Meteoric ion layers in the ionospheres of venus and mars: Early observations and consideration of the role of meteor showers

Layers of metal ions produced by meteoroid ablation have been known in Earth's ionosphere for decades, but have only recently been discovered at Venus and Mars. Here we report the results of a search for meteoric layers in earlier datasets from Venus and Mars. We find 13 candidates at Venus in Mariner 10, Venera 9/10, and Pioneer Venus Orbiter data that augment the 18 previously identified in Venus Express data. We find 8 candidates at Mars in Mariner 7 and Mariner 9 data that augment the 71 and 10 previously identified in Mars Global Surveyor and Mars Express data, respectively. These new findings extend the ranges of conditions under which meteoric layers have been observed, support studies of the temporal variability of meteoric layers, and (for Venus) independently confirm the existence of meteoric layers. One of the proposed causes of temporal variations in the occurrence rate of meteoric layers is meteor showers. This possibility is controversial, since meteor showers have minimal observed effect on meteoric layers in Earth's ionosphere. In order to aid progress towards a resolution of this issue, we present a series of tests for this hypothesis.