Alumni News - Department of Earth Sciences, University of Toronto

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Planetary Science In the Remote Sensing Laboratory By Rebecca Ghent, Assistant Professor of Geology Figure 1. 70-cm radar mosaic of the Northern Imbrium region of the lunar nearside. Regions with high radar returns appear bright; low returns appear dark. Radar-dark haloes are marked with black arrows. 8 Introduction Planetary science has long captured the imaginations of the scientific community and the public. The past decade has seen an unprecedented acceleration in international collaboration in the design and operation of planetary exploration missions, and analysis of resulting data. There has rarely been a more exciting time for those interested in planetary science, with active missions at Mars, the Moon, Venus, Mercury, and Saturn numbering in the double digits, and with the revival of interest in the Moon rivaling the Apollo era. Research in the Geology Department’s Remote Sensing Laboratory focuses on problems in lunar geology, largely using Earth-based radar observations. Earth-based radar: how does it work? Together with colleagues from the Smithsonian Institution and Cornell University, we have used the NSF Arecibo Observatory (AO) radio telescope in Puerto Rico and the Green Bank Telescope (GBT) in West Virginia to image the entire nearside of the Moon, providing both synoptic and high spatial resolution (up to 40 m / pixel) coverage. We use the Arecibo telescope to transmit pulsed radar signals at either 70-cm or 12.6-cm wavelengths; these signals are reflected from the Moon, and we receive and record them at GBT. We can associate a given reflected pulse with a particular location on the Moon by measuring the pulse’s delay (travel time to and from the Moon) and Doppler shift (caused by slight motions of the Moon relative to the receiver). The strength of a radar return is a function of the roughness, attitude, and/or composition of the target. Using a series of geometrical transformations, we produce maps of the Moon in selenographic coordinates, with strong returns appearing bright and weak returns appearing dark (Fig. 1). Radar is a particularly useful tool for the study of the lunar regolith, which is the layer of pulverized debris that blankets the Moon’ surface. The particles that make up the regolith range from micron-sized glass spherules to housesize boulders ejected during large bolide impacts. Radar signals can penetrate the regolith to depths up to several tens of meters, with long-wavelength signals penetrating more deeply than short-wavelength ones. In addition, the polarization of the reflected waves yields information about the size and distribution of scatterers (blocks on the order of the radar wavelength) both on the surface and within the upper volume of regolith. Surface or subsurface blockiness causes the reflected pulses to contain some energy with the same sense of circular polarization as the transmitted signal, and some with the opposite sense. At GBT, we measure and record these two polarizations separately, allowing us to use them independently and in concert (e.g., by calculating their ratio). By comparing radar returns with both polarizations and at multiple wavelengths, we gain information about the properties of regolith materials. A Few Current Results Work in the Geology Department’s Remote Sensing Laboratory has focused on understanding the physical properties
of impact ejecta (the material excavated from an impact crater and redeposited on the surface). This field of study is important to our understanding of lunar geological processes, because the characteristics of impact ejecta deposits – their spatial extent and continuity, the size distribution of their component particles, and their physical state at the time of emplacement – provide links to the physics of the impact process. Furthermore, an understanding of the processes by which these materials are produced and then emplaced, and their effects on the surrounding landscape, is required to unravel the impact history of a given region on the Moon. Finally, a knowledge of the geotechnical properties of the regolith are vital to efforts to explore the Moon, and eventually, to establish a permanent lunar base. Using Earth-based radar, we have discovered that large lunar impacts (craters >10 km diameter) produce an ejecta facies characterized by an absence of particles or blocks 10 cm or greater in size. These finegrained materials form low-radarreturn “haloes” around many nearside craters (Figs. 1, 2). The radar-dark haloes must be on the order of 10 meters thick at their outer edges to be detectable; thus, these block-poor deposits represent a large volume of material, and are significant contributors to the fine fraction of the lunar regolith. Thermal energy dissipated in a large impact commonly results in production of “impact melt,” which can line the interior of the cavity and/or flow across the surface, inside or outside the crater rim, until it cools and solidifies. Examination of the large multiringed impact basin Orientale, located at the Western limb of the Moon (Fig. 2), showed that melt-rich ejecta from this impact are distributed over much of the lunar nearside. This material shows a radar signature indicative of high roughness on the wavelength (12.6- to 70-cm) scale, with the roughest signatures associated with small (hundreds of meters) impacts. This signature indicates a source of competent material at shallow depths (tens of meters) that later, smaller impacts can excavate, producing abundant blocks. Distal Orientale melt-rich ejecta were deposited in dicontinuous linear streamers across the surface, reaching to the south pole and beyond, into the South Pole-Aitken (SPA) basin. This finding is significant because independent evidence suggests the possible presence of water ice in permanently shadowed regions near the pole; therefore, the SPA basin is a very likely target for near-future landed missions. Knowledge of the nature of surface and near-surface materials in SPA is a key goal in current lunar science. Future work NASA plans to launch its Lunar Reconnaissance Orbiter (LRO) spacecraft in 2009. The Remote Sensing Laboratory will be involved in processing and analyzing data from the DIVINER thermal mapper on that spacecraft. DIVINER and the other LRO instruments will provide new opportunities to understand the lunar regolith. More information about the Geology Department’s Remote Sensing Laboratory is available at www.geology.utoronto. ca; information about LRO is available at http://lunar.gsfc. nasa.gov/. Figure 2. 70-cm radar mosaic of the southern hemisphere of the Moon; orthographic projection centered on the South pole. Ratio of same-sense to opposite-sense polarized signals overlain in color on opposite-sense image. Orientale is visible at lower left. Radar-dark haloes, accentuated in the ratio image, are marked with white dashed outlines. The margins of Orientale’s dark halo are marked with yellow arrows; here, it can be seen to partially obscure, or mute, the high polarization ratio streamers (yellow and green salt-and-pepper texture) that radiate outward from Orientale. 9
Page 1 and 2: Issue 18 February 2009 Alumni NEWS
Page 3 and 4: Canadian Mining Hall of Fame Emerit
Page 5 and 6: In addition Andrew has completed th
Page 7: Connaught Fellowship Neil Bennett C
Page 11 and 12: with procuring steady research fund
Page 13 and 14: 2009 JOINT ASSEMBLY The Department
Page 15 and 16: The undergraduate geology program o
Page 17 and 18: and led the international initiativ
Page 19 and 20: The Explorers Fund has been establi

Alumni News - Department of Earth Sciences, University of Toronto

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