How large is the cosmic dust flux into the Earth's atmosphere?
Abstract
Cosmic dust particles are produced in the solar system from the sublimation of comets as they orbit close to the sun, and also from collisions between asteroids in the belt between Mars and Jupiter. Current estimates of the magnitude of the cosmic dust input rate into the Earth's atmosphere range from 2 to well over 100 tons per day, depending on whether the measurements are made in space, in the middle atmosphere, or at the surface in polar ice cores. This nearly 2 order-of-magnitude discrepancy indicates that there are serious flaws in the interpretation of observations that have been used to make the estimates. Dust particles enter the atmosphere at hyperthermal velocities (11 - 72 km s ^{-1}), and mostly ablate at heights between 80 and 120 km in a region of the atmosphere known as the mesosphere/lower thermosphere (MLT). The resulting metal vapours (Fe, Mg, Si and Na etc.) then oxidize and recondense to form nm-size particles, termed "meteoric smoke". These particles are too small to sediment downwards. Instead, they are transported by the general circulation of the atmosphere, taking roughly 5 years to reach the surface. There is great interest in the role smoke particles play as condensation nuclei of noctilucent ice clouds in the mesosphere, and polar stratospheric clouds in the lower stratosphere. Various new estimates of the dust input will be discussed. The first is from a zodiacal dust cloud model which predicts that more than 90% of the dust entering the atmosphere comes from Jupiter Family Comets; this model is constrained by observations of the zodiacal cloud using the IRAS, COBE and Planck satellites. The cometary dust is predicted to mostly be in a near-prograde orbit, entering the atmosphere with an average velocity around 14 km s ^{-1}. The total dust input should then be about 40 t d ^{-1}. However, relatively few of these particles are observed, even by the powerful Arecibo 430 MHz radar. Coupled models of meteoroid differential ablation, ionization and radar detection can be used to compute the probability of detecting a specified meteoroid in the Arecibo beam; an upper limit to the cosmic dust input of 16 t d ^{-1} has been obtained from the radar observations. Underpinning this modelling work is a novel laboratory experiment at the University of Leeds, where a novel Meteor Ablation Simulator is used to study the evaporation of metals from cosmic dust particles that are flash heated to over 3000 K. Finally, rocket-borne measurements of charged meteoric smoke particles indicate that about 5 t d ^{-1} of this cosmic dust ablates in the atmosphere, and another 6 t d ^{-1} fall to the surface as cosmic spherules.
- Publication:
-
41st COSPAR Scientific Assembly
- Pub Date:
- July 2016
- Bibcode:
- 2016cosp...41E1569P