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Cosmic Dust in the Terrestrial Atmosphere

Final Report Summary - CODITA (Cosmic Dust in the Terrestrial Atmosphere)

The CODITA project was designed to answer two overarching questions: how much cosmic dust enters the Earth’s atmosphere every day, and what impact does the dust have throughout the atmosphere? Estimates of the rate of input of cosmic dust into the Earth’s atmosphere vary from about 3 to 300 tonnes per day, depending on how this is measured. The highest values tend to come from spaceborne measurements and the accumulation rates of “cosmic” elements such as Iridium, Platinum and Osmium in deep-sea sediments and ice cores. In contrast, measurements within the atmosphere indicate much lower input rates: however, these are actually measurements of the fraction of the incoming dust that ablates during atmospheric entry, and so this fraction needs to be known in order to determine the total input. All these estimates require assumptions to be made about the size and velocity distributions of the dust. This is not straightforward because the dust has several sources: Jupiter Family Comets, which are short-period comets captured by the gravitational field of Jupiter; longer period Halley Type and Oort Cloud Comets with periods from many decades to centuries; and the asteroid belt between Mars and Jupiter.

In CODITA we used an astronomical model which launches dust particles from these sources and tracks their evolution through the solar system. The model is constrained by observations of the Zodiacal Cloud. Once the particles fall into a planetary atmosphere, their fate is modelled by our Chemical Ablation Model, which predicts where in the atmosphere individual elements from each particle are injected. During the project we developed a unique instrument, the Meteoric Ablation Simulator, to measure the evaporation rates of different metals in meteoritic fragments undergoing realistic heating profiles that simulate atmospheric entry. This enabled the ablation model to be properly tested, before combining it with the astronomical model to predict the contribution of each dust source to metals in the upper atmosphere, and cosmic spherules at the surface (spherules are cosmic dust particles that melt but do not completely evaporate during entry, and so can be identified as glassy spheres). By constraining the extra-terrestrial dust sources using the measured fluxes of sodium and iron atoms at about 88 km in the atmosphere, and the spherule flux at South Pole, we showed that the total dust input is around 43 tonnes per day, of which 18% ablates during entry. Most of this dust (about 80%) comes from Jupiter Family Comets.

A significant part of CODITA involved developing novel laboratory techniques for measuring the rates of reactions which the meteor-ablated metals undergo in the upper atmosphere. The resulting rate coefficients were then incorporated into a global “whole atmosphere” chemistry-climate model extending from the Earth’s surface to about 140 km. This model satisfactorily reproduces the layers of metal atoms which occur between 80 and 105 km, though only if the meteoric injection rates are reduced by about an order of magnitude. This reflects the inability of global models to resolve atmospheric waves which increase vertical transport. We showed that the potassium layer is particularly sensitive to the long-term cooling that is occurring in the upper atmosphere because of increasing greenhouse gases and ozone depletion. Below the layers, the metal atoms become oxidized and combine to form tiny meteoric smoke particles. These particles are the “seeds” on which water ice forms during summer at high latitudes, creating the noctilucent clouds which are almost certainly visible evidence of climate change. The smoke particles then descend into the stratosphere over the polar regions during winter. We demonstrated that smoke particles raise the temperature at which droplets of nitric acid and water freeze to make polar stratospheric clouds, explaining for the first time how these “mother-of-pearl” clouds form and catalyze ozone depletion. The smoke particles then enter the lower atmosphere at mid-latitudes and provide a significant source of bio-available iron to the Southern Ocean around Antarctica, which has a potential climate feedback.

Within CODITA we also explored impacts in other atmospheres: formation of metallic layers in the Martian atmosphere, and nucleation of CO2-ice clouds; oxidation of CO in the hot lower atmosphere of Venus; and formation of benzene from acetylene in the cold lower atmosphere of Titan.