I study physical processes on the surfaces of Mars, the Earth, the Moon, and asteroids, mostly processes that involve H2O. Most of my time is spent on computations, small and large, but I also carry out field work, data analysis, and purely theoretical work.
Slides from a seminar about the theory of ice ages on Mars
most recent version: 2013
|Slides from a seminar about the lifetime of ice on main belt asteroids
based on Schorghofer, ApJ 682, 697–705 (2008) and more recent work.
most recent version: 2011
Mars Orbiter Camera observations of seasonal frost at latitude 37°S:
Movie in wmv
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
For a detailed description see Schorghofer & Aharonson, JGR 110, E05003 (2005).
|Acausal slope-area relations: a short presentation about a topic in quantitative geomorphology
most recent version: 2002
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. Kenn Edgett and I studied white patches in imagery, from an orbiting spacecraft, that come and go with season. They turned out to be carbon dioxide frost, at latitudes almost inside the tropics. The surface temperature of 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. Oded Aharonson and I made detailed model predictions of that. Work by others added two unexpected twists to this story. 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). And there is another, even more exciting development. 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 a number of implications.
For microclimates with three-dimensional topography, slopes and shadowing play a major role too, and modeling is computationally far more intensive 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 melt 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. I have built on this work with additional theoretical studies and participated in a series of dedicated laboratory investigations at the "CalTech Icelab", conducted by Troy Hudson and others. I have also developed a numerical method that is fast enough to track ice volume changes over geologic times, ten thousand times faster than previous models.
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, especially when the ice sheets are waning. 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:
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.
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, or more 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.
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. The mean temperature at most other latitudes is dominated by obliquity variations.
Ice age cycle on Mars. The currently "best", or at least most coherent, theory of Martian ice ages has been worked out by a number of planetary scientists, including Francois Forget, Jim Head, Misha Kreslavsky, and Benjamin Levrard. When the planet's axis tilt is high, lots of ice sublimes from the north polar cap and precipitates on the western slopes of the tall tropical volcanoes. As the axis tilt becomes smaller, the ice is redistributed, again by precipitation, in form of a mantle poleward of about 30° latitude on both hemispheres. This mantle subsequently retreats to about 60° and ice is deposited on one or both poles. As the axis tilt oscillates with a period of about 120 ka, the climate becomes periodically more humid, but not humid enough for any precipitated snow to last through the summer. Instead, ice forms periodically in the form of pore ice.