It all depends on density of your body vs. the surrounding water. If it's high/low you would be more affected by gravity than if it is low/high, or both were the same.

For example, if you lived in water and had an air-filled bladder (like most bony fish), the gravity wouldn't matter. The bladder would allow you to control how dense you are relative to the surroundings (by compressing it or relaxing it), and therefore how much you "weigh". If you were a human and the water was REALLY salty (like in the black sea) you would actually float to the surface. If it was fresh water, you could sink to the bottom, but gravity wouldn't affect you too much because most of your body is made out of water.

... so in general, you're right. Gravity wouldn't have such a big impact underwater on life as we know it.

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"As long as the prerequisite for that shining paradise is ignorance, bigotry and hate, I say the hell with it." Henry Drummond, inherit the wind.

Isn't what you're talking about more related to density than gravity?

Gravity would have a major effect on the pressure (psi) of water at 2.2X relative to the pressure on Earth.

Swimming at 50 feet there for a human would be like diving to 110 feet here.

Well yeah. But if your body is less dense than the water surrounding it, you will float up and therefore you wouldn't need strong bones or whatever to deal with gravity pushing you down.

Hmm. Hadn't thought of that. Of course, the exact numbers would also depend on how much atmosphere that planet has...

but anyway, human-like organisms could live in shallow seas (assuming there are any) or just generally near the surface, and the pressure changes wouldn't affect them too much. Or there could be organisms similar to deep-sea fish, or those which migrate (here on earth) from the deep ocean to the surface each night (or like sperm whales, which dive down deep in order to find Squid and other things to eat)... They'd have to be careful about pressure, but I don't think the changes would be too drastic...no? I still think organisms would be less affected by gravity than if they lived on land.

And they could certainly have soft bodies and be huge.

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"As long as the prerequisite for that shining paradise is ignorance, bigotry and hate, I say the hell with it." Henry Drummond, inherit the wind.

Sorry for quoting myself, but I feel we haven't really given this all the debate it deserves - so I'll make another try.

For me, the most fascinating thing about Gliese 581c (apart form potentially holding liquid water, of course)
is the mind-blowingly exotic conditions in that system.
(e.g. SEVERAL planets INSIDE Mercury's orbit )

And the fact that maybe THOSE are the 'normal' planetary systems, since brown dwarfs are so much more frequent than our type of star (Sun).

1) This really defies all our earlier conceptions about planetary systems, doesn't it? Does this maybe represent the typical solar system?

2) I'm trying to imagine how weird those conditions might actually 'feel' like on Gliese 581c (see my quote above). Any thoughts?

I'm a new listener to the Astronomy Cast, and new reader here, but I thought this would be an appropriate place to mention a link concerning Gliese 581c.

The following link goes to an article in the online journal The Geochemical News, published by the Geochemical Society. The article provides a speculative but geologically plausible description of possible conditions on this newly-discovered planet (including some speculation about possible life there). It's sort of a literary analog to an 'artist's conception' of the planet.

http://gnews.wustl.edu/gn131/gliese581c.htm

Anyway, thanks for the great podcast.

Assumes that the planet will, despite "somewhat lower metal contents but substantially higher volatile contents than the inner worlds of our own star system", have only limited water, not enough to cover the entire surface. Also doesn't seem to understand that H2O is a greenshouse gas. A thick CO2 atmosphere would make both sides of the planet extremely hot (cf Venus, the 120 day night makes no difference to the temperature).

Water is a greenhouse gas, it's true, and on a per molecule basis has a similar magnitude of infrared absorbance - and therefore radiative forcing potential - as CO2. However, the effect of water vapor on global atmospheric heat trapping is strongly dependant on its average atmospheric concentration. That, in turn, tends to be strongly heterogeneous regionally, varying on Earth from about 0 to 4% depending on latitude, average surface temperature, wind patterns, and availability of evaporating liquid water at the surface.

On a planet as described in the narrative, absolute global H2O vapor concentration would be likely limited by freeze-out on the night side and low availability of surface liquid on the day side. So the relative humidity, and therefore the absolute concentration of water vapor, would be globally low, with a relative maximum near the terminator (over a liquid ocean) and a minimum at both poles. So, the average global effect of H2O on absolute radiative forcing would likely be minimal.

As far as a higher volatile content producing a global ocean, that is one potential model, and it may be more accurate. However, the model described in the narrative seems to suggest that due to tidal locking effects, at any given moment much of the available H2O reservoir is held as a solid on the night side. This could be a realistic model, given that the rate of viscous relaxation of ice would be the rate-limiting step in delivery of liquid H2O to the terminator environment. That step would likely be slower than the rate of H2O vapor delivery to the night side, resulting in a low average global concentration of H2O vapor and a limit to the amount of liquid water there could be on the day side. It would probably take some rheological calculations of the rate of viscous relaxation of ice in the heavier gravity, along with the significant distance (Gliese 581c is probably a larger planet than Earth) the glaciers would have to travel to reach the terminator, and some estimate of the total reservoir of surface H2O, to make a definitive conclusion.

