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