Research

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.

Selected Presentations

Ice on Mercury and the Moon
Slides for a general 1 hour seminar
most recent version: 2014
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
 

Movies

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).

Science

Snow and ice in Hawaii

Studying ice and snow on the Hawaiian Islands might sound like an obscure idea, because there is so little of it, but the topic is actually fascinating, unexplored, and rewarding. Currently, there are three ongoing research projects that deal with snow and ice in Hawaii:

Permafrost and microclimates in the craters of Mauna Kea: There are numereous cinder cones with craters in the summit region of Mauna Kea. Some of these crater interiors are colder than the summit and even contain permafrost. This permafrost might be melting away due to climate warming. We study these exceptionally cold areas with temperature probes, data loggers, and infrared imaging, and conduct geophysical surveys with minimal disturbance to the environment. When there is no wind, cold air pools inside the craters at night; this is where one finds the coldest temperatures ever measured on the Hawaiian Islands (about -18°C).

The study of historical documents about snow on Mauna Kea: In the past, Mauna Kea and Mauna Loa had snow more frequently than today [SKN14]. 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, and because the cave is ventilated it could rapidly continue to lose ice [PSSH14]. 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 documenting and monitoring the ice caves.

Planetary microclimates

On Mars, topography can dramatically lower surface temperatures, and can lead to seasonal frost or perennial ice far from where it is found otherwise. Pole-facing slopes are colder than equator-facing slopes. Seasonal carbon dioxide frost is observed on pole-facing slopes near tropical latitudes; the temperature on these slopes falls below 150K when CO2 from the atmosphere solidifies. This is remarkable, as carbon dioxide frost otherwise only forms in polar winter. And it is even more remarkable that the sun rises above these cold slopes every single day of the Mars year, unlike the carbon dioxide ice that forms in the polar regions during polar winter [SE06]. Low-latitude seasonal CO2 ice may be responsible for the formation of linear gullies, originally believed to be due to the action of liquid water [movie clip]. In addition to seasonal frost, slope effects also allow patches of permanent subsurface water ice to exist far from where otherwise expected [AS06].

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. We study temperatures inside a crater on the high and barren volcano of Mauna Kea in Hawaii. My collaborator Brendan Hermalyn has developed 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].

Subsurface-atmosphere interactions on Mars

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.

Stability with respect to sublimation: When the (long-term 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. Troy Hudson has reproduced this process in the laboratory [H09]. Remarkably, the ice produced by vapor deposition contains bubbles, which may trap old martian air.

I1/4-Milankovitch Theory: Milankovitch theories connect orbitally-driven changes in insolation I (incoming solar radiation) with changes in ice volume. 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. 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 (maybe 5 Myr ago), 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 are 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 top half meter mostly consists of massive ice [S07,SF12].

Water on the Moon

Water molecules migrate in the lunar exopshere by ballistic hops and in the Moon's subsurface by diffusion. When they migrate on the surface, they pile up around the morning terminator, but not at the evening terminator [S14]. DHO and H2O molecules reach the permantly dark and cold crates near the lunar poles with nearly the same probability (esimated 10%), and thus the D/H ratio of any ice deposits can be used to identify the source of the water [S14]. The pumping process for water vapor, discussed above for Mars, can also occur on the lunar surface, but is very inefficient on the Moon [SA14].

Numerical methods for planetary processes

Accurate calculations of ice loss from planetary surfaces require numerical time steps much smaller than a solar day, but we want to know the changes in ice volume over millions of years (on Mars) or billions of years (on icy asteroids). In computational science, this is known as a multi-scale problem. Numerical methods have been developed that can efficiently solve this two-scale problem, and they have been applied to the Ice Age Cycle on Mars [S07,S10,SF12] and to ice loss on asteroids, such as Elst-Pizarro and Ceres.

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 scientific models that would greatly benefit from GPU acceleration.