Update on Gliese 581d's habitability

May 9, 2011 By Jon Voisey, Universe Today
An artist’s impression of Gliese 581d, an exoplanet about 20.3 light-years away from Earth, in the constellation Libra.

When last we checked in on Gliese 581d, a team from the University of Paris had suggested that the popular exoplanet, Gliese 581d may be habitable. This super-Earth found itself just on the edge of the Goldilocks zone which could make liquid water present on the surface under the right atmospheric conditions. However, the team’s work was based on one dimensional simulations of a column of hypothetical atmospheres on the day side of the planet. To have a better understanding of what Gliese 581d might be like, a three dimensional simulation was in order. Fortunately, a new study from the same team has investigated the possibility with just such an investigation.

The new investigation was called for because Gliese 581d is suspected to be tidally locked, much like Mercury is in our own solar system. If so, this would create a permanent night side on the planet. On this side, the temperatures would be significantly lower and gasses such as CO2 and H2O may find themselves in a region where they could no longer remain gaseous, freezing into ice crystals on the surface. Since that surface would never see the light of day, they could not be heated and released back into the atmosphere, thereby depleting the planet of greenhouse gasses necessary to warm the planet, causing what astronomers call an “atmospheric collapse.”

To conduct their simulation the team assumed that the climate was dominated by the greenhouse effects of CO2 and H2O since this is true for all rocky with significant atmospheres in our solar system. As with their previous study, they performed several iterations, each with varying atmospheric pressures and compositions. For atmospheres less than 10 bars, the simulations suggested that the atmosphere would collapse, either on the dark side of the planet, or near the poles. Past this, the effects of greenhouse gasses prevented the freezing of the atmosphere and it became stable. Some ice formation still occurred in the stable models where some of the CO2 would freeze in the upper atmosphere, forming clouds in much the same way it does on Mars. However, this had a net warming effect of ~12°C.

In other simulations, the team added in oceans of which would help to moderate the climate. Another effect of this was that the vaporization of water from these oceans also produced warming as it can serve as a greenhouse gas, but the formation of clouds could decrease the global temperature since water clouds increase the albedo of the planet, especially in the red region of the spectra which is the most prevalent form of light from the parent star, a red dwarf. However, as with models without oceans, the tipping point for stable atmospheres tended to be around 10 bars of pressure. Under that, “cooling effects dominated and runaway glaciation occurred, followed by atmospheric collapse.” Above 20 bars, the additional trapping of heat from the water vapor significantly increased temperatures compared to an entirely rocky planet.

The conclusion is that Gliese 581d is potentially habitable. The potential for surface water exists for a “wide range of plausible cases”. Ultimately, they all depend on the precise thickness and composition of any atmosphere. Since the planet does not transit the star, spectral analysis through transmission of starlight through the atmosphere will not be possible. Yet the team suggests that, since the Gliese 581 system is relatively close to Earth (only 20 lightyears), it may be possible to observe the spectra directly in the infrared portion of the spectra using future generations of instruments. Should the observations match the synthetic spectra predicted for the various habitable planets, this would be taken as strong evidence for the habitability of the planet.

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not rated yet May 09, 2011
Ultimately, they all depend on the precise thickness and composition of any atmosphere.

Who would have thought that habitability is largly dependent on the thickness and composition of an atmosphere (as well as distance from the host star)?
3 / 5 (4) May 09, 2011
And why is anyone running scenarios assuming that any atmosphere such a planet like this has is static?

Has there ever been a planet or moon with an atmosphere so lacking in mixing

Venus has planet wide circulation despite nearly be tidally locked.

The freezing out of atmosphere would create a vast low pressure area that would suck atmosphere from the day side of Gliese 581d at many times hurricane wind speed.

This would of course melt much of the atmosphere that froze out creating a rebound effect and quickly result in an atmosphere that is constantly on the move.

Far far more likely is a planet that has a near uniform temperature on both the day and night side, with the only difference being the day side is a bit warmer than the night side.

Any dramatic temperature difference would immediately kick in hypercane force winds and megastorms that would continue until the overall temperature of the planet reached a sort of balance, but even then the winds would only slow, not stop
not rated yet May 10, 2011
nice study but i think they are trying to hard,if you know what i mean.i expect in a few years a real "second Earth" will turn up a places like Gliese with be like our first look at the back of the Moon.
2.3 / 5 (3) May 10, 2011

I mostly agree.

At the surface, winds should blow from night side to day side, as cold air sinks, until it gets to the hottest region near the Normal with the star's incident radiation.

Then as the air heats up, it would rise rapidly and speed around to the opposite side of the planet, where it would sink as it encounters any permanent ice caps or just the darkness itself.

Essentially, the entire planet would be a gigantic "hurricane", with the surface of the planet itself being the "eye" the inflow being the surface winds and low level winds racing into the hotspot on the day side, and the "outflow" being the upper level winds ventilating back onto the cold side of the planet.

In terms of temperature, the only safe place for humans to live would likely be between plus or minus 45 degrees from the terminator. However, this is likely to be the region where straight line winds at the surface are highest, having had the longest time to accelerate.
2.3 / 5 (3) May 10, 2011
The potential wind speed from the center of the day side to the center of the night side, based on thermodynamics, could exceed 300m/s (675mph).

And since the hypothetically "stable" atmosphere was at least 10 bar, this means the density and pressure at the surface is obviously 10 times greater than on Earth, thus the mass component of wind's destructive force is ten times greater.

So, due to the square of wind speed formula for destructive potential, a 30mph wind there would be as destructive as a 95mph wind here on Earth.

So the destructive force (per second of exposure) of those 675mph winds would be 73 times greater than the strongest of the tornadoes that hit Alabama and Mississippi a few weeks ago.
2.3 / 5 (3) May 10, 2011
Actually, overall, Gliese 581d would probably be more suitable for human habitation if it's amosphere has frozen out.

After all, then you wouldn't have wind to deal with. You coudl build pressurized habitats on the day side just near the terminator where the incident radiation is low enough to not be dangerous, but high enough to provide heat and energy. Then import water and oxygen from the ice caps on the night side.

If the atmosphere is stable at 10bar or more, based on the wind speeds that would exist, it would be impossible for any man-made machine to ever survive a landing on the surface. It would be utterly destroyed in seconds, and actually bounced from Pole to pole every few hours by the wind...

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