Rebecca Rose Ghent

Diviner lunar radiometer observations of cold traps in the moon's south polar region

Watering the Moon About a year ago, a spent upper stage of an Atlas rocket was deliberately crashed into a crater at the south pole of the Moon, ejecting a plume of debris, dust, and vapor. The goal of this event, the Lunar Crater Observation and Sensing Satellite (LCROSS) experiment, was to search for water and other volatiles in the soil of one of the coldest places on the Moon: the permanently shadowed region within the Cabeus crater. Using ultraviolet, visible, and near-infrared spectroscopy data from accompanying craft, Colaprete et al. (p. 463; see the news story by Kerr; see the cover) found evidence for the presence of water and other volatiles within the ejecta cloud. Schultz et al. (p. 468) monitored the different stages of the impact and the resulting plume. Gladstone et al. (p. 472), using an ultraviolet spectrograph onboard the Lunar Reconnaissance Orbiter (LRO), detected H2, CO, Ca, Hg, and Mg in the impact plume, and Hayne et al. (p. 477) measured the thermal signature of the impact and discovered that it had heated a 30 to 200 square-meter region from ∼40 kelvin to at least 950 kelvin. Paige et al. (p. 479) mapped cryogenic zones predictive of volatile entrapment, and Mitrofanov et al. (p. 483) used LRO instruments to confirm that surface temperatures in the south polar region persist even in sunlight. In all, about 155 kilograms of water vapor was emitted during the impact; meanwhile, the LRO continues to orbit the Moon, sending back a stream of data to help us understand the evolution of its complex surface structures. A controlled spacecraft impact into a crater in the lunar south pole plunged through the lunar soil, revealing water and other volatiles. Diviner Lunar Radiometer Experiment surface-temperature maps reveal the existence of widespread surface and near-surface cryogenic regions that extend beyond the boundaries of persistent shadow. The Lunar Crater Observation and Sensing Satellite (LCROSS) struck one of the coldest of these regions, where subsurface temperatures are estimated to be 38 kelvin. Large areas of the lunar polar regions are currently cold enough to cold-trap water ice as well as a range of both more volatile and less volatile species. The diverse mixture of water and high-volatility compounds detected in the LCROSS ejecta plume is strong evidence for the impact delivery and cold-trapping of volatiles derived from primitive outer solar system bodies.

Focused 70-cm Wavelength Radar Mapping of the Moon

We describe new 70-cm wavelength radar images of the lunar near-side and limb regions obtained via a synthetic-aperture-radar patch-focusing reduction technique. The data are obtained by transmitting a circularly polarized pulsed waveform from the Arecibo telescope in Puerto Rico and receiving the echo in both senses of circular polarization with the Robert C. Byrd Green Bank Telescope in West Virginia. The resultant images in both polarizations have a spatial resolution as fine as 320 m 450 m near the lunar limb. The patch-focusing technique is a computationally efficient method for compensating for range migration and Doppler (azimuth) smearing over long coherence times, i.e., 983 s, which is needed to achieve the required Doppler resolution. Three to nine looks are averaged for speckle reduction and to improve the signal-to-noise ratio. At this long wavelength, the radar signal penetrates up to several tens of meters into the dry lunar surface materials, thus revealing details of the bulk loss properties and decimeter-scale rock abundance not evident in multispectral and other remote-sensing data. Application of the new radar images to the analysis of basalt flow complexes in Mare Serenitatis shows that the long-wavelength radar data are sensitive to differences in both flow age and composition, and may be particularly useful for studies of smaller deposits that do not have robust crater statistics. The new 70-cm lunar radar data are archived at the National Aeronautics and Space Administration Planetary Data System.

