“Cold capture” of micrometeorites in Archean and Quaternary atmospheres: Effects of dilute exospheres

Abstract

Micrometeorites (MM) that undergo low heating could have provided a source of organic material to the Earth during the Archean (4-2.5 Ga ago) before life emerged, given that the density of interplanetary dust and larger grains were much higher than today. Amino acids are destroyed on atmospheric entry if the temperature rises above the pyrolysis temperature of few hundred degrees Celsius, depending on type of amino acid. A numerical study was carried out to obtain temperature statistics along relatively rare grazing angle trajectories in the Quaternary (modern) and Archean atmospheres to determine the probability of ‘‘cold capture’’ below pyrolysis temperatures. Effects of the thermospheric temperature and density was considered for the Quaternary atmosphere, and an extended hydrogen/helium envelope remnant from the protosolar nebula was considered for the Archean atmosphere. An important result for the Archean is an elevated ‘‘cold capture’’ probability (twice the capture probability of the modern atmosphere, up to 7%–8%) for low heating below 500 ◦C of small asteroidal grains around 20 μm in diameter, and geocentric velocities in the range 3–5 km/s, provided that there was a remnant envelope. Cometary 20 μm grains of higher geocentric velocities did not have such an elevated capture probability. If the Archean atmosphere did not have an envelope, it was found that these capture probabilities were lower than for the modern atmosphere for both cometary and asteroidal grains, due to smaller density scale height of the lower Archean atmosphere leading to faster heating rate. Radiative degradation of amino acids in these relatively small grains should be considered more closely since the X-ray and XUV-flux from the Sun was larger by a factor of about 5–10 in the Archean. Very low maximum temperatures were found for 20 μm asteroidal and cometary grains in the Quternary atmosphere, with temperatures in the range 150–200 ◦C, but with a very small capture probability in this range of typically less than 0.3%. All 300 μm asteroidal grains were heated to temperatures above 500 ◦C for all atmosphere models. The probability of heating to temperatures < 500 ◦C of 100 μm asteroidal grains, was estimated to 0.3% or less for all models. Most 100 μm cometary grains were heated to temperatures > 500 ◦C for all models.