Impacts of Cosmic Dust in the Atmospheres of Mars and Venus

Cosmic dust particles are produced from the sublimation of comets and by collisions between asteroids. Because the particles enter a planetary atmosphere at hypersonic velocities, collisional heating with air molecules causes a fraction of them to melt, leading to vaporization of their metallic constituents. The injection of these elements causes a wide variety of atmospheric phenomena in the terrestrial atmosphere, including the formation of global layers of metal atoms between 80 and 105 km; airglow emissions; layers of metallic ions which affect radio communications; and the production of meteoric smoke particles which enable the nucleation of mesospheric ice clouds and the freezing of polar stratospheric clouds. Certain metal atoms can be observed very precisely by ground-based lidar and from satellites, providing an excellent tracer of dynamics and chemistry at the edge of geospace.

The input rate of cosmic dust to the Earth’s atmosphere has been very uncertain. A new estimate of around 28 tonnes per day globally will be discussed; this was obtained using an astronomical dust model to provide the size and velocity distributions of dust in the inner solar system, combined with the Leeds Chemical Ablation Model (CABMOD) to determine the rate of injection of metals into the atmosphere. CABMOD is itself benchmarked using a novel Meteoric Ablation Simulator to measure the evaporation rates of metals from meteoritic particles that are flash heated, simulating atmospheric entry. The dust inputs into the atmospheres of Mars (2 t d^-1) and Venus (31 t d^-1) can now be constrained using the terrestrial input.

The chemistry of the four most abundant meteoric ablation elements - Mg, Fe, Si and Na - has been constructed from laboratory measurements and theoretical calculations of the rate coefficients for over 120 individual reactions involving neutral and ionized species. This chemistry, together with the relevant metal injection rates as a function of height, location and time, has been inserted into the Planetary Climate Models for Mars and Venus developed in Paris (LMD) and Granada (IAA). For Mars, model simulations generally compare well against observations of metallic ions made by instruments (IUVS and NGIMS) on NASA’s MAVEN spacecraft. In particular, the diurnal, latitudinal and seasonal variations of the Mg⁺ layer centred around 95 km are captured well. However, there are several interesting differences higher in the ionosphere that are currently unexplained.

In the case of Venus, metallic species have never been observed. However, the PCM-Na model predicts that the atomic Na layer around 110 km should be observable at the dawn terminator by a terrestrial telescope-spectrometer. Metallic carbonate species are also predicted to act as ice nuclei, forming transient CO₂-ice clouds above 110 km. CO₂-ice clouds may also form in the upper haze layer between 80 and 90 km, in cold pockets produced by gravity waves. The sedimentation and subsequent evaporation of these large particles would lead to the downward transport of various atmospheric constituents. Finally, a strong candidate for the mystery absorber in the Venusian clouds is iron trichloride (FeCl₃), produced by the extra-terrestrial input of Fe. In contrast, the SO dimer OSSO, which has been favoured in the past to explain the absorber, should make a negligible contribution.