a large subset of the available photometric data on Saturns E Ring, including Voyager images and star tracker data, Earth-based CCD images, photometry, and spectrophotometry. In order to compare data sets with vastly different vi viewing geometries on this broad, vertically extended ring, we have calibrated every observation based on a common three-dimensional model. The ring shows a density peak at a location indistinguishable from the Enceladus orbit, with a radial offset of <3000 km. We have found that a very simple power-law model describes the rings normal optical depth profile with orbital radius. The ring shows a general increase in vertical thickness with distance from the planet, ranging from 6000 km FWHM at its inner bound to nearly 40, 000 km at its outer. However, this trend is violated near the density peak, where a localized decrease in thickness by ∼30% is observed. The ring is also displaced northward from the planets equatorial plane by 310 ± 25 km in one set of Voyager images, although the overall nature of the rings vertical asymmetry is unclear. No power-law size distribution is compatible with the available photometry. A narrow distribution of slightly nonspherical particles of radius 1.0 ± 0.3 μm provides the best fit to the data. This highly peculiar size distribution clearly indicates that the ring does not originate from collisional or disruptive processes, and is therefore unlike any other known ring. Hence, we can give some credence to the possibility that the E Ring originates in “geyser-like” eruptions from the surface of Enceladus. Depending on the precise particle size chosen, the rings peak normal optical depth is found to be 1.5(±0.4) × 10−5, corresponding to a geometric cross-section per unit area of 5.3(±1.3) × 10−5. Macroscopic bodies comprise less than 1% of the rings optical depth.

Observations of Saturn's Ring-Plane Crossings in August and November 1995

Observations of Saturns ring system with the Hubble Space Telescope during the 10 August 1995 Earth crossing and the 17 to 21 November 1995 solar crossing indicate that the F ring dominates their apparent edge-on thickness of 1.2 to 1.5 kilometers. The F ring is slightly inclined with respect to the A ring, which may explain the approximately 50-minute difference in apparent crossing times for the east and west ring ansae in August. Prometheus lags its predicted position by about 19 degrees in longitude. The faint G ring is neutral or reddish in color and is confined to a radial range of 2.72 to 2.85 Saturn radii. The broad, distinctly blue E ring flares outward to a maximum thickness of about 15,000 kilometers at 7.5 Saturn radii and appears to have a spatially uniform particle size distribution.

Remote sensing of potential lunar resources: 1. Near‐side compositional properties

New telescopic CCD multispectral imaging of the lunar near side and 330–870 nm spectroscopy of selected regions are presented as aids in estimating compositional differences relevant to locating potential lunar resources such as ilmenite (FeTiO3) and solar wind-implanted 3He and H. Conversion of 400/560 nm CCD ratio images to weight percent TiO2 using a version of the Charette et al. (1974) empirical relation allows construction of a new TiO2 abundance map for the lunar maria accurate to ±2 wt % HO2. This map provides a consistent view of TiO2 distribution among mature mare soils and can be used to estimate areas potentially rich in ilmenite. Western Mare Tranquillitatis exhibits the highest TiO2 abundances (>8 wt %), followed by regions near Flamsteed P and in northern Oceanus Procellarum. A 950/560 nm CCD ratio mosaic of the full Moon provides estimates of relative surface maturity on local scales as defined by the degree of agglutinate production due to micrometeorite impacts. Since high 3He concentrations are correlated with mature ilmenite-rich soils, a combination of relative surface maturity maps and TiO2 abundance maps may be useful for estimating 3He (and possibly H) distribution on local scales. While dark mantle materials are also potential sources of ilmenite, 3He, and H, their compositional differences from mare soils prevent accurate remote mapping of their TiO2 abundance and relative maturity. Reflectance spectra (relative to a more rigorously defined MS-2 standard region) of dark mantle materials and high-TiO2 mare areas exhibit previously undocumented near-UV (<350 nm) changes in spectral slope apparently related to compositional variations. Determination of the primary lunar sites for resource utilization will be dictated by the future goals and priorities of the lunar resource program.

Spatially resolved methane band photometry of Saturn. I - Absolute reflectivity and center-to-limb variations in the 6190-, 7250-, and 8900-A bands

Abstract Spatially resolved measurements of Jupiters absolute reflectivity in methane bands at 6190, 7250, and 8900 A and nearby continuum regions are presented. The data were obtained with a 400 × 400 pixel charge-coupled device (CCD) at the 1.54-m Catalina telescope near Tucson, Arizona. Jupiter was imaged on the CCD through narrow-band interference filters. Photometric standard stars were also measured. Calibration data were obtained to remove instrumental effects. Uncertainty in the absolute reflectivity is ±8%. Uncertainty in the relative (across the disk) reflectivity is 1 or 2%. Uncertainty in the geomtry is ±1 pixel (0.22 arcsec) for centering and ±1% in scale. Intensity and scattering geometry are tabulated for points across 10 axisymmetric cloud bands and the Great Red Spot. Because of their high spatial, photometric, and time resolution, these data provide strong constraints on models of the Jovian cloud structure.

DISINTEGRATING ASTEROID P/2013 R3

Splitting of the nuclei of comets into multiple components has been frequently observed but, to date, no main-belt asteroid has been observed to break up. Using the Hubble Space Telescope, we find that main-belt asteroid P/2013 R3 consists of 10 or more distinct components, the largest up to 200?m in radius (assumed geometric albedo of 0.05) each of which produces a coma and comet-like dust tail. A diffuse debris cloud with total mass ~2 ? 108?kg further envelopes the entire system. The velocity dispersion among the components, ?V ~?0.2-0.5?m s?1, is comparable to the gravitational escape speeds of the largest members, while their extrapolated plane-of-sky motions suggest a break up between 2013 February and September. The broadband optical colors are those of a C-type asteroid. We find no spectral evidence for gaseous emission, placing model-dependent upper limits to the water production rate ?1?kg s?1. Breakup may be due to a rotationally induced structural failure of the precursor body.