Structures in tessera terrain, Venus: Issues and answers

Many workers assume that tessera terrain, marked by multiple tectonic lineaments and exposed in crustal plateaus, comprises a global onionskin on Venus. Because tesserae are exposed mostly within crustal plateaus, which exhibit thickened crust, issues of tessera distribution and the mechanism of crustal plateau formation (e.g., mantle downwelling or upwelling) are intimately related. A review of Magellan data indicates that tessera terrain does not form a global onionskin on Venus, although ribbon-bearing tesserae reflect an ancient time of a globally thin lithosphere. Individual tracts of ribbon-bearing tessera terrain formed diachronously, punctuating time and space as individual deep mantle plumes imparted a distinctive rheological and structural signature on ancient thin crust across spatially discrete 1600-2500 km diameter regions above hot mantle plumes. Plume-related magmatic accretion led to crustal thickening at these locations, resulting in crustal plateaus. Crustal plateau surfaces record widespread early extension (ribbon structures) and local, minor perpendicular contraction of a thin, competent layer above a ductile substrate. Within individual evolving crustal plateaus the thickness of the competent layer increased with time, and broad, gentle folds formed along plateau margins and short, variably oriented folds formed in the interior; late complex graben cut folds. Local lava flows accompanied all stages of surface deformation. In contrast to these conclusions, Gilmore et al. ( 1998) summarized post-Magellan arguments in favor of downwelling models for crustal plateau formation. In light of this discrepancy, we reexamine the regions investigated by these workers and evaluate their arguments against upwelling models. We show that Gilmore et al. ( 1998) did not differentiate ribbons from graben and therefore their proposed temporal relations are invalid; they disregarded shear fracture ribbons, thus invalidating their criticism of ribbon models; they misunderstood previous radargrammetric work that constrains ribbon geometry; and they relied solely on geometrical relations to constrain timing, violating kinematic analysis methodology. Their stratigraphic constraints on ribbon-fold temporal relations are invalid because they (1) misinterpreted implications of map relations; (2) did not isolate radar artifacts due to local radar slope effects from proposed material units; (3) chose a region for analysis that clearly shows the effects of younger tectonism and volcanism; and (4) presented map relations that cannot be reproduced. Their attempts to discount upwelling models of crustal plateau formation fail because they combine fundamentally different pre-Magellan and post-Magellan upwelling models. These misconceptions about the upwelling model and processes responsible for global warming (Phillips and Hansen, 1998), lead to serious errors in Gilmore et al.s (1998) criticism. Furthermore, we show that the data of Gilmore et al. ( 1998) are actually more consistent with upwelling than downwelling models, consistent with arguments that tessera terrain is not global in spatial distribution.

Tessera terrain and crustal plateaus, Venus

Many workers assume that tessera terrain—marked by multiple tectonic lineaments and exposed in crustal plateaus—comprises a global “onion skin” on Venus. A growing body of structural, mechanical, magmatic, gravitational-topographic, and geologic evidence indicates that tesserae record the local interaction of individual deep-mantle plumes with an ancient, globally thin Venusian lithosphere, resulting in local regions of thickened crust.

Formation of lunar swirls by magnetic field standoff of the solar wind

Lunar swirls are high-albedo markings on the Moon that occur in both mare and highland terrains; their origin remains a point of contention. Here, we use data from the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer to support the hypothesis that the swirls are formed as a result of deflection of the solar wind by local magnetic fields. Thermal infrared data from this instrument display an anomaly in the position of the silicate Christiansen Feature consistent with reduced space weathering. These data also show that swirl regions are not thermophysically anomalous, which strongly constrains their formation mechanism. The results of this study indicate that either solar wind sputtering and implantation are more important than micrometeoroid bombardment in the space-weathering process, or that micrometeoroid bombardment is a necessary but not sufficient process in space weathering, which occurs on airless bodies throughout the solar system.

Constraints on the recent rate of lunar ejecta breakdown and implications for crater ages

We present a new empirical constraint on the rate of breakdown of large ejecta blocks on the Moon based on observations from the Lunar Reconnaissance Orbiter (LRO) Diviner thermal radiometer. We find that the rockiness of fresh crater ejecta can be quantified using the Diviner-derived rock abundance data set, and we present a strong inverse correlation between the 95 th percentile value of the ejecta rock abundance (RA 95/5 ) and crater age. For nine craters with published model ages derived from crater counts on their continuous ejecta, RA 95/5 decreases with crater age, as (age [m.y.]) −0.46 . This result implies shorter rock survival times than predicted based on downward extrapolation of 100 m crater size-frequency distributions, and represents a new empirical constraint on the rate of comminution of large rocks not previously analyzed experimentally or through direct observation. In addition, our result provides a new method for dating young lunar craters.

