Rebecca Ghent

Ghent_heli1

 Associate Professor

University of Toronto
Department of Earth Sciences

22 Russell St., Toronto
Ontario, Canada M5S 3B1

Tel.:  416 978 0597
Fax.: 416 978 3938
e-mail: ghentr@es.utoronto.ca

 

 Publications

 

Research Interests

I am a planetary scientist who focuses on geological processes on the terrestrial planets at a number of spatial scales.

Lunar science

Current focus: Regolith characteristics and evolution, impact cratering

Over the past five years, my work in lunar geology has focused on understanding the physical properties of the lunar regolith, and the ways in which impact crater ejecta contribute to regolith formation and evolution. Regolith characteristics provide clues to fundamental questions in lunar geology, because the regolith bears a record of processes occurring in the neighborhood of Earth over the past 4.5 billion years. As part of this work, I have completed or contributed to a number of studies addressing the physical characteristics of lunar impact ejecta (Ghent et al., 2005, 2008), the emplacement and distribution of fine-grained ejecta on the Moon, Mars, and Venus (Ghent et al., 2010), the subsurface geology of the Aristarchus plateau (Campbell et al., 2008), variations in regolith thickness across the southern lunar nearside highlands (Thompson et al., 2009), and the geology of the northern rim of the Imbrium basin (Thompson et al., 2006).

Among the tools I use are Earth-based radar observations. I am part of a collaborative project (USGS, NASA Goddard, Cornell University, University of Toronto; led by Dr. Bruce Campbell) to image the nearside of the Moon with high-resolution, bistatic, dual-polarization Earth-based radar observations at 12.6-cm and 70-cm wavelengths using the Arecibo and Green Bank radiotelescopes. Radar observations at these wavelengths provide a means of probing the upper several meters of lunar regolith, and analysis of the polarimetric properties of the radar returns in conjunction with orbital multispectral and other remote datasets allow detailed characterization of the physical and chemical properties of lunar surface materials. We have completed a map of the complete lunar nearside at 70-cm wavelength and 400m spatial resolution, which is now archived in the PDS (http://pdsgeosciences.wustl.edu/). Additional high-resolution 12.6- and 70-cm observations continue for targeted studies of particular regions of interest. Among the questions we are addressing are: What is the distribution and extent of buried mare deposits? How thick are lunar pyroclastic deposits, and what lies beneath them? How does the thickness of the regolith vary, and can we use it to establish the provenance and stratigraphy of ejecta from large basins?

I am also a member of the LRO Diviner thermal mapper science team. Diviner is a nine-channel infrared radiometer that has produced the first global temperature maps of the Moon. My focus is on characterizing the thermophysical properties of the lunar regolith using Diviner’s four thermal channels. I have collaborated in efforts to calculate a rock abundance index (led by Josh Bandfield, Univ. of Washington) derived from spectral variations in the nighttime radiances arising from the presence of mixtures of warm rocks and cool “soil” within a single instrument field of view (e.g., Bandfield et al., 2010; 2011). This dataset is a natural complement to our ongoing Earth-based radar measurements, and comparisons between the two datasets promise to provide new and valuable quantitative information about regolith physical characteristics. Currently, I am investigating the process of regolith development via the thickness of mantling deposits covering impact ejecta blocks. 

Near-Earth asteroids

Current focus: Regolith properties, tectonic features

This represents a new area of interest for me. I am a member of the OSIRIS REx mission science team, assigned to the laser altimeter group. OSIRIS REx was recently selected as NASA’s latest New Frontiers mission, and will return samples from the asteroid 1999 RQ36, with launch planned for 2016. This asteroid was chosen because it is spectrally unique, with the lowest observed albedo of any known asteroid, and provides the first opportunity to observe a primitive B-class carbonaceous body in detail. It is also in an Earth-crossing orbit, and therefore merits study because of the potential hazard it represents. Based on previous radar observations from Arecibo, there is abundant regolith present, which presents a valuable opportunity to study the processes by which regolith forms on a small body. The OSIRIS REx Laser Altimeter (OLA) will provide: geological context for the returned samples; bulk 3-D properties of the asteroid, including shape, volume, and total mass; and geophysical characteristics such as surface slope and gravity.

