Re: Snowball Earth at 2.3 gya


"yahooterry@xxxxxxxxx" <terryhilleman@xxxxxxxxxxx> wrote in message news:ddmh11$1bob$1@xxxxxxxxxxxxxxxxxxxxxx
> This is just my opinion, but with an early-earth atmosphere similar to
> Jupiter, methane was likely the primary atmospheric greenhouse gas (25X
> as potent as CO2), keeping water in liquid form at this time of lower
> sun intensity. Methane and ammonia seem to be stable on Jupiter and
> the other gas giants, in spite of the storms on the surface.

I just reread Kasting's 1993 review article "Earth's Early Atmosphere".
(Science 259, Feb 12, 1993, pp920-926). It reminded me of the main
argument that Earth either never had a Jupiter-like reducing accretion
atmosphere, or that it lost it early. That argument is based on neon.

Neon is a noble gas with a fairly high solar and cosmic abundance. It
is actually slightly more common than nitrogen, and comparable in
abundance to carbon. If the Earth once had an accretion atmosphere
of H2 and CH4, it would also have contained a lot of neon. Several
bars of Neon. But Earth's current atmosphere has very little neon.
Did we lose it? But any process that causes the Earth to lose neon
would also lose H2 and CH4! A look at ratios of Ne(20) and Ne(22)
provides more info. We lost that Neon in one fell swoop rather
than gradually leaking it away.

However, unlike the gasses H2, CH4, and Ne, the Earth would have
received H2O, NH3, and oxidized forms of carbon in solid form -
as ices and as adsorbed materials in chondrites. This kind of
stuff could be buried, protected from whatever process dissipated
our original atmosphere of H2, CH4, and Ne. Then, as the Earth's
interior gradually reorganized itself, it was outgassed.

As early as 4.4 Ga, Earth's atmosphere was primarily CO2, CO, and H2O.
There would have been some H2 (hot metallic meteorites react with
water to form H2) and some methane. But CO2 and CO dominated, with
combined partial pressures of something like 0.5-3 bar. And the
CO fraction of that carbon would have decreased over time due to
photolysis of water and escape of hydrogen. Similarly, NH3 would
decrease in favor of N2.


.

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