In 2010, we haven’t the vaguest idea what will be discover in the near future and beyond. Theories bounce around like balls and we are just in kindergaren on this subject.

I think it would be marvelous to live for thousdand of years and see the discoveries that appear, then of course, being the curious person I am, I’d want more time.

JO

A habitable exo-Earth? Not so fast.

Indeed: the article is confusing both atmospheres and the metrics.

First, on the atmospheres and their mixing, the comments have mentioned what is observed, well mixed atmospheres. It should also be noted that if this didn’t happen, there would be little greenhouse effect as it happens (mostly) throughout the entire mixed troposphere.

Second, on the metrics, the habitability prediction is using observed gravity and surface temperature, so is safe from atmospheric compositions which are already folded in.

But even so, if you start to include atmospheric composition you will differentiate between inhabited habitable planets (nitrogen/oxygen atmosphere in Earth analogs, nitrogen/methane in putatively inhabited Titan analogs, no atmosphere in putatively inhabited Europa analogs) and uninhabited habitable planets. That is something else entirely.

Earth, too, likely had an atmosphere composed chiefly of CO2 in its youth, and if it still had that atmosphere, it would be too hot for life. Earth was “lucky”, though: in its infancy Earth was struck by a Mars-sized object and stripped of its Venus-like atmosphere. What are the odds that events like this are common throughout the galaxy?

The odds with your model is 1/3, extrapolating from observations of our system.

All four terrestrials were hit with massive last impactors scaled to their final size. (Moon sized for Mars, potentially the one creating both the North cap low region and the Moon analog Phobos & perhaps Deimos impact ejecta assemblages. There’s an interesting update on the later prediction circulating right now.) Mercury atmosphere was lost anyway, too close to the Sun, so we don’t know either way.

Venus last impactor was likely larger than ours since it retrograded Venus. The difference is that the retrograde motion would destabilize any Venus moon impact ejecta assemblages by tidal effects, to crash back on Venus. (See for example Wikipedia on Venus for references.)

But that isn’t what is thought happened. The main model today seem to be that early Earth retained a massive CO2/H2O atmosphere after the Theia impact. In collision models Theia atmosphere can contribute to Earth’s volatile supply, at that, explaining why Earth/Moon refractories are well mixed but Earth got plenty volatiles. And certainly our carbon supply supports that hypothesis. (See astrobiology textbooks.)

The difference between Earth and Venus then comes down to plate tectonics that locked away our carbon. Earth retained enough water after the putative early water hydrolysis/hydrodynamic hydrogen loss stage to form seas making the crust malleable to plate tectonics by modifying viscosity such as by helping granitification along. Venus did not, probably because it was closer to the early sun massive CMEs and lost too much hydrogen.

What this means is that the habitable zone for migrating planets is initially smaller (outwards Venus) than later (inwards Venus).

To conclude, Earth locked away substantial carbon quickly since the early crust formation was quick. (Some papers now say less than ~ 30 My, because of the new find of material from an early differentiated crust reservoir.) And since its initial turn around was an order of magnitude faster. (~ 5 times larger internal heat flow, and no large plates somewhat lidding.)

We know from both fossil water isotope analysis and and an ingenious new chert isotope analysis method that Earth seas was less than 40 degC @ ~ 3 Ga. Certainly the stromatolite cyanobacteria assemblages @ ~ 3.5 Ga was living in less than 73 degC, the upper limit for aerobic photosynthesis such as they use. Finally, isotope analysis of diamonds locked in the oldest found zirconium crystals put the seas as below 100 degC @ ~ 4.2 Ga, because they saw a substantial liquid water supply. At that time the solar radiant flux was ~ 70 % today’s level, mind.

In sum these upper temperature limits tests the carbon dioxide lock in prediction well, seeing the initial available carbon supply and the resulting one predicts the process in the first place.

David – they’re saying that the Earth would have been too hot for life to begin in the first place had the collision not reset its atmosphere early on.

Why is it thought that this sort of event is required? Post hoc, ergo propter hoc…

2. Venus may be hot at the surface, but it cools off as you increase in altitude, and gases can’t escape directly from the surface into space:
http://www.datasync.com/~rsf1/vel/1918vpt.htm

3. “Most gases” can’t reach escape velocity. Hydrogen can, but the rest are too heavy.

What can happen is that H2O at high altitudes is dissociated into hydrogen and oxygen so the hydrogen is lost. On Earth the tropopause is cool enough that almost all water freezes out before it can get that high leading to a very dry stratosphere. The UV needed to dissociate water molecules doesn’t reach deep enough into our atmosphere to find much water to attack. If not for this, Earth might have lost most of its water by now.