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Australia: The Land Where Time Began |
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Early Earth and an Atmospheric Solar Shock
In the Early solar system the young Sun is believed to have ejected
energetic particles that compressed the magnetosphere of the Earth. It
is suggested by simulations that the atmosphere of the Earth was
penetrated by the particles which initiated reactions that warmed the
planet and fertilised life.
Earth of the present is in a position in the solar system where it is
far enough from the Sun, without being too far from the Sun, for the
conditions being correct for the water to be in a stable state on the
Earth’s surface, and the other environmental requirements for life to
flourish. Ramirez suggest similar conditions may have existed on the
Earth as far back as 4 Ga, even when the Sun was 30 % less bright than
it is at present. The so-called ‘faint young Sun paradox, a problem that
was raised initially by Carl Sagan and George Mullen (Sagan & Mullen,
1972, Science, 177, 52-56),
the paradox being that the Earth was not permanently glaciated under the
Sun that was less luminous. It is suggested (Airapetian,
et al., 2016) that if the
young Sun was as stormy as other Sun-like young stars, a habitable Earth
could have been sustained by such stormy solar weather.
Concentrations of greenhouse gasses in the atmosphere that were higher
than those of the present have been invoked by most of the proposed
solutions to the paradox. The probable most obvious candidate is CO2,
carbon dioxide, though several indicators, such as data from
ancient soils, suggest that CO2 would have been too low to
prevent global freezing (Sheldon, 2006). Another suggestion is ammonia
(NH3) is a strong warming agent, but it soon rains out of the
atmosphere, and also it is photochemically unstable (Kuhn & Atreya,
1979). Another possibility is methane (CH4), though it can
form anti-greenhouse hazes at high CH4/CO2 ratios
(Haqq-Misra et al., 2009).
The atmosphere is also warmed by methane, which partially offsets
warming of the surface (Ramirez et
al., 2014). Hydrogen (H2)
has also been evoked as a warming agent of the Early Earth (Wordsworth &
Pierrhumbert, 2013), though it is difficult to achieve such high
concentrations with an early terrestrial mantle that was oxygenated
(Ramirez et al., 2014).
Nitrous oxide (N2O) is a final possibility, though not much
is known about its levels in atmospheres of the past. In order to
generate nitrous oxide molecular nitrogen (N2) in the
atmosphere is required, but molecular nitrogen is chemically relatively
inert and energetic processes are needed for it to react and generate
compounds that are biologically usable. These processes include
lightning, UV radiation and atmospheric shock caused by falling
meteoroids, though it is not clear if these mechanisms could have
resulted in enough nitrous oxide to warm the early climate
significantly.
It is suggested by measurements of magnetic activity on young stars that
the magnetic fluxes generated by the Sun about 3.8 Ga may have been more
than 10 times as powerful as those that are produced at the present.
Powerful solar storms with energies greater than 1015 atomic
bombs exploding would have been triggered by these fluxes.
It is suggested by Airapetian et
al. that these super solar storms could have frequently impacted the
Earth, and under certain circumstances, possibly more than once per day.
Intense shocks that accelerated energetic particles away from the Sun
would have been produced by this constant onslaught. It was found by
numerical modelling of the interactions between such an event and the
magnetosphere of the Earth carried out by Airapetian et
al. that the magnetosphere is
compressed by the surging particle cloud, opening large gaps around the
regions of the polar caps. The result is that particles that are
normally shielded by the magnetosphere are allowed to penetrate into the
atmosphere through the polar cap region.
An atmospheric chemistry model was then used by Airapetian et
al. to compute the rates at
which the solar-atmospheric interactions excited and decomposed the
atmospheric molecules. Reactions that use nitrogen to produce other
useful atmospheric constituents result from these interactions. The
chemical reactions resulting from repeated solar storms are predicted by
the model to have maintained the concentrations of carbon dioxide,
methane and hydrogen at modest levels, which is consistent with
observations. Mineral assemblages that are not observed in the rock
record would have been produced by high concentrations of carbon dioxide
and methane (Rosing et al,
2010).
Also, it was found by Airapetian et
al. that the
solar-atmospheric interactions produce abundant NO and NH, which then
take part in chemical reactions that produce both N2O and
hydrogen cyanide (HCN), potentially quite efficiently, if the solar
storms are as frequent as the researchers suggest.
Concentrations of N2O
of 20 parts per billion by volume (PPBV) at an altitude of 30 km are
predicted by the model of Airapetian et
al., if it is assumed the
atmospheric pressure is equal to that of the present (1 bar). It has
been suggested, however, that the pressure of the early atmosphere may
have at least twice as high (Goldblatt et
al., 2009) and nitrous oxide
concentrations can reach 3,000 PPBV if the pressure is doubled. Nitrous
oxide in the upper atmosphere would have been transported downwards to
lower altitudes by vertical motions in the atmosphere, where it could
warm the surface by the greenhouse effect.
The implications are no less important for the early Earth of HCN, the
other gas produced in the simulations. HCN is an important molecule for
life. Chains of HCN are known to produce various amino acids, the
building blocks of proteins. Concentrations greater than 10 ppbv are
produced by the simulated 2-bar atmosphere of Airapetian et
al. These HCN levels, as well
as other nitrogen-bearing products of the reactions in the atmosphere,
could have helped to fertilise early surface biology.
Ramirez suggests that one potential question is could this mechanism
have counteracted a younger Sun that was dimmer at later times as there
was a weakening of the solar magnetic activity. The processes proposed
by Airapetian et al. may,
nevertheless, be critical to the environment of the Earth as well as
influence planetary evolution elsewhere. It is also suggested by
geological evidence that Mars was also paradoxically warm and wet around
the same time (Craddock & Howard, 2002). Estimates of the inventory of N2
on early Mars range from 3-300 mb (Mackay & Stoker, 1989), which could
potentially fuel similar solar-atmospheric interactions to those seen in
models that are Earth-base.
Airapetian et al. (Airapetian
et al., 2016) have
demonstrated that there may have been significant impact on both warming
of the surface and biology almost 4 Ga as a result of super solar storms
on the active young Sun. According to Ramirez these findings may suggest
implications for the climate and potential biology of terrestrial
exoplanets that orbit very young stars that are Sun-like, particularly
stars that have exceptionally high magnetic fluxes and very intense
super stellar storms.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |