The atmospheres of water worlds

October 23, 2017
The atmospheres of water worlds
Artist's illustration of a hypothetical ocean planet with two natural satellites. Astronomers have calculated the rates of evaporation of water from ocean planets under a variety of stellar wind scenarios, and conclude that ocean exoplanets around M stars are likely to loose their water in a relatively brief time. Credit: Lucianomendez, 2011

There are currently about fifty known exoplanets with diameters that range from Mars-sized to several times the Earth's and that also reside within their stars' habitable zone – the orbital range within which their surface temperatures permit water to remain liquid. A "water world" is an extreme case, an exoplanet defined as being covered by a deep ocean, perhaps as deep as hundreds of kilometers, and among these fifty are several that might be candidates for this category. Astronomers note that at least two of the terrestrial planets in our solar system, Earth and Venus, may possibly also have been water worlds early in their evolution.

One of the critical factors in determining if a planet could really be habitable is the presence of an enduring atmosphere. The deep oceans on a water world offer a reservoir for water vapor for its atmosphere, and so scientists have been trying to calculate how stable an exoplanet's ocean and atmosphere are, especially to effects like evaporation by winds from the star. Since most of the fifty known examples orbit close to their small, host M stars, they are heavily exposed to stellar winds and related stellar even though their temperatures may be moderate.

CfA astronomer Manasvi Lingam was a member of a team of astronomers who modeled the effects of the on a water world under a variety of possible scenarios. They include effects of stellar magnetic fields, , and atmospheric ionization and ejection. Their computer simulations are in good agreement with the current Earth-Sun system, but in some of the more extreme possibilities, as for example might exist on the set of exoplanets around M-stars, the situation is very different and the escape rates may be as much as or more than one thousand times greater. The result means that even a water world, if it orbits an M-dwarf star, could lose its atmosphere after about one billion years, a relatively brief time for possible development of life. Lingam has also recently co-authored two related articles on the same topic with CfA astronomer Avi Loeb.

Explore further: The space weather forecast for Proxima Centauri B

More information: Chuanfei Dong et al. The Dehydration of Water Worlds via Atmospheric Losses, The Astrophysical Journal (2017). DOI: 10.3847/2041-8213/aa8a60

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eric96
not rated yet Oct 23, 2017
Gotta love it each time they talk about the habitable zone with absolutely zero forethought of radiation. Bunch of fools.
Mark Thomas
not rated yet Oct 23, 2017
"habitable zone – the orbital range within which their surface temperatures permit water to remain liquid."

The "habitable zone" is thought to be the best place to look for life because all known life on Earth requires liquid water. Scientists would love to know what the actual limits for life are for every environmental factor, such as radiation, both separately and in combination. However, complete knowledge of the limits on life on many other worlds is going to require we boldly go where no one has gone before.
rrwillsj
4 / 5 (4) Oct 23, 2017
Not going to get an argument from me about the dangers of radiation on biology. I rank it number 2 hazard after low to zero-gravity environment.

However, it is the Earth's magnetic field that (usually) shields us from such consistent danger.

I was wondering, if one of the more distant research satellites searching for exoplanets. If it was turned around towards the Earth? What would it detect to indicate non-technological life? And could those findings assist discovering living worlds?
Mark Thomas
5 / 5 (2) Oct 23, 2017
rrwillsj, I believe the answer is yes. More recent probes with working spectrometers would probably identify both oxygen and methane in our atmosphere (gases which would not exist simultaneously very long without biology). It is also possible the spectral signature of chlorophyll could be detected, given how much of the Earth has plant life.

Others have proposed using these indicia of life (bio-markers or bio-signs) to look for life on exoplanets. This will become much more important once the James Webb Space Telescope (JWST) is launched in a year or two. The JWST has a huge collecting area for concentrating light compared to the Hubble Space Telescope and far greater sensitivity to infrared, where many absorption lines lie.

Hopefully we will be able to obtain some high quality spectrographic info from nearby exoplanets. If the signals look favorable, but are not definitive, I suspect we may all be discussing the finer points of spectrographic analysis in a couple years.
rrwillsj
3 / 5 (2) Oct 24, 2017
Thanks for the head-up MT. I keep an eye out for reports from the JWST.

However, I encourage people to be cautious about accepting click-bait headlines that make assumptive claims based on early, unsubstantiated and unfiltered data.

It is mt own, unsubstantiated opinion that early proclamations of discovering earth-type worlds will turn out to be misdiagnosed series of wishful thinking.

If we discover any exoplanets as having the longevity to achieve a visible biosphere? The innate randomness of this Universe dictates those worlds will be few and far between. And this early in this era of our Galaxy? Most likely to be archaic-type micro-biological lifeforms.

That will not be at all, in any way, compatible with Earth's varieties of biology.
Mark Thomas
1 / 5 (1) Oct 24, 2017
rrwillsj, you make some good points and click-bait headlines are an unfortunate plague.

I believe one of the biggest limiting factors is creating a well-oxygenated, breathable atmosphere, then keeping it long enough for large, multi-cellular life to evolve. This may be a somewhat Earth-centric point of view, but life existed here for billions of years before macroscopic multi-cellular life arose, so it is a good bet this is a difficult thing to do. I expect there will be many surprises out there, but I also expect terraformable worlds will vastly outnumber naturally terraformed worlds like ours. Time will tell.
john_snodgrass
not rated yet Oct 24, 2017
I think we may be missing something here. If a water-world orbiting a M-class star could lose its atmosphere after about one billion years and we have detected about 50 of them. due to stellar time-frames most of these should have lost their water by now?
rrwillsj
1 / 5 (1) Oct 25, 2017
MT, I think I understand the points you are making. However, I have to disagree with your conclusion that we will find worlds that could be rehabilitated for sustaining an introduced Earth biosphere.

