Early Earth and an Atmospheric Solar Shock

Australia: The Land Where Time Began

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 CO_2,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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