One thing the author didn't mention but which would probably be a factor on Gliese 581c; crustal strain produced by tidal effects. On a sphere experiencing anisotropic stress, you'd expect strain fractures along the equator perpendicular to the axis of stress (in this case the tidal pull direction, oriented to the day/night 'poles'). Along the terminator there would be latitudinal rifting of the crust, forming immense grabens into which a liquid ocean would tend to accumulate. The deepest lake on Earth is Lake Baikal (Russia), formed over a graben resulting from continental rifting. It's likely a similar process would occur on Gliese 581c, but there resulting in an equatorial ring of graben-bound seas.

With so many variables, it seems the best we can do without direct evidence is come up with a suite of possible models constrained by known factors. That still leaves room for some fun speculation, for now.

I think you'd need some pretty special topography to make that work. What's stopping a massive water flow from the cold to the hot side? Unless you assume all the highland is on the day side and all the ocean basins are on the cold side. If there is an ocean basin that spans the terminator then you're going to have massive evaporation/boiling there. You only have to evaporate a small percentage of an ocean to add massively to the atmospheric greenhouse, and evaporates that condensed out as snow and rain are likely to fall where they are most available for flow back to the hot side, close to the terminator, not at the pole of cold.

Exactly what happened along the way to replace the primordial hydrogen, helium, and methane atmosphere with carbon dioxide isn't explained.

But remember that the intensity of solar insolation would be weakest near the terminator, and you'd have the nearby glaciers acting as a thermal sink. The result should be a dry, cool environment. You wouldn't expect to see high surface temperatures anywhere except near the dayside pole. As far as topography goes, I think the article's model assumes that there isn't much vertical exaggeration because of the higher gravity. In any event, ice would just follow whatever regional topographic gradients already exist, and there would undoubtedly be nightside regions of tectonic uplift where massive continental glaciers simply flow around the rocky massifs. The key variable would be total available surface H2O: too little and all of it builds up permanently on the night side, flattens itself out, but none ever relaxes completely to the terminator. In such a model, oceans would be precluded.

It's a good point about global temperatures in a thick atmosphere, with Venus as an example. We don't know the actual atmospheric composition of Gliese 581c, so its atmosphere remains a free parameter in any model of that world's global geochemistry. However, it's important to note that Venus has a very heavy atmosphere, about 93 bars at the datum, and is mostly CO2. Also its cloud cover (the formation of which is still something of an unresolved issue) assists in heat retention, and most importantly Venus receives a great deal more insolation than would a planet orbiting Gliese 581, even at the proximity of planet c.

If Gliese 581c had an atmosphere very similar to Venus, it might also have a runaway greenhouse environment. If its atmosphere is substantially thinner, or contains a large partial pressure of infrared-inactive components (e.g. N2), a runaway greenhouse would be unlikely, as would a global distribution of intense heat. Gliese 581 is a very dim star; only about 1% as bright as the Sun. At 1 AU distance, the blackbody temperature of an Earth duplicate would be low enough to freeze out most of our atmosphere. Even as close as Gliese 581c is to its primary, its dayside pole would still receive only a fraction of the energy that the planet Mercury receives from the Sun.

About primordial H2, etc.: the gravity of Gliese 581c might make retention of even the lightest gases (H2, He) possible, and if the planet formed farther from its primary and wandered closer at a later date, this would be even more plausible. However, in our system the inner planets were stripped of their primordial atmospheres by intense solar winds during the T-Tauri phase of solar formation. A red dwarf would have a weaker T-Tauri phase, but would still exhibit a more energetic phase of solar activity as it formed. Gliese 581 is a variable star, which probably contributed even more violence to its birth. It's probable that inner worlds in the Gliese 581 system would have undergone some atmospheric stripping, making a modern H2 and He atmosphere around a terrestrial planet unlikely.

I look forward to future spectroscopic observations of Gliese 581c's atmosphere. Such data would settle many of these questions, and teach us a lot about planetary formation processes around red dwarf stars.

WHOAAA, now that is a COOL link!

Might be a bit speculative, but hey,
we're talking about the first earth-like planet we've found
- if that doesn't make our imagination run a little wild, what else will ever?!!

Thank you for sharing it Nimwe!!!

PS: can't wait to read through the rest of the posts tonight
- finally we're getting into the debate
(this thread was going a bit astray at first)

Oh, emos, stop crying, these babbles about the end of the universe are just babbles , enyoy living , the live is short, so why not enyoy it , and please dont trust that babbles about the end of universe these are blablas

OMG YOU WANT "EVIDENCE" FOR EVERYTHING! Some things are true without evidence.