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