In my opinion, it is still far too early in the timescale of our Galaxy to have any such expectations. Perhaps over the next hundred billion years such opportunities will become more common. Perhaps not.

Any world we would consider habitable, will already have it's own sustained biosphere. The odds are, such nativist biologies, evolved over billions of years, will be quite inimical to colonization.

If you are considering habilitating dead worlds similar in mass and materials to Earth. Within a sustained Goldilocks Zone.

I just gotta wonder what went wrong that prevented or disrupted the establishment of a native biosphere?
Mark Thomas
1 / 5 (1) Oct 25, 2017
rrwillsj, because terraforming technology must be global in scale, it may be considerably harder to implement than to develop. For example, we already know adding greenhouse gases to Mars will warm and thicken the atmosphere, but adding enough is going to be challenging. Eventually I believe it will be possible to transform the surface of Mars. I also suspect Mars has a subsurface biosphere, but the surface appears to be generally to hostile for life.

I imagine there are plenty of Mars-like planets in the galaxy whose conditions could be improved with terraforming.

It is incredibly ironic and poetic justice that one of the greatest threats to the human race, i.e., global warming, could be destined to become one of our greatest allies in the very long run.
Mark Thomas
1 / 5 (1) Oct 25, 2017
rrwillsj, another way to answer your question is that we might find an Earth analog planet, but one that is a couple billion years behind us. We could help it along by introducing plants other life to accelerate the terraforming process instead of waiting for those lifeforms to evolve.

Suppose this Earth analog were to only have another billion years in the habitable zone, and let us say this is too short to evolve land-based plants on its own. We would be fostering a living biosphere where none would otherwise develop.
yaridanjo
1 / 5 (1) Oct 26, 2017
To much guesswork to be credible, especially given evidence that Proxima b hosts an intelligent space mobile species that has been visiting Earth for at least 4300 years now.
rrwillsj
3 / 5 (2) Oct 26, 2017
Sorry MT but I am about to sound like a moralist prig, Please do not take offense, this is not personal, This is exactly the reason for a pragmatic argument.

Turn your speculations around. How would you feel? How would you react? If some BEM/LGM came along and decided to dump an alien biosphere on the Earth?

After all, they know whats best usage for this real estate. Obviously superior cause they told us they were superior.

This is your justification for devastating a defenseless alien world and dictating their future. We have the power, we must be right!

Well buddy, tumbleweed and asian carp and the formosa termite and plague carrying rats all over this world should clue you in to the Law of Unintended Consequences.
Mark Thomas
1 / 5 (1) Oct 26, 2017
rrwillsj, no offense taken, I would rather hear your points. I certainly fear the Law of Unintended Consequences (LOUC) when it comes to doing anything with biology. I am well aware of many missteps people have made over time. I think terraforming Mars is technically doable, but it is going to going to be very risky because of LOUC. We are novice terraformers at best, so we will probably make mistakes.

From the morality perspective, we have a moral imperative to survive too, and sooner or later that means living on other planets. I personally don't have any problem with terraforming dead or bacteria-only worlds. In fact, one of the unintended consequences of terraforming Mars may be to improve conditions for subsurface bacteria by making the planet warmer, wetter and more nutrient rich. We may find martian bacteria running rampant on the surface of Mars once terraforming takes hold. At the other end of the spectrum, as a Star Trek fan, the prime directive applies.
rrwillsj
1 / 5 (1) Oct 27, 2017
Personally I have always admired the courage of pacifists who practice the level of morality they espouse.

Wish I had that level of courage!

It really isn't sensible to claim a moral standard, that when the moment arrives for your gain? You toss those standards aside for personal profit.

We certainly have no standing to dictate the future of other worlds. Until we make a realistic effort to resolve all the problems we have inflicted on this poor, old world. It chokes in our filth.

The claim that we 'deserve' a refuge from the very disaster we caused? Is a bloated sense of affluenzic entitlement.

A disease-bearing vermin, a cancer, a blight, a gangrene has no superior right to existence than you or I. Yet the standard you advocate. That you have a superior claim to survival displays the inherent contradictions of the code of Survival of the Fittest.

If you honestly believed that, you would be submitting to domination by termites or even tardigrades.
Mark Thomas
1 / 5 (1) Oct 29, 2017
rrwillsj, building a truly advanced, space-faring civilization from beings evolved from animals with no external guidance has got to be one of the hardest things to do. You need to be cutting the human race a whole lot more slack than you are. I am also hopeful we have a shot at redemption. There are at least three aspects to humanity redeeming itself for the damage done. 1.The majority of people want to find ways to reverse the damage done here and we may eventually accomplish some of that. 2. Mankind may prevent the next deadly asteroid impact,thereby saving life on Earth to some large degree. 3. With terraforming, perhaps we can spread life from Earth onto other worlds, ensuring their survival as well.
rrwillsj
1 / 5 (2) Oct 30, 2017
MT, for your first assumption? That the Human Race will make some stupendous effort and massively redirect the funding and resources to salvage this world's biosphere?

I do not share that optimistic wish.

Without a means to reproduce Real Gravity and some sort of protective field to redirect heavy radiation? There will be no long-term biology beyond the Van Allen Belt. Defending ourselves from falling rocks is a job for robots and drones.

As for your third proposal? Genocide of alien biospheres is saving them from nothing!

Please to consider for your edification. Falling rocks and ices, over billions of years, have played an important part in constructing this world and energizing evolution.

Changing the rules to temporarily (at best!) benefit yourself has no moral or ethical standing. And may well give credence to those who share your lack of any standards. While you are at the bottom of the gravity well and they are above it.

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