2009 CEDAR Workshop Report: Comparative meteor science - The effects of meteoroids on planetary atmospheres and ionospheres

Paul Withers (Boston University) and Meers Oppenheim (Boston University)

A workshop entitled "Comparative meteor science - The effects of meteoroids on planetary atmospheres and ionospheres" was held on Tuesday 30 June as part of the 2009 CEDAR Meeting in Sante Fe, New Mexico. Approximately 30 people attended the late evening workshop. The aims of the workshop were to discuss recent observations of metal ion layers in the ionospheres of Venus and Mars, compare these layers to their well-studied terrestrial cousins, and identify ways in which comparative studies of metal ion layers can improve understanding of the environments of the Earth and other planets. 

Meers Oppenheim (Boston University, USA) began the workshop by posing two questions. How can Earth help us understand extra-terrestrial metal ion layer? How can Venus and Mars challenge and validate Earth-centric theory and models?

Paul Withers (Boston University, USA) discussed observations of metal ion layers across the solar system. Low altitude plasma layers that are sporadically seen in electron density profiles from Venus and Mars have been identified as metal ion layers. These layers occur at neutral densities and pressures that are one order of magnitude greater on Venus than on Earth or Mars. Some of the planetary properties that may affect metal ion layers, and cause them to differ from planet to planet, include the chemical composition of the atmosphere, magnetic field environment, rotation rate, atmospheric dynamics and position in the solar system. 

Joe Grebowsky (NASA Goddard Space Flight Center, USA) described the physics and chemistry associated with metal ion layers. Due to orbital dynamics, planets in the inner solar system and satellites in the outer solar system have a relatively wide range of meteoroid entry speeds, unlike planets in the outer solar system. The dominant mechanism for the production of metal ions from ablated material varies. Charge exchange with ambient ion species is most important on Earth, photoionization is most important on Mars and impact ionization, whose rate scales as meteoroid entry speed to the power 3.5, is most important on Jupiter. The predicted pressure levels at which metal ion layers occur is close to 1 microbar for all solar system planet and satellite atmospheres. 

John Plane (University of Leeds, UK) discussed the chemical significance of replacing N2, abundant on Earth, with CO2, abundant on Venus and Mars, in three body reactions involving metal ions. Plane presented the results of laboratory studies of reaction rates. The measured reaction coefficient for Mg+ + CO2 + CO2 -> Mg+.CO2 + CO2, which is the rate-limiting step in the loss of metal ions, is 200 times greater than that assumed by analogy with reactions in N2 by modellers. The inferred lifetime of metal ions in the martian ionosphere is reduced to on the order of one hour, suggesting that metal ion layers on Mars should be much less dense and broader than the observed low altitude plasma layers. A spirited discussion ensued. 

John Mathews (Pennsylvania State University) discussed the possibility of planetary analogues to sporadic E layers, which on Earth are formed by wind shear in a strong magnetic field and are largely composed of atomic metal ions. Since Venus has no magnetic field, sporadic E layers as we know them will not form. Since Mars has a weak, inhomogeneous magnetic field, sporadic E layers as we know them may form. Uncertainties in their neutral winds make it challenging to predict the existence and properties of extra-terrestrial sporadic E. Neptune has strong winds and magnetic fields, making it likely to possess analogues to sporadic E layers at altitudes where meteoroids ablate. 

David Hysell (Cornell University) discussed observations of sporadic E layer instability above Arecibo. These observations point to neutral dynamics and dynamical instabilities as the source of midlatitude sporadic E layer structuring. Planetary analogues to sporadic E layers, if they exist, are likely to display a wide range of plasma instabilities driven by the neutral atmosphere.

Michael Mendillo (Boston University) concluded the workshop by synthesizing and summarizing the preceding presentations, reminding the audience of the success of earlier comparative studies in other aspects of aeronomy, and predicting that an ionospheric radar operating on the surface of Mars would have tremendous value.