Comet Enthusiast

The current model is that there is “an ocean below for an ocean above”, I believe.

I.e. current Earth water mass may be twice the visible, or twice the ocean ~ 70 % surface or twice ~ 3*10^8*10^6*10^3 m^3 [Wikipedia & assuming ~ 1 km mean depth] @ 10^3 kg/m^3 or ~ 10^20 kg.

This comes out at ~ 10^20/5*10^24 [Wikipedia] or a mere ~ 2*10^-5 of Earth mass. So meteorites could well have supplied the water. The maximum cometary delivery figure may vary with accuracy, I believe I’ve seen 20 % mentioned; in any case a now rather certifiable non-dominant amount.

Certainly we can believe that volatiles survive what seems to be the last stage in planet accretion, when a large planetoid is busted up with another planetoid an order of magnitude smaller. Our own Earth had the Moon creating Mars sized impactor. Venus seems to have been retrograded by something similar. (And Mars north polar region low and flat is hypothesized to be due yet another analogous Moon sized impactor.) Both retain plenty of volatiles.

To pitch in with Uncle Fred wish & data, it seems to me the LHB vaporization case isn’t all that good after said accretion was done with.

First, absent ~ 400 km crust busters, at worst the oceans vaporized if at all for a mere 10-1000 years. Even with crust busters it is now IIRC claimed in some papers that there was a global Goldilocks crustal zone ~ 1 km down where liquid water survived, and life could have survived, outside of the impact. (Sorry, I seem to have misplaced that one reference.)

Second, on “a few rocks that survived the late bombardment” I can repeat what I commented last week:

“Another data point [besides Jack Hill zircons] comes from the recent found faux-amphibolite in Nuvvuagittuq, an igneous or meta-igneous rock derived from material @ 4.28 Ga (at least). It has undergone several thermal events @ 4.0, 3.8 and 3.66 Ga, but it follows that somewhere between 4.28 – 3.8 Ga:

1) There were reservoirs of liquid water. (Wet produced minerals.)
2) Archaean plate tectonics may have existed in the Hadean. (Both wet and dry produced minerals.)
3) LHB impactors didn’t seriously affect the environment. (No oxygen anomalies.)
4) Photosynthesis existed. (Preoxygen atmosphere sulfur photosynthesis drove a biological sulfur cycle, as witnessed by several isotope ratios.)

While the ambiguities doesn’t allow us to test that life existed @ ~ 4.25 Ga as in the parsimonous [sic] hypothesis, this means that it isn’t a safe assumption to claim that life didn’t exist then!

So life may have existed a mere 250 My after the Moon creating impact @ ~ 4.5 Ga, a reasonable amount of time. (About the same time it took from the first multicellular body plans to the first land living terapods.)”

How quickly did Earth become temperate?

That is a tougher one.

There is an orphan data point from a rock water inclusion indicating ocean temp ~ 40 degC @ ~ 3.1 Ga from salinity data, IIRC.

Shoring that one up is a new paper on understanding chert data, that rather persuasively (this layman thinks) shows maximum ocean temp ~ 40 degC @ ~ 3.2 Ga. (I have that reference somewhere, if you need it.)

And of course we know that photosynthezisers, which today works at max ~ 70 degC and may be suspected to earlier have been able to sustain a lot lower temperature, thrived @ ~ 3.5 Ga.

Putting that together, I would claim that ~ 3.5 Ga the temperature likely would have been close to ~ 40 degC by continuity.

Before that, who knows? Assuming the carbon and sulfur data together indicates life, and seeing that the oldest developed protein folds works best @ ~ 20 degC (yet another paper; we drown in them, don’t we?), it may well be that ~ 4.25 Ga when photosynthesis and so proteins putatively existed, the temperature had dropped in the shine of the young and weak sun to a balmy 20-40 degC. Just what the doctor ordered!