nai

Project Progress

A telescopic search for extrasolar planets in our local neighborhood will likely encounter planetary systems at different ages and stages of planetary and ecological evolution. Extrasolar terrestrial planets may therefore have environments that more closely match the ancient Earth, rather than any present environments in our solar system. Understanding the environments of the Early Earth helps to constrain models of possible extrasolar planet atmospheres.

In this report, we describe significant progress in understanding the evolution of Earth’s early atmosphere via understanding the significance of two different isotopic records:

1. O isotopes: It has long been recognized that ancient cherts and carbonates are depleted in ¹⁸O relative to ¹⁶O. Taken at face value, this depletion suggests that ocean temperatures were as high as 70°C in the Archean (prior to 2.5 Ga) and that they remained significantly elevated until as recently as 400 Ma. This interpretation would not be correct, however, if the oxygen isotope composition of seawater itself has changed with time. We have proposed a mechanism for causing just such a change. It involves changes in ridgecrest depth, and concomitant changes in the nature of water-rock interactions within the midocean ridge hydrothermal vents. A paper on this topic has been submitted to EPSL:

2. S isotopes: Multiple S isotopes have been used to infer the timing of the rise of O₂ in Earth’s atmosphere. Deviations from the standard mass-dependent fractionation line, termed “mass-independent fractionation”, or MIF, can occur when O₂ levels are low (< 10^-5 times present). The by now generally accepted story is that O₂ levels first increased to significant levels around 2.4 Ga, based (partly) on the disappearance of the sulfur MIF signal at that time.

   The S isotope story has recently become more complicated, as new data show low-MIF values also between 2.8 and 3.2 Ga. Either atmospheric O₂ concentrations went up and then went back down, which seems unlikely, or something else happened to cause this change. The S-MIF anomaly also appears to correlate with putative evidence for glaciation at 2.9 Ga. We have suggested two different mechanisms that may have caused these anomalies. These mechanisms are described in two papers submitted to Phil. Trans. Royal. Soc.Lond.

   The second, and more likely, mechanism involves the creation of organic haze from CH₄ photolysis. This haze simultaneously reduces the SO₂ photolysis that causes MIF and also creates an anti-greenhouse effect that may trigger glaciation. An additional paper on the organic haze hypothesis is in preparation.