A rainy day 2.7 billion years ago reveals surprising notions of the early Earth's atmosphere

Air density limited 2.7 billion years ago limited to twice present levels by fossilized raindrop imprintsSanjoy Som, David Catling, Jelte Harnmeijer, Peter Polivka, Roger Buick

Earth and Space Sciences and Astrobiology ProgramUniversity of Washington

Atmospheres are a window to the planet2Atmospheres: a window to the planet

Kasting, Sci. Am., 20043Early Earth Atmosphere: poorly constrained

Kasting, Sci. Am., 2004Greenhouse forcing: which gases and how much?

4Early Earth Atmosphere: poorly constrained

Kasting, Sci. Am., 2004Greenhouse forcing: which gasses and how much?

Would be nice to know the pressure5Using imprints to infer atmospheric density was first suggested by Charles Lyell in 1851From such data we may presume that the atmosphere of one of the remotest periods known in geology corresponded in density with that now investing the globe,

Charles Lyell, Proceedings of the Geological Society, 1851.

Thin section

Scale bar: 5 mm~sea level + semi arid environment

Maximum raindrop size is independent of air density

Vuillermaux and Bossa, Nature Phys. 2009

Mathews and Mason, 1964Clift et al, 1978Raindrop imprint air density puzzle

Given that maximum raindrop size is independent of air density,

Given that raindrop impact momentum controls imprint size,

II. How does raindrop impact momentumchange with air density?

I. What is the relationship between impact momentum and imprint size in volcanic ash?Impact momentum controls imprint sizeHuang et al. 1983

A dozen imprints per drop size

Imprints in 10% moist (by mass) ashImprints were measured with 3D laser scanner

High-resolution 3D laser scanner(vertical accuracy: 0.0002)Relationship between imprint size and impact momentum

Som et al. 2012

Impact momentum is obtained from first principlesLorenz, 1993

Som et al. 2012

Air density depends on what raindrops size created the largest imprints

Som et al. 2012Assume that semi-arid modern rainfall statistics apply to the early EarthLebel et al. 1997

99%78%When the imprints were formed, rainfall fell at a rate < 100 mm hr-1 with a probability of 78%-99%Assume that largest imprints were formed by largest raindrops

Som et al. 20120.2 % of totalBinning algorithm: Scott, D. On optimal and data-based histograms, Biometrika, 1979Raindrop size distribution and rainfall rate are related by empirical relationships

Som et al. 20120.2 % of total

R4 = 100 mm hr-1Som et al. 2012(after Willis and Tattelman, 1989) Maximum raindrop diameters were between 3.8mm and 5.3mm with 78%-99% probability

Som et al. 2012Air density was between 0.6 kg m-3 and 1.3 kg m-3 with 78% - 99% probability

Som et al. 2012