Pale Blue Dot III.3 - Live Long and Prosper

Pale Blue Dot trucks on, some very interesting discussion on biomarkers and remote sensing, and ongoing plenary sessions on science and media relations and the current state and evolution of science journalism. UScentric, but very, very interesting.
Good turnout by science journalists, btw, both old pros and students.

Raymond Pierrehumbert is now giving a very interesting talk on long term habitability and the long term evolution of the atmosphere, prospects for continued habitability and implications for exoplanet detection.

The issues are the usual. The atmosphere evolves, and there needs to be feedback so that habitability persists as the luminosity of the star evolves, slowly.
While life on Earth arose very rapidly, as far as we can tell, it took some 3 billion years for complex multicellular animal life to get going, and the probable remaining habitable time (without drastic planetary engineering) is likely less than a billion years.
So, there is a statistical concern that evolution to complex life is "touch-n-go" compared with the typical timescale for a planet to actually be habitable. Statistics of a single object, but still.

One concern is that "snowball Earth" episodes occurring intermittently could hit at just the wrong time and choke off incipient animal life, as distinct from single cell organisms that may ride out the snowball.
Fortunately for us, some eukaryoted did survive the pre-cambrian snowball, maybe cold resistant algae, or some critters huddled around a geothermal oasis, or there was a warm stripe near the equator, maybe due to contingent continental configurations keeping a pool of open water.

There is of course also the concern that a snowball would either become permanent or be severe enough that the planet is sterilised (unlikely, I think, life, once it gets going, is pretty tough).
Snowball Earth occurs when cooling leads to runaway glaciation. Growing ice cover leads to more radiation reflected less absorbed, so there is further cooling, hence more ice and repeat.
Snowballs break, we think, when greenhouse gases from volcanism accumulates to the point where the ice melts (presumably at the equator) and you get a reverse albedo feedback as dark ocean is exposed and warms by absorbing radiation and melts more ice.

The Earth appears to have gone through maybe 2-3 snowball phases. Particularly ~ 2 billion years ago
and about 700 million years ago, interestingly, just before the cambrian explosion...
The earlier snowball may have coincided with the (permanent) rise in atmospheric oxygen.

Another interesting topic covered at the meeting is whether there were early "pulses" of oxic atmosphere - ie that oxygen was released in substantial amounts and then drew down rapidly back to an anoxic phase, either through reduction chemistry in the ocean, or through self-poisoning of beasties lucky enough to come up with oxygen cycle photosynthesis, as distinct from anoxic photosynthesis, but not lucky enough to have a anti-oxidant scheme to protect its own metabolism.
Tricky that: may in fact be one of the two or three evolutionary hurdles that take serious time to work around. Be interesting to know for sure.

Interesting new point (new to me).
To break the snowball, models suggest you need to get to ~ 1-200,000 ppm of CO2.
You then get a "fireball" feedback, you go from sub-zero mean tempertures to temperatures that are maybe 60K or more higher (100 F). That is a large temperature jump.
Simulations suggest mean continental land temperatures go to ~ 70C (160-170 F), which is a hard shock for cold adapted organisms, and likely to sterilise land, if anything survived the ice; so you have to survive in the ocean only. It is also a strong enough a swing that going straight to a Venus greenhouse runaway effect could happen directly, Probably cloud feedback stops that.
Again it'd be nice to know for sure,.
I like Raymond's suggestion that we do a 2001AD on Europa just to see it go from a snowball to a Venus directly...

PS: Kirschvink followed and made some interesting points, arguing strongly for a single oxidation episode occuring rapidly after onset of oxygenic photosynthesis, and incidentally triggering the third and worst of the Huronic glaciations.
He also argued that the older banded iron formations came from biological oxidation of Fe(II) rather than free oxygen. He made the point well, will have to think about it.

Also an interesting talk on the Goldilocks problem and Venus/Earth/Mars early comparative planetology, a interestingly different, but I suspect controversial case made for a persistent moist greenhouse on Venus, with a long lived (~ 2 billion year!) warm ocean. In which case Venus is a candidate for having had life. Be hard to find traces, unless some persists in the upper atmosphere, which, not surprisingly, has been argued in a refereed paper as a possibility.
Who knew.