Wednesday, August 15, 2012

Snows of Tharsis Montes

On Earth, tropical glaciers are a rarity and they are only found near the Equator on the mountains of the Indonesian province of Papua, East Africa and in the Andean mountains of South America. The total area of these tropical glaciers represents only 4 percent of the combined area of Earth’s mountain glaciers. Tropical glaciers are unique because they experience very minimal seasonal variations and without a colder winter season where ice can accumulate, these glaciers are more susceptible to climate change. The only point exactly on the Earth’s Equator with permanent snow cover is found on the south slope of Mount Cayambe in Ecuador and at 4690 metres, this is also the highest point in the world crossed by the Equator.

On Mars, huge fan-shaped deposits of glacial origin are known to cover the northwest flanks of the massive Tharsis Montes volcanoes located within the tropics. The Tharsis Montes are three enormous shield volcanoes and Ascraeus Mons is the tallest of them, with a summit elevation of over 18 km above the Martian datum. Based on the sizes of these fan-shaped deposits, each glacier once covered an area of over 100,000 square kilometres. Climate models of Mars have shown that during periods when the axial tilt of Mars was much larger, the climate would have been conducive for the formation of such tropical glaciers. Like the Earth, Mars also experiences Milankovitch cycles which causes the axial tilt of Mars to vary over a 120,000 year cycle. However, Mars’ axial tilt has a larger variation than the Earth’s since Mars does not have a large moon like the Earth to provide a stabilizing influence. The axial tilt of Mars can exceed 45 degrees. In comparison, the current axial tilt of Mars is 25.2 degrees, which is almost the same as the Earth’s.

Fan-shaped deposits (yellow) on the northwest flanks of the Tharsis Montes interpreted to represent tropical ice-age glaciation (Head and Marchant, 2003) (top, Ascraeus Mons; middle, Pavonis Mons; and bottom, Arsia Mons).

With a much larger axial tilt, the climate of Mars would have been quite different as water will sublimate at the polar ice caps and be transported to the tropics. During such periods in Mars’ history, the presence of prevailing winds blowing west to east over the Tharsis Montes volcanoes would have generated strong upwelling and cooling of moist air as the air mass is forced up the slopes of these enormous mountains. This allows the moisture to precipitate as snow on the western flanks of the Tharsis Montes volcanoes to form extensive tropical mountain glaciers. In climate models, the volume of water ice currently in the polar ice caps of Mars served as the source of moisture for the development of these glaciers. On Mars, the extremely cold and hyper-arid environment means that these tropical mountain glaciers are quite different compared to glaciers on Earth. For these Martian glaciers, sublimation serves as the dominant mechanism for mass removal, with buoyancy-driven sublimation and turbulence-driven sublimation being the two sublimation mechanisms. On Earth, melting is the primary mechanism through which a glacier loses mass.

In Mars’ carbon dioxide dominated atmosphere, buoyancy-driven sublimation depends on the density difference between water vapour and carbon dioxide while turbulence-driven sublimation depends on the amount of air blowing over the surface of the glacier. The ability for a glacier to develop and grow on Mars depends on the sublimation and accumulation rates of ice. There must be net accumulation of ice for a glacier to grow. As temperature declines, the sublimation and accumulation rates also decline. Just as on Earth, temperature decreases with altitude on Mars and this means that sublimation and accumulation rates decline with increase in altitude. However, sublimation rates decline less rapidly than accumulation rates and this lead to a negative net accumulation of ice at higher elevations. As a result, contrary to what is seen on Earth, snow-capped mountains are not expected since ice is more likely to accumulate on the flanks than on the summits of the towering Tharsis Montes volcanoes.

At the lowest elevations, increase in turbulence-driven sublimation and possible melting can also lead to a negative net accumulation of ice. Therefore, two equilibrium lines can exist on Mars where one equilibrium line denotes the lowest altitude and the other denotes the highest altitude where ice can accumulate. Estimations of the total ice volume of the tropical mountain glaciers through simulations have shown it to be comparable to the volume of the current north polar ice cap on Mars, which is estimated to be about 1.6 million cubic kilometres. In comparison, the ice sheet covering Greenland on Earth has a volume of 2.85 million cubic kilometres. As such, the inventory of water ice locked in the current polar ice caps on Mars is sufficient to produce the large tropical mountain glaciers during epochs of high axial tilt in Mars’ history.

1. James L. Fastook, et al., “Tropical Mountain Glaciers on Mars: Altitude-dependence of Ice Accumulation, Accumulation Conditions, Formation Times, Glacier Dynamics, and Implications for Planetary Spin-axis/Orbital History”, Icarus 198 (2008) 305-317
2. Seth J. Kadish, et al., “The Ascraeus Mons Fan-shaped Deposit: Volcano-Ice Interactions and the Climatic Implications of Cold-based Tropical Mountain Glaciation”, Icarus 197 (2008) 84-109