Remote sensing of potential lunar resources: 2. High spatial resolution mapping of spectral reflectance ratios and implications for nearside mare TiO2 content

High spatial resolution maps illustrating variations in spectral reflectance 400/560 nm ratio values have been generated for the following mare regions: (1) the border between southern Mare Serenitatis and northern Mare Tranquillitatis (including the MS-2 standard area and Apollo 17 landing site), (2) central Mare Tranquillitatis, (3) Oceanus Procellarum near Seleucus, and (4) southern Oceanus Procellarum around Flamsteed. We have also obtained 320–1000 nm reflectance spectra of several sites relative to MS-2 to facilitate scaling of the images and provide additional information on surface composition. Inferred TiO2 abundances for these mare regions have been determined using an empirical calibration which relates the weight percent TiO2 in mature mare regolith to the observed 400/560 nm ratio. Mare areas with high TiO2 abundances are probably rich in ilmenite (FeTiO3) a potential lunar resource. The highest potential TiO2 concentrations we have identified in the nearside maria occur in central Mare Tranquillitatis. Inferred TiO2 contents for these areas are >9 wt % and are spatially consistent with the highest-TiO2 regions mapped previously at lower spatial resolution. We note that the morphology of surface units with high 400/560 nm ratio values increases in complexity at higher spatial resolutions. New telescopic spectra of landing sites successfully reproduce the Charette relation, although we find that the 400/560 nm value is strongly a fimction of the sample area size. The increased spectral contrast of the 400/730 nm ratio over the 400/560 nm ratio demonstrates the potential increased precision with which the 400/730 nm ratio might be used to estimate TiO2 abundances. Comparisons have been made with previously published geologic maps, Lunar Orbiter IV, and ground-based images, and some possible morphologic correlations have been found between our mapped 400/560 nm ratio values and volcanic landforms such as lava flows, mare domes, and collapse pits.

Hubble Space Telescope Observations of Comet P/Shoemaker-Levy 9 (1993e)

The Hubble Space Telescope observed the fragmented comet P/Shoemaker-Levy 9 (1993e) (P indicates that it is a periodic comet) on 1 July 1993. Approximately 20 individual nuclei and their comae were observed in images taken with the Planetary Camera. After subtraction of the comae light, the 11 brightest nuclei have magnitudes between ∼23.7 and 24.8. Assuming that the geometric albedo is 0.04, these magnitudes imply that the nuclear diameters are in the range ∼2.5 to 4.3 kilometers. If the density of each nucleus is 1 gram per cubic centimeter, the total energy deposited by the impact of these 11 nuclei into Jupiters atmosphere next July will be ∼4 x 1030 ergs (∼108 megatons of TNT). This latter number should be regarded as an upper limit because the nuclear magnitudes probably contain a small residual coma contribution. The Faint Object Spectrograph was used to search for fluorescence from OH, which is usually an excellent indicator of cometary activity. No OH emission was detected, and this can be translated into an upper limit on the water production rate of ∼2 x 1027 molecules per second.

The Extraordinary Multi-tailed Main-belt Comet P/2013 P5

Hubble Space Telescope observations of main-belt comet P/2013 P5 reveal an extraordinary system of six dust tails that distinguish this object from any other. Observations two weeks apart show dramatic morphological change in the tails while providing no evidence for secular fading of the object as a whole. Each tail is associated with a unique ejection date, revealing continued, episodic mass loss from the 0.24 ± 0.04 km radius nucleus over the last five months. As an inner-belt asteroid and probable Flora family member, the object is likely to be highly metamorphosed and unlikely to contain ice. The protracted period of dust release appears inconsistent with an impact origin, but may be compatible with a body that is losing mass through a rotational instability. We suggest that P/2013 P5 has been accelerated to breakup speed by radiation torques.

A reevaluation of spectral ratios for Lunar Mare TiO2 mapping

The empirical relation established by Charette et al. [1974] between the 400/560 nm spectral ratio of mature mare soils and weight percent TiO2 has been used extensively to map titanium content in the lunar maria [e.g., Johnson et al., 1977; Johnson et al., 1991b]. Relative reflectance spectra of mare regions show that a reference wavelength further into the near-IR, e.g., > 700 nm, could be used in place of the 560 nm band to provide greater contrast (a greater range of ratio values) and hence a more sensitive indicator of titanium content. An analysis of 400/730 nm ratio values derived from both laboratory and telescopic relative reflectance spectra suggests that this ratio provides greater sensitivity to TiO2 content than the 400/560 nm ratio. The increased range of ratio values is manifested in higher contrast 400/730 nm ratio images compared to 400/560 nm ratio images. This potential improvement in sensitivity encourages a reevaluation of the original Charette et al. [1974] relation using the 400/730 nm ratio.

Multi-cycle fluorescence: Application to S2 in comet IRAS-Araki-Alcock 1983VII

Abstract Fluorescence calculations have been made to analyze the B-X system of S 2 which appeared in IUE spectra of Comet IRAS-Araki-Alcock 1983VII. Single- and multiple-cycle fluorescence calculations indicate that fluorescent equilibrium accounts for the observed S 2 spectra despite the fact that the S 2 lifetime against solar ultraviolet radiation is relatively short. This analysis confirms that emission peaks in the 3000–4000 A range of ground-based spectra are due to the B-X bands of S 2 . The fluorescence calculation indicates that the previous S 2 production rates should be reduced at least a factor of two. We also argue that collisional effects on the S 2 spectra by neutrals, electrons, and ions are negligible compared with the effects of pure fluorescence.