Volcanic and impact deposits of the Moon's Aristarchus Plateau: A new view from Earth-based radar images

Lunar pyroclastic deposits reflect an explosive stage of the basaltic volcanism that filled impact basins across the nearside. These fine-grained mantling layers are of interest for their association with early mare volcanic processes, and as possible sources of volatiles and other species for lunar outposts. We present Earth-based radar images, at 12.6 and 70 cm wavelengths, of the pyroclastic deposit that blankets the Aristarchus Plateau. The 70 cm data reveal the outlines of a lava-flow complex that covers a significant portion of the plateau and appears to have formed by spillover of magma from the large sinuous rille Vallis Schroteri. The pyroclastics mantling these flows are heavily contaminated with rocks 10 cm and larger in diameter. The 12.6 cm data confirm that other areas are mantled by 20 m or less of material, and that there are numerous patches of 2 cm and larger rocks associated with ejecta from Aristarchus crater. Some of the radar-detected rocky debris is within the mantling material and is not evident in visible-wavelength images. The radar data identify thick, rock-poor areas of the pyroclastic deposit best suited for resource exploitation.

Ribbon spacing in Venusian tessera: Implications for layer thickness and thermal state

[1] Wereportonmeasurementsofcharacteristicextensional wavelength represented by ribbons in Venusian tessera terrain. Fourier power spectra for ribbons in 35 areas from 9 geographic regions show dominant wavelengths of 2 to 6 km. We used these values to estimate mechanical layer thickness during ribbon formation at 0.6 to 2.9 km. Because ribbons accommodate extension of a single strong layer overlyingaductilesubstrate,weconcludethatthebaseofthis mechanical layer corresponded to the local brittle-ductile transition (BDT) during ribbon formation. Maintaining a BDTat <3 km depth for a significant length of time requires a locally hot environment, as over a plume impinging on thin lithosphere. These results indicate that locally hot conditions prevailed at widely distinct locations in the past. INDEX TERMS: 6295 Planetology: Solar System Objects: Venus; 8015 Structural Geology: Local crustal structure; 8010 Structural Geology: Fractures and faults; 5475 Planetology: Solid Surface Planets: Tectonics (8149); 8159 Tectonophysics: Evolution of the Earth: Rheology—crust and lithosphere. Citation: Ghent, R. R., and I. M. Tibuleac, Ribbon spacing in Venusian tessera: Implications for layer thickness and thermal state, Geophys. Res. Lett., 29(20), 2000, doi:10.1029/2002GL015994, 2002.

Earth-based radar data reveal extended deposits of the Moon's Orientale basin

We present new Earth-based radar observations of ejecta associated with the lunar Orientale impact basin. We can distinguish (1) a block-poor ejecta facies composing a concentric halo of mantling material 10 m or greater in thickness that extends more than 1000 km from the basin center, and (2) a melt-rich deposit that forms a discontinuous but areally extensive stratigraphic marker across the southern highlands. The melt-rich component likely extends well into the South Pole–Aitken basin, a key target for future landed and sample return missions. The observation of these two ejecta facies and their distribution across the southern nearside yields new insight into the types and distribution of material contributed by large basin-forming impacts to the highlands megaregolith.

Rugged crater ejecta as a guide to megaregolith thickness in the southern nearside of the Moon

The southern highlands of the Moon comprise superposed ejecta layers, individually as thick as a few kilometers, from the major basins. Smaller (1–16-km-diameter) impact craters that penetrate this layered megaregolith and excavate material from depth have radar properties that provide insight into the variability of megaregolith thickness above a postulated basement of large crustal blocks. We observe a significant difference in the population of radar-bright craters, 1–16 km and larger in diameter, between regions of the southeastern near-side highlands north and south of ~lat 48°S. There are about one-third more radar-bright craters north of this line than to the south, broadly coincident with the mapped boundary between southern deposits mapped as pre-Nectarian age and those of Nectarian–Imbrian age to the north. The radar-bright crater population is consistent with a megaregolith thickness of ~1.5 km in the north and ~2.5 km in the south, a difference we attribute to South Pole–Aitken basin ejecta.