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Department of Geology
Bryn Mawr College
101 North Merion Avenue
Bryn Mawr, PA 19010
Phone: (610) 526-7392
Fax: (610) 526-5086

Selby Cull

Research

Mars vs. Earth

I study the geochemistry and mineralogy on Mars, with an eye toward understanding how water and ice have interacted with Mars’ past and present environments.  As a near-infrared spectroscopist, I do this by mapping surface physical properties and mineralogy, seeking to understand modern mineral-environment interactions and to reconstruct past environmental conditions.

Modern Geochemical Interactions

Much of my work to date has focused on the most important and elusive Mars cycle: the modern water cycle.  Modern Mars has water reservoirs in its atmosphere, two polar ice caps, seasonal ices, and vast subsurface areas beneath the northern lowlands.  Water exchange among these reservoirs is poorly understood. 

Phoenix Lander LogoIn previous work, I charted water exchange between the atmosphere, seasonal ices, and subsurface ices at the landing site for NASA’s Phoenix mission, using near-infrared spectra (e.g., Cull et al. 2010a, 2010b).  One of the most important findings from this work was the discovery of thin films of liquid water at the site today (Cull et al. 2010c).  These thin films form when seasonal ices interact with perchlorate, a highly soluble mineral deposited on the surface by atmospheric interactions.  

This type of modern water-mineral interaction is an exciting new field of research, and I will continue to explore the many questions it provokes; for example: how widely dispersed are perchlorates, how prevalent thin films, and are there other salt-ice interactions on modern Mars?

Potential Student-Led Projects: 

  • Detailed mapping of perchlorate distribution in the soil column at the Phoenix landing site
  • Detailed morphological analysis of perchlorate crusts and clods at the Phoenix landing site
  • Spectral analysis of perchlorate-bischoffite mixtures
  • Systematic spectral survey of perchlorate salt distribution across the northern plains
  • Development of a perchlorate spectral index

Reconstruction of Past Environments

Currently, I am working on reconstructing past environments on Mars using three types of deposits: subsurface water ice, the polar ice layers, and hydrated mineral deposits.  Each is a different window into Mars’ climate history: subsurface ices were deposited ~400,000 years ago (Byrne et al. 2009), the polar cap ice layers record climate conditions from the past 100,000 to 10 million years (Byrne 2009), and phyllosilicate and sulfate deposits on Mars are almost all older than 3.5 billion years (Bibring et al. 2005).

Subsurface Water Ice Water Ice Buried Beneath the Surface of Mars

A vast deposit of water ice underlies the northern plains of Mars.  Several mechanisms have been proposed to explain the origin of the ice, including vapor diffusion from the atmosphere, buried frozen surface water, buried glaciers, and accumulation and burial of snow pack (e.g., Mellon et al. 2004, Carr et al. 1990, and Mischna et al. 2003, respectively).  In papers I have led (Cull et al. 2010d) and co-authored (Byrne et al. 2009), I have used near-infrared spectroscopy to test these hypotheses by estimating the composition of the buried ice.  Interestingly, the buried ice is not heterogeneous: the composition varies on a scale of meters.  This work has provoked many questions that I will continue to pursue: is the bulk of the ice pure or mixed with dust?  What were the timescales for deposition and evolution of the subsurface ice?  What role did brine-producing salts, such as perchlorate, play in the evolution of the ices?

Polar Cap Ice LayersNorthern Polar Layered Deposits

The northern polar cap of Mars is made of cyclic layers of dusty-ices that record climate conditions at the time of deposition.  By modeling the ice-to-dust ratio in each of these layers, we can read Mars’ recent climate history as we would with an ice core.  Using near-infrared spectra from orbiting satellites, I plan to map the ice-to-dust ratio in layers exposed by chasmata in the ice cap.  By combining these results with age estimates (e.g., Phillips et al. 2008), the ice layers will illustrate the changing interactions between the water and dust cycles over the past several thousand years.

Phyllosilicate and Sulfate Mineral Deposits

Recently, I’ve been working with the landing site selection effort for the Mars Science Laboratory mission to map mineral distributions at four potential landing sites for the lander, which will launch in Fall 2011.  These sites are rich in hydrated minerals (e.g., Bishop et al. 2008, Wray et al. 2008).   To understand deposition conditions for these sites, I use near-infrared spectra to map distributions, assemblages, and stratigraphic relationships of major minerals.  In particular, I use the spectra to estimate mineral abundances.  I do this by constructing mathematical models of how light interacts with different materials and mixtures of minerals.  By estimating mineral abundances, we can model deposition conditions.

Potential Student-Led Projects

  • Detailed mineralogical mapping of Mars Science Laboratory candidate landing sites: Are there correlations with distinct geomorphic units?
  • Detailed spectral analysis of subsurface ices exposed at the Phoenix landing site
  • Mapping the relationship between patterned ground and seasonal ice distribution: Is there a thermal inertia control?
  • Mapping of stratigraphic relationships at Mars Science Laboratory candidate landing sites

References

Bishop, J. et al. (2008) Phyllosilicate Diversity and Past Aqueous Activity Revealed at Mawrth Vallis, Mars. Science 321: 830-833.

Byrne, S. et al. (2009) Distribution of mid-latitude ground-ice on Mars from new impact craters. Science, 325(5948): 1674-1676.  doi: 10.1126/science.1175307

Byrne, S (2009) The polar deposits of Mars.  Annual Review in Earth and Planetary Science 2009.

Cull, S et al. (2010a) The Seasonal H2O and CO2 Ice Cycle at the Mars Phoenix Landing Site: I. Pre-Landing CRISM and HiRISE Observations. Journal of Geophysical Research, 115: doi:10.1029/2009JE003340.

Cull, S. et al. (2010b) The Seasonal Ice Cycle at the Mars Phoenix Landing Site: II. Post-Landing CRISM and Ground Observations. Journal of Geophysical Research, 115, E00E19, doi:10.1029/2009JE003410.

Cull, S. et al. (2010c) Concentrated perchlorate at the Mars Phoenix landing site: Evidence for thin film liquid water on Mars.  Geophysical Research Letters, submitted August 2010.

Cull, S. et al. (2010d) Compositions of subsurface ices at the Mars Phoenix landing site.  Geophysical Research Letters, submitted September 2010.

McSween, H et al. (2009) Elemental composition of the Martian crust.  Science 324: 736-739.

Morris, RV et al. (2010) Identification of Carbonate-Rich Outcrops on Mars by the Spirit Rover. Science 329: 421-424.

Wray, J. et al. (2008) Compositional stratigraphy of clay-bearing layered deposits at Mawrth Vallis, Mars. Geophysical Research Letters 35.