Using raindrops to constrain past atmospheric density




Kavanagh, Lucas
Goldblatt, Colin

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Earth and Planetary Science Letters


There exists a dearth of constraints on the physical properties of the early Earth atmosphere. The Som palaeopycnometry method estimates an upper limit on ancient atmospheric density based on the size of lithified raindrop imprints preserved in ancient strata, with the assumption that the largest imprint was made by the largest possible raindrop. Using this technique Som et al. (2012) proposed a constraint on Archean atmospheric density of less than View the MathML source 2.3 kgm−3. Applying this method to modern raindrop imprints, the upper bound on surface density produced is View the MathML source 0.9 kgm−3, lower than the actual value of View the MathML source 1.2 kgm−3, refuting the method. We propose several changes to the method, the most important of which is increasing the maximum possible drop size from 6.8 to 10 mm6.8 to 10 mm to be consistent with new large datasets of raindrop observations. With these changes, our upper bound on modern surface density becomes View the MathML source 2.7 kgm−3, a valid limit. The upper bound on Archean atmospheric density is then revised to View the MathML source 11.1 kgm−3. In general, we find that raindrop imprint size distribution depends much more strongly on rainfall rate than atmospheric density, which translates into large errors. At best, the precision of raindrop palaeopycnometry will be a factor of a few to an order of magnitude.



palaeobarometry, raindrops, Archean, pressure, density, faint young Sun


Kavanagh, L. & Goldblatt, C. (2015). Using raindrops to constrain past atmospheric density. Earth and Planetary Science Letters, 413, 51-58.