A Seasonal Feature in Mercury’s Exosphere Caused by Meteoroids from Comet Encke

Abstract:

The planet Mercury is enveloped in a tenuous atmosphere, the result of a delicate balance between poorly understood sources and sinks (Killen et al, 2007). Meteoroid impacts are a contributing source process (eg Wurz et al, 2010), but their importance compared to other production mechanisms is uncertain.

Killen and Hahn (2015) found that seasonal variations in Mercury's calcium exosphere as observed by the MASCS spectrometer onboard the MESSENGER spacecraft (Burger et al, 2014) may be due to impact vaporization of surface material by the infall of interplanetary dust. However, an additional dust source was required to explain a Ca excess at a True Anomaly Angle (TAA) of 25±5 deg. Killen and Hahn suggested that dust from comet 2P/Encke, crossing Mercury's orbital plane at TAA=45 deg, may be the culprit.

We have simulated numerically the stream of meteoroids ejected from Encke to test the Killen and Hahn conjecture. We find that Encke particles evolving solely under the gravity of the major planets and the Sun encounter Mercury at TAA=50-60 deg, well after the peak of the Ca excess emission. However, the addition of Poynting-Robertson (P-R) drag in our model couples the age and size of the meteoroids to the TAA at encounter, causing smaller, older particles to encounter Mercury progressively earlier in the Hermean year. In particular, mm-sized grains ejected between 10 and 20 kyr ago impact on the nightside hemisphere of Mercury at TAA = 350−30 deg, near the observed peak time of the exospheric feature.

During this presentation, we will describe our model results and discuss their implications for the physical mechanism that injects impact-liberated Ca into sunlight as well as the origin and evolution of the Encke stream of meteoroids.

Astronomical research at the Armagh Observatory is funded by the Northern Ireland Department of Culture, Arts and Leisure (DCAL). RMK was supported by NASA Grant NNX07AR78G-S01 as a Participating Scientist on the NASA MESSENGER mission to Mercury and by STROFIO, a NASA Mission of Opportunity on the BepiColombo mission. AAC acknowledge the SFI/HEA Irish Centre for High-End Computing (ICHEC) as well as the Dublin Institute for Advanced Studies (DIAS) for the provision of computational facilities and support.