I study physical processes on the surfaces of Mars, the Earth, the Moon, and asteroids, mostly processes that involve water ice. Most of my time is spent on computations, small and large, but I also carry out theoretical work, data analysis, and field work.
Theory of Ice Ages on Mars
Slides for a ~1.5 hour talk
first version: 2007
most recent version: 2013
Lifetime of Ice on Main Belt Asteroids
based on Schorghofer, ApJ 682, 697–705 (2008) and more recent work.
most recent version: 2011
|Acausal slope-area relations: a short presentation about a topic in quantitative geomorphology
most recent version: 2002
Mars Orbiter Camera observations of seasonal frost at latitude 37°S
Movie in wmv format.
For a detailed description see Schorghofer & Edgett, Icarus 180, 321–334 (2006).
Model calculations of the accumulation of ground ice from
atmospherically derived water vapor under Mars conditions
Movie in wmv format.
For a detailed description see Schorghofer & Aharonson, JGR 110, E05003 (2005).
For microclimates inside craters, slopes and shadowing play an even greater role than for planar slopes. In addition, stagnant air can accumulate on the floor of a crater. To get a good grasp of this problem, we study temperatures inside a crater on the high and barren volcano of Mauna Kea in Hawaii. My collaborator Brendan Hermalyn has built a time-lapse infrared camera system for field work, which provides space and time resolved infrared-derived temperature maps. What makes the cinder cones on Mauna Kea even more interesting is that some of them actually do contain permafrost [W74].
Permafrost and microclimates in the craters of Mauna Kea: As mentioned above, some of these crater interiors are colder than the summit and even contain permafrost. This permafrost might be melting away due to climate warming.
The study of historical documents about snow on Mauna Kea: In the past, Mauna Kea and Mauna Loa had snow more frequently than today. And snow was common on the island of Kauai in the early 19th century.
Ice-filled lava tubes on Mauna Loa: A cave with permanent ice has been briefly described in 1979 by Kempe and Lockwood. It has lost a significant volume of ice since then. Another, much larger ice cave was discovered in 2009 by Steven Smith, who named it "Arsia Cave", after a feature on Mars. This cave system has two large permanent ice lakes. The ice contains huge bubbles, indicative of slow freezing rates, and is probably layered. These are the world's geographically most isolated ice caves. Steven Smith, Andreas Pflitsch, and I are working on basic documentation and monitoring of the ice caves.
H2O on present-day Mars is never liquid (with perhaps extremely rare exceptions), but changes between solid and vapor directly. This is unlike Earth, where ice is lost primarily through melting rather than through sublimation. Many permafrost processes relevant for Mars occur rarely, if at all, anywhere on Earth. A series of studies has been devoted to the exchange of water vapor between the Martian atmosphere and a porous subsurface. A few of the relevant concepts are outlined here:
Stability with respect to sublimation: When the time-averaged absolute humidity in the atmosphere exceeds that of buried ice, the ice is "stable" with respect to sublimation loss, because no net loss occurs. This factor, and specifically the atmospheric water vapor content, may also play a significant role for the persistence of extremely old "fossil" ice in one of the Dry Valleys of Antarctica [S09], although other factors, such as nearly impermeable salt crusts, may also be important there.
Thermal pumping and in-situ growth of ice from the vapor phase: Temperature cycles can pump water vapor into a porous subsurface. The amplitude of seasonal temperature variations decays quickly with depth, and the nonlinear dependence of saturation vapor pressure on temperature causes a net downward flux. This process has been studied by Mellon and Jakosky in 1993, and only occurs when the atmosphere is humid enough. It leads to the formation of ice in the void spaces. The process has been reproduced in the laboratory by Troy Hudson [H09]. Remarkably, the ice produced by vapor deposition contains bubbles, which may trap old martian air.
I1/4-Milankovitch Theory: The symbol I stands for insolation (incoming solar radiation). When ice is buried, it responds to mean surface temperature more than to peak surface temperature. One may think that mean temperature is related to mean insolation, but this is not always the case on Mars. In the idealized case of a body without atmosphere and negligible thermal conductivity, the surface temperature is proportional to I1/4, based on the Stefan-Boltzmann law. The annual time averages <.> of insolation can be studied as a function of latitude and orbital parameters. <I1/4>, compared to <I>, depends more on precession (or, more precisely, on a factor known as Milankovitch index) than on obliquity. On Mars at around 60° latitude, annual mean temperature changes with the periodicity of precession cycles and varies little with obliquity cycles [S08].
Three-layered ice distribution: After the last climate event that deposited snow on the surface of Mars, the snow compacted and was gradually buried. Humid climate periods deposited interstitial pore ice in the soil layer, by the "pumping" process mentioned above. Hence, three layers can be expected: dry soil, pore ice (from vapor deposition / pumping), and massive (nearly pure) ice. A three-layered structure also explains why the Phoenix Lander mainly saw pore ice, but the neutron spectrometers suggest the ice content of the top half meter is more consistent with massive ice [S07,SF12] .
Another technique to achieve several orders of magnitude of computational speedup is the use of programmable GPUs (Graphics Processing Units). These massively parallel processors (single instruction multiple data architectures) can schedule and process many computational threads in parallel, if an algorithm is suitably structured. My role is to know just enough about GPU hardware and GPU programming to identify numerical problems with an algorithmic structure that is suitable for acceleration on GPUs.