In a bonus research synopsis this week, we take a look at some Mars climate simulations that produce glaciers on the slopes of Mars’ great volcanoes: Olympus Mons, Elysium Mons, and Arsia, Pavonis and Ascraeus Mons (known collectively as Tharsis Montes). These massive volcanoes all exhibit surface features that, on Earth, are attributed fairly exclusively to glacier action. On the left is one example of such a feature found down in the Hellas Basin region; it’s an hourglass shaped crater filled with what looks like a glacier pouring from one side into the other (image credit: ESA).
These volcanoes are all located at the mid-latitudes of present-day Mars and as such receive too much sunlight for exposed surface ice to be stable (see this map of Mars from NASA for surface features and topography). However, we know that the tilt (or obliquity) of Mars spin axis varies greatly and that within the last few tens of millions of years, it may have been as much as 45 degrees (see General Explanations, below). At this angle, surface ice on these massive volcanoes would be stable, and the authors of the paper below use climate models to show that ice could indeed have built up there.
Citation (online at CiteULike.org):
Forget, F., Haberle, R.M., Montmessin, F., Levrard, B., and Head, J.W. (2006). Formation of Glaciers of Mars by Atmospheric Precipitation at High Obliquity. Science 311(5759), pp. 368-371.
Synopsis:
Forget and others use a climate model of Mars developed to simulate present-day conditions, but change the obliquity of the planet to 45 degrees, rather than its current 25.2 degrees. This change allows the North and South polar caps to receive a great deal more sunshine during the summer months, which releases more water into the atmosphere than observed on Mars today. This atmospheric water then is free to precipitate elsewhere on the planet, including on the flanks of Mars’ giant volcanoes. The figure on the right from their paper shows modeled ice accumulation regions in parts (b) and (c). Ice accumulates in these regions because as air rises over the volcanoes it cools by 10 or 20 degrees and precipitates out ice.
Also in the figure, part (a) is a geologic map of Mars with suspected glacier-deposited landforms shaded in yellow. Notice the general agreement between the locations of the features. The glacial features highlighted in yellow are giant fans of boulders and sand deposited by the scraping action of glaciers as they flowed down the slopes of the volcanoes. For the three Tharsis Montes volcanoes, the location of ice accumulation according to Forget and others’ climate model is at the source of these fan deposits.
The ice deposited on the volcanoes came from sublimation (solid->gas phase transition, dry ice sublimates rather than melting) of the northern polar ice cap. Water from the southern polar ice cap (if indeed there truly is one, rather than just a small ice layer as is suspected today) deposits glaciers in a different place, namely along the rim of the Hellas Basin. Atmospheric circulation in the south of Mars forms a circum-polar air current that mostly does not mix with currents in the northern hemisphere. However, the depth of the Hellas Basin disturbs this air current and allows a jet of moist air to flow up towards the equator. That jet would then deposit ice along the rim of the Hellas Basin in a location that very nicely coincides with observed glacial features.
Part (a) of the figure on the left marks observed glacial features (the different symbols indicate different types of features, the hourglass crater shown at the top of this entry is the red dot), while part (b) shows where Forget and others’ model predicts ice would accumulate. The red arrow shows the direction of the moist air jet coming from the Southern polar cap.
Context:
As soon as researchers started looking at images from Viking, and later the Mars Global Surveyor, Mars Odyssey, and Mars Express, they saw what looked to be glacial features. Explanations for the origins of these features have included gradual creep of permafrost, or even outflow from groundwater reservoirs. Some of these features may be explained by those two processes, but many of them were consistent with true glacial erosion. We see u-shaped valleys, lateral and end moraines, and outflow debris aprons, all characteristic of true atmospherically-deposited flowing glaciers.
Forget and others note that they did not include some atmospheric processes that may have been important in a wetter Martian atmosphere because their model was intended to simulate present day Mars. Also, each cell in the model (explained in a previous synopsis) is approximately 2×2 degrees, which is large enough to miss many smaller possible ice deposits. Thus they justify looking at only the largest of the observed glacier-deposited features and suggest that more refined versions of their model would likely explain many of the smaller features seen across the Martian surface.
The significance of the presence of mid-latitude glaciers on Mars extends beyond mere explanation of orbital photographs. Those glaciers may have fed groundwater aquifers and buried massive chunks of ice that could be vital sources of drinking water and fuel for Martian explorers. Though the polar caps do have large volumes of ice, the temperatures in those regions are so cold that human exploration would be much more dangerous than near the equator where summertime highs can be well above freezing (though the daily temperature variation because of the lack of clouds means night time temperatures are still well below freezing).
General Explanations:
Obliquity, and Obliquity Variation
Obliquity is the angle between the equator of a planet or moon and the plane of its orbit. For the Earth and Mars the plane of the orbit around the sun is the Ecliptic. Below is a diagram I put together showing the angles of obliquity and orbital planes of Mars and the Earth/Moon system. The spin axis of the Earth “wobbles” periodically by about 1.5 degrees every 41,000 years, while Mars’ axial tilt varies chaotically by more than 35 degrees. The reason for this enormous difference is that the orbit of our Moon provides a gyroscopic stabilizing force on the direction of our spin axis (Notice that the plane of the lunar orbit is inclined by about 5.2 degrees to the ecliptic. Also, currently the moon’s obliquity is about 1.5 degrees relative to its own orbit and wobbles by about 3 degrees. This wobble influences the Earth and induces a motion known as nutation). Also, note that historically the most likely value of Mars’ obliquity is approximately 40 degrees, so its current shallow obliquity and its similarity to our own is something of a fluke. At these higher obliquities, Mars’ polar caps would have received a great deal more sunshine during the summer than they do presently.
