[1] We have used multispectral images from Clementine and data from Lunar Prospector's magnetometer to conduct a survey of lunar crustal magnetic anomalies, prominent lunar swirls, and lesser known swirl markings to provide new information on the nature of swirls and their association with magnetic anomalies. We find that all swirls and swirl-like albedo patterns are associated with areas of magnetized crust, but not all areas of magnetized crust are colocated with swirl-like albedo anomalies. All observed swirls exhibit spectral characteristics similar to immature material and generally have slightly lower FeO values compared with their surroundings as determined with a multispectral iron-mapping method. We discuss these results in relation to the various hypotheses for swirl formation. The comet impact hypothesis for lunar swirls would not predict a difference in the spectrally determined FeO content between swirls and nearby ordinary surfaces. The compositional difference could be explained as a consequence of (1) magnetic shielding of the surface from the solar wind, which could produce anomalous space weathering (little darkening with limited reddening) and potentially alter the predictions of the multispectral iron-mapping algorithm while the compositional contrast could be enhanced by delivery of lower-FeO ejecta from outside the swirl; and (2) accumulation of fine plagioclase-rich dust moving under the influence of electric fields induced by solar wind interactions with a magnetic anomaly. Therefore, we cannot at present clearly distinguish between the solar wind shielding and electrostatic dust accumulation models for swirl formation. We describe future measurements that could contribute to solution of the puzzle of swirl origin.

[1] Observations of rayed craters at optical and radar wavelengths provide insight into the processes that lead to ray formation and degradation on terrestrial planets. We have compared optical and S-Band radar data for several large (> 20 km diameter), young craters on the Moon and Mercury and find evidence that secondary cratering plays a significant role in the formation of crater rays. Regions where rays appear bright to optical and radar sensors correspond to dense concentrations of secondary craters, and the observed radar enhancement appears to be a result of the deposition of blocky, immature ejecta from the secondary craters and/or the rocky, immature interior walls of the secondary craters. We define a new optical maturity index for Mercury and find that rays in radar and optical images correspond closely, indicating that the rays are rich in centimeter-to decimeter-sized clasts. Rays on the Moon are less prominent at radar wavelengths, suggesting that they are currently composed of smaller clasts, centimeter sized or less. This difference suggests that secondary craters are larger on Mercury and capable of excavating more decimeter-sized clasts. Furthermore, observations of rayed craters provide an opportunity to assign relative ages to the youngest craters on the Moon and Mercury. Although rayed craters on Mercury appear most similar to the youngest craters on the Moon, the apparent ages are more likely influenced by inherent differences in impact velocity, surface gravitational acceleration, and target properties that result in the formation of larger secondary craters on Mercury.

We investigate the depth, variability, and history of regolith on asteroid Vesta using data from the Dawn spacecraft. High‐resolution (15–20 m pixel−1) Framing Camera images are used to assess the presence of morphologic indicators of a shallow regolith, including the presence of blocks in crater ejecta, spur‐and‐gully–type features in crater walls, and the retention of small (<300 m) impact craters. Such features reveal that the broad, regional heterogeneities observed on Vesta in terms of albedo and surface composition extend to the physical properties of the upper ~1 km of the surface. Regions of thin regolith are found within the Rheasilvia basin and at equatorial latitudes from ~0–90°E and ~260–360°E. Craters in these areas that appear to excavate material from beneath the regolith have more diogenitic (Rheasilvia, 0–90°E) and cumulate eucrite (260–360°E) compositions. A region of especially thick regolith, where depths generally exceed 1 km, is found from ~100–240°E and corresponds to heavily cratered, low‐albedo surface with a basaltic eucrite composition enriched in carbonaceous chondrite material. The presence of a thick regolith in this area supports the idea that this is an ancient terrain that has accumulated a larger component of exogenic debris. We find evidence for the gardening of crater ejecta toward more howarditic compositions, consistent with regolith mixing being the dominant form of “weathering” on Vesta.

Silicate glasses are an important constituent in the regolith of airless planetary bodies, and knowledge of glass reflectance characteristics is important for remote-sensing studies of the Moon, Mercury, and asteroids. We recovered reflectance spectra for 20 vacuum-melted lunar glass simulants measured by Wells (1977), which cover a wider range of Fe and Ti contents (0-17.5 wt % FeO and 0-15 wt % TiO 2 ) and a wider wavelength range than those of the better-known Bell et al. (1976) study. We examine the spectra in terms of known absorptions, explore the relationship between ultraviolet spectral parameters and composition, and apply the Hapke radiative transfer model to predict the reflectance spectra of the Wells glasses. The imaginary part of the refractive index (k) at each wavelength was computed based on the Ti and Fe composition using the linear relationship presented by Wilcox et al. (2006) and with a new linear-exponential hybrid relationship. Comparison of the model spectra with the measured spectra reveals that the samples rich in Fe and Ti are best modeled by the linear relationship, because the linear model was developed using the Fe-and/or Ti-rich Bell et al. (1976) glasses. For Fe-and Ti-poor glasses, the hybrid model provides a better fit to the measured spectra, because this model for k is based on the wider compositional range of the Wells glasses. In the future, better linear model fits might be obtained if optical parameters were derived for a wider compositional range, from low-Fe/low-Ti to the higher-Fe/higher-Ti compositions of Apollo volcanic glasses.