Upon OSIRIS REx’s rendezvous with the asteroid, I will be involved in analyzing the laser altimeter data, focusing on tectonic features and impact craters and their implications for the body’s mechanical properties and stress-strain history, and the surface roughness and block size distribution. I am particularly interested in understanding the bulk mechanical nature of the body (e.g., rubble pile vs. coherent object), and investigating regolith-forming processes. Before launch, I will conduct dielectric permittivity measurements of regolith analog materials. This is important because radar is a common method used to observe asteroids, and measurements of the dielectric properties of a primitive carbonaceous body will provide previously unavailable “ground truth” for observations of other similar objects.

Mapping, structural analysis, and numerical modeling of tectonic structures.

I am interested in both brittle and ductile tectonic deformation at a variety of scales, and on various planets.  Previous work has involved analysis of complexly deformed terrain characteristic of highland plateaus on Venus using both photogeologic analysis and numerical modeling (Ghent and Hansen, 1999; Hansen et al., 1999, 2000; Ghent and Tibuleac, 2002; Ghent et al., 2005).  Most of this work was focused on distinguishing between competing mechanisms for Venus’ complexly deformed “tessera terrain” and the highland plateaus on which it is commonly found.  The two most commonly argued mechanisms involved formation of the highland plateaus by a mantle drip mechanism, in which ductile flow of lower crustal material resulted in localized plateaus with elevated topography and thick, buoyant crust; and formation of the same plateaus by a plume mechanism, producing elevated plateaus with thick crust by intrusion, extrusion, and underplating.  Each mechanism predicts its own structural evolution, but existing data are insufficient to allow unique determination of the kinematic and structural history at most locales.  My modeling work (Ghent et al., 2005) was aimed at testing the plume model via a finite element simulation of  compression of Venusian crust with uniform composition and temperature-dependent viscosity.  The results showed that to form the structural elements we observe in Venus’ highland plateaus by upwelling requires a mechanically (and hence, probably compositionally) layered crust.

Terrestrial radar investigations in planetary analog environments

During summer 2010, I conducted a reconnaissance field project at Ellesmere Island, Nunavut, focused on ground-penetrating radar investigations of massive ground ice exposed at permafrost thaw slumps.  Following on that project, and in collaboration with industrial partners MDA and Sensors and Software and researchers from McGill University, the University of Western Ontario, and U of T, I am involved in a Canadian Space Agency-funded project involving development and field testing of a remotely operated rover-based GPR, intended to develop planetary analog and eventual flight capabilities.

Mission involvement

Science team, NASA Lunar Reconnaissance Orbiter Diviner thermal mapper

Diviner is one of seven instruments aboard NASA’s Lunar Reconnaissance Orbiter, launched on June 18, 2009.  It was designed to map compositional variations in lunar surface material and to obtain the first global maps of surface temperature.  From these basic measurements, the team is pursuing a host of scientific questions.  I am primarily interested in the thermophysical properties of lunar materials, including crater ejecta, and their implications for the processes of regolith formation and evolution.

Science team, NASA OSIRIS-REx laser altimeter

OSIRIS-REx is a sample return mission, recently selected for flight as NASA’s next New Frontiers mission, that will return a substantial mass of pristine carbonaceous regolith from asteroid 1999 RQ36 to Earth for study.  This mission will investigate the origin of volatiles and organic materials residing on the asteroid, provide ground truth for future observations of B-type carbonaceous asteroids, to explore the regolith at high spatial resolution, and evaluate the hazard represented by objects in Earth-crossing orbits.  I am part of a group of researchers in Canada working on OSIRIS-REx under the auspices of the Canadian Space Agency.  Following launch in 2016, I will be involved in geological studies of the asteroid.

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