Re: Habitability of Gliese 876c & d

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).

> 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 neccessary 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.

> 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.

> 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 enviroment, 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).

> 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.

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

--
Brian Davis

.

Relevant Pages

  • Re: Habitability of Gliese 876c & d
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