Mission to Planet Earth: The First Two Billion Years

Abstract

Solar radiation and geological processes over the first few million years of Earth’s history, followed soon thereafter by the origin of life, steered our planet towards an evolutionary trajectory of long-lived habitability that ultimately enabled the emergence of complex life. We review the most important conditions and feedbacks over the first 2 billion years of this trajectory, which perhaps represent the best analogue for other habitable worlds in the galaxy. Crucial aspects included: (1) the redox state and volatile content of Earth’s building blocks, which determined the longevity of the magma ocean and its ability to degas H2O and other greenhouse gases, in particular CO2, allowing the condensation of a water ocean; (2) the chemical properties of the resulting degassed mantle, including oxygen fugacity, which would have not only affected its physical properties and thus its ability to recycle volatiles and nutrients via plate tectonics, but also contributed to the timescale of atmospheric oxygenation; (3) the emergence of life, in particular the origin of autotrophy, biological N2 fixation, and oxygenic photosynthesis, which accelerated sluggish abiotic processes of transferring some volatiles back into the lithosphere; (4) strong stellar UV radiation on the early Earth, which may have eroded significant amounts of atmospheric volatiles, depending on atmospheric CO2/N2 ratios and thus impacted the redox state of the mantle as well as the timing of life’s origin; and (5) evidence of strong photochemical effects on Earth’s sulfur cycle, preserved in the form of mass-independent sulfur isotope fractionation, and potentially linked to fractionation in organic carbon isotopes. The early Earth presents itself as an exoplanet analogue that can be explored through the existing rock record, allowing us to identify atmospheric signatures diagnostic of biological metabolisms that may be detectable on other inhabited planets with next-generation telescopes. We conclude that investigating the development of habitable conditions on terrestrial planets, an inherently complex problem, requires multi-disciplinary collaboration and creative solutions.

Depleted carbon isotope compositions observed at Gale crater, Mars

“…Also, direct observation of CO in the Martian atmosphere by spectroscopy from Earth has not revealed a large 13 C depletion in CO, with the results appearing to be within 250‰ of the telluric value ( 82 ) and probably slightly 13 C enriched relative to Earth ( 66 , 68 ). Without further information with which to deconvolve the details of where and how carbon was fractionated, we can conclude the photochemical production of organic material from 13 C-depleted CO is a possible scenario on Mars for deposition of highly depleted organic material on to an exposed surface ( 74 ) that should not be rejected without further investigation of Martian CO and the photochemical processes that influence its carbon isotopic composition.…”

Magma Ocean Evolution of the TRAPPIST-1 Planets

“…The amount of CO 2 present in the atmosphere determines whether condensation of water to an ocean can occur. The higher the CO 2 partial pressure the more difficult it is for the water to condense (Lebrun et al, 2013;Massol et al, 2016;Salvador et al, 2017;Stüeken et al, 2020). To fully understand the early evolution of rocky planets, it is necessary to include CO 2 into this model.…”

“…The latter, reducing atmosphere mixture would have facilitated atmospheric escape of hydrogen leading to a higher D/H ratio. Similarly, the D/H ratio implies a shortlived steam atmosphere, that is, a short solidification time of the magma ocean (Stüeken et al, 2020). Both circumstances, the small effect of atmospheric escape of hydrogen and the short solidification times, are represented in our model.…”

A revised lower estimate of ozone columns during Earth’s oxygenated history

“…This was originally based on passing the threshold for full UV screen limits of ≈ 210 DU proposed by Berkner & Marshall [95] and ≈260 DU proposed by Ratner & Walker [96]. More recently, atmospheric [16,22], biogeochemical [36,131], biological [55,126,132], and astrobiological/ exoplanet work [74,131,[133][134][135][136][137] have cited one-dimensional results in figure 6, often with the statement that at least 1% PAL of O 2 is needed to establish a full UV shield. Therefore, prior studies in figure 6 show that at 1% the present atmospheric level of O 2 , the fully UV shielding range is between 120-185 DU for the mean O 3 column, whereas our 1% PAL simulation gives a mean O 3 column of just 66 DU, roughly half the lower end of the 120-185 DU range.…”