Saturday, August 24, 2013

Ice-Albedo Feedback on Terrestrial Planets

The ice-albedo feedback is expected to play an important role in determining the climates of many terrestrial planets. It is based on the positive feedback between decreasing surface temperatures, an increase of snow and ice cover, and an associated increase in the planet’s overall reflectivity, which then further decreases surface temperature. Basically, the ice-albedo feedback describes the possible runaway cooling of a planet’s surface. This can cause a terrestrial planet to be locked in a snowball state where ice and snow completely covers the planet from Pole to Pole.

Previous studies have shown that the strength of the ice-albedo feedback is reduced for terrestrial planets around M-dwarf stars. This is because M-dwarf stars emit mainly in the near-infrared where the snow/ice reflectivity is low. In comparison, our Sun emits primarily in the visible where the snow/ice reflectivity is high. A recent study by Paris et al. (2013) investigates the influence of a planet’s atmosphere on the ice-albedo feedback.

Artist’s impression of a frozen terrestrial planet with ice covering much of the planet. Credit: Scott Richard

The study shows that for a planet with a dense carbon dioxide atmosphere, the ice-albedo feedback is suppressed whereby the difference in the planet’s reflectivity between the ice and ice-free cases is strongly reduced. A terrestrial planet located towards the outer edge of the habitable zone around its host star can be expected to have a very dense atmosphere comprising up to several bars of carbon dioxide. As a result, a suppressed ice-albedo feedback can allow such a planet to remain habitable by keeping it from entering a snowball state via runaway cooling.

The type of star around which a terrestrial planet orbits also has a large effect on the planet’s overall reflectivity. For a planet with a dense carbon dioxide atmosphere, its overall reflectivity is found to be 2 to 3 times higher if it were orbiting around a Sun-like star instead of an M-dwarf star. This implies that terrestrial planets around M-dwarf stars can be 10 to 20 percent further away and still receive the same net stellar energy input into their atmospheres. It could mean that the habitable zone around an M-dwarf star is widened with respect to the habitable zone around a Sun-like star.

Trace gases such as water vapour, methane and ozone in a terrestrial planet’s atmosphere can affect the strength of the ice-albedo feedback. The presence of small amounts of water vapour and methane can weaken the ice-albedo feedback by several percent for planets around both Sun-like and M-dwarf stars. For planets with dense carbon dioxide atmospheres around Sun-like stars, the presence of significant amounts of ozone can strongly suppress the ice-albedo feedback.

Paris et al. (2013), “The dependence of the ice-albedo feedback on atmospheric properties”, arXiv:1308.0899 [astro-ph.EP]