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 purely theoretical work, data analysis, and field work.
The Theory of Ice Ages on Mars
Slides for a ~1.5 hour talk
first version: 2007
most recent version: 2013
The 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).
On Mars, topographic slope has a strong effect on surface temperature. Pole-facing slopes can be dramatically colder than equator-facing slopes. This has implications for seasonal frost and for permafrost. Seasonal frost is visible in the form of white patches in imagery from orbiting spacecrafts, at near tropical latitudes. The temperature on these slopes falls below 150K when CO2 from the atmosphere solidifies. In addition to seasonal frost, slope effects also allow patches of permanent subsurface water ice to exist far from where otherwise expected. Work by others added two unexpected twists to this story. First, the exact time of the appearance of near-tropical CO2 frost can only be explained if there is water ice beneath (which has relatively high thermal conductivity and causes the seasonal surface temperature to change more slowly). Second, the activity of linear gullies, originally believed to be due to the action of liquid water, occurs in the cold season and is connected with CO2 activity. Thus, low-latitude seasonal CO2 frost has significant implications.
For microclimates with three-dimensional topography, slopes and shadowing play an even greater role than for planar slopes. In addition, meteorological effects may be important inside a crater with stagnant air. 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 marvelous time-lapse infrared camera for field work, which provides space and time resolved infrared-derived temperature maps of the crater interior. What makes the cinder cones on Mauna Kea even more interesting is that some of them actually do contain permafrost. This permafrost might be melting away due to climate warming. We are investigating where exactly the buried permafrost is located and why these locations are exceptionally cold.
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. Hence, 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.
For Mars, the term "ice age" refers to the redistribution of polar ice to lower latitude reservoirs. One element of this cycle is the regolith ice reservoir, and its exchange with the atmosphere. The loss of ice to the atmosphere through a sublimation lag is a rate-limiting factor in the ice-age cycle, which is otherwise driven by orbital changes. A few of the relevant concepts are outlined here:
Stability with respect to sublimation: When the downward pumping balances or exceeds the upward loss, the ice is "stable" with respect to sublimation loss, i.e. no net loss occurs at all. In other words, the humidity of the atmosphere balances that of the buried ice. 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, although other factors, such as nearly impermeable salt crusts, may also be important there.
Pumping: 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 Hudson et al. in 2009.
In-situ ice growth from vapor phase: The geometric structure of ice that grows directly in the subsurface from the vapor phase has only been incompletely studied. One-dimensional models show that there are two deposition modes, a volumetric mode where ice gradually grows at a range of depths, and a vertical mode, where the ice table moves purely upward. In the volumetric mode, the ice initially grows in the form of vertical tendrils. These tendrils then merge and they contain bubbles.
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 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. At most other latitudes, the mean temperature is dominated by obliquity variations.