Despite only faint light from the young Sun, the early Earth enjoyed fairly equable climates. These balmy temperatures could be explained by extremely high levels of carbon dioxide and other greenhouse gases. Measurements of carbon isotopes in 2.7-billion-year-old palaeosols, however, suggest that the carbon dioxide concentrations would only have been high enough to maintain a temperate climate if the atmospheric nitrogen concentration was twice that of today's levels.

The nitrogen hypothesis is attractive, but without any measurements of atmospheric pressure during the Archaean eon, it remains speculative. Sanjoy Som of the University of Washington, Seattle, and colleagues (Nature http://dx.doi.org/10.1038/nature10890; 2012) propose that fossilized raindrop imprints allow a reconstruction of ancient atmospheric pressure from the rock record. They identified raindrop imprints in a layer of 2.7-billion-year-old volcanic tuff found near Prieska, South Africa.

Credit: © NPG 2012

The size of a raindrop is independent of atmospheric pressure. However, the terminal velocity of the raindrop is controlled by air density, and thus can provide bounds on atmospheric pressure. The team experimented on two different types of volcanic ash — the precursor to tuff — to identify the relationship between raindrop terminal velocity and the imprint left. Using the assumption that rain during the Archaean followed the same size distribution as modern rainfall, they find that the absolute upper limit of air density is 2.3 kg m−3, sufficient for the nitrogen hypothesis to be plausible.

However, the most probable estimate for Archaean air density is less than 1.1 kg m−3, similar to modern values, a pressure that precludes a nitrogen-enhanced efficacy of greenhouse gases. These atmospheric pressure values also rule out exceedingly high concentrations of carbon dioxide. There are, of course, other ways to explain Archaean warmth. A hazy atmosphere consisting of the greenhouse gas methane and fractal hydrocarbons would allow ammonia — a potent greenhouse gas — to persist in quantities sufficient to promote warming (Science 328, 1266–1268; 2010). The search is on for other greenhouse gases that, although readily broken down in today's oxygenated atmosphere, may have been stable under more reducing conditions.

Identifying the components of the atmosphere billions of years ago is challenging, at best. And so it seems that despite decades of scientific effort, the faint young Sun paradox may remain just that for the foreseeable future.