Re: Habitability of Gliese 876c & d

<brdavis@xxxxxxxx> wrote in message
news:1124986104.839927.312290@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Logan Kearsley wrote:
>
>>> tidal heating... [might not] generate
>>> terrestrial-style tectonics
>>
>> It doesn't need to generate terrestrial-style tectonics
>
> The thing about the carbonate-silicate feedback is that it depends
>on weathering mountain ranges, so you need mountain ranges, and the
>easiest way to do that is fold belts from plate-style tectonics. And
>then you need some very large potential sink for carbonates, and
>without recycling off the ocean floor, I'm not sure how that works (to
>put it another way, I've really never tried to think it through; it may
>work rather well).

It may be the easiest way, but who says it's the only way? Those giant
hot-spot volcanoes or tidal-rift vents I mentioned might do the job. And
that is, of course, assuming that the carbonate-silicate cycle is in fact
necessary, which, although very convenient for a life-bearing planet to
have, may not be the case.

>> But is 'enough' [tidal heating] the same as 'a lot'?
>
> I'm not sure, again (I much prefer those sort of answers, BTW - it
>means I'm still learning, and thus not dead ;-) ). Io does not seem to
>have plate-style tectonics, but "why" here has more than one answer -
>for instance, Io is rather completely dehydrated, and water is likely
>another necessary component for plate tectonics.
>
>>> ...where upon it builds up in the atmosphere over time,
>>> leading to an increasing greenhouse effect, skyrocketing
>>> temperatures, boiling away the oceans, and... voila', baked
>>> planet.
>>
>> Why? Where would [the CO2] come from to build up?
>
> Terminal vulcanism, like Mars (or Venus?) seems to have experienced,
>where CO2 is released but not fixed back out of the atmosphere.

Aha. I was thrown by the 'over time' bit. So we just have to assume that the
amount of CO2 that is released and cannot be sequestered by other means is
less than would be detrimental. Since the amount that would be detrimental
is probably rather high, that shouldn't be a problem.

>> when most of the incoming radiation is infrared, CO2
>> could act as a cooling agent rather than a heating agent.
>
> Well, it means the atmosphere would be heated at a different
>location rather than at ground level. But again, it's not something
>I've work through as yet.

As long as it doesn't get heated at ground level, that's all we're looking
for. Who cares, as far as the viability of surface life is concerned,
whether the temperature is unbearable twenty kilometers up?

>> If there's too much CO2, or it gets too hot from increasing stellar
>> luminosity, plant growth speeds up to take the extra CO2 out of the air.
>> Of course, one then has to wonder what happens to the extra oxygen....
>
> It is used by other organisms to oxidize the fixed carbon back to
>CO2 as an energy source, the same way it does now on Earth. Build up a
>high-pressure O2 environment, and you get fires (and, honestly, long
>before that you get highly accelerated decomposition) that redress the
>balance. You need to lock up the CO2 as CO2 in this case (note to
>greenhouse warming critics - no, this doesn't have anything to do with
>anthropogenic effects, for instance. What humans do to the terrestrial
>CO2 balance is very temporary... heck, the oceans are not even in
>equilibrium with atmospheric CO2 as yet).

Well, that would be the default situation, yes. Perhaps I should rephrase my
statement: One then has to wonder by what process we could remove that extra
oxygen from the atmosphere. Around a flare star, or a hot with lots of UV, I
might posit that most of it ends up as nitrous oxides. Perhaps, though,
there's a lot of sulphur laying around on this world- the free oxygen
combines with that to produce SO2 and SO3. In either case, that also
provides a handy solution for where the acidic seas come from.

>> surface temperature --> watts radiated per meter --> use the
>> radius [to get] total luminosity [etc.]
>
> OK. I just used the bolometric luminosity from on-line sources. I
>was wondering if you were trying to correct for the non-point-source
>nature of the star (particularly when it's that close to the planet).
>I've yet to come up with a good way to understand that issue.

So've I. Is an interesting problem to contemplate. The best thing I can
think of is to figure out the angle of a line tangent to both the surface of
the sun and the planet, and use that to calculate exactly what fraction of
the planet is lit at once, rather than just assuming it to be 1/2.

>> Just dividing them [advective timescale vs. radiative relaxation
>> timescale], unfortunately, doesn't work
>
> Well, first, nobody said the relationship had to be linear with
>respect to such a dimensionless index. Second, it's quite possible that
>there is no even semi-accurate correlation - after all, this is a very
>complicated system we're trying (wishing?) to model with some simple
>equations.

It doesn't say that the relationship is linear, but the paper does say that
delta-T_dn is determined by the relative values of t_r and t_d.

> But, I'd love to hear what Joshi thinks on the subject :-).

So now, we shall just wait for his response....

-l.
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