5th MESSENGER Bepi-Colombo Science Workshop Berlin, Germany Seasonal variations in Mercury’s Calcium Exosphere Modeled by Impact Vaporization from Dust Disk Plus Meteor Shower Rosemary Killen NASA Goddard Space Flight Center, Greenbelt, Maryland Joe Hahn Space Science Institute (SSI) and Center for Space Research (CSR) University of Texas, Austin, Texas THE MASCS Team

Observations UVVS Data taken with MESSENGER MASCS over 9 Mercury Years Persistent Seasonal pattern Dawn-centered Ca Exosphere Additional enhancement at 30° TAA

OBSERVATIONAL GEOMETRY

Observational Geometry Burger et al., 2014

MESSENGER MASCS Ca DATA

MODEL Assumed Impact Vaporization is the sole source for Ca Used the Planar Impact Approximation (Melosh, 1989); model (Killen et al. 2005) Modeled dust disk with Hahn et al. (2002) model of interplanetary dust disk Mercury Orbital parameters i=7° e=0.2

Calcium Source Rate at Mercury

IMPACT VAPORIZATION MODEL Dust Disk Model - Hahn et al., 2002, fit to Clementine Data DUST DISK INCLINATION BELIEVED TO BE i~3.5° but was allowed to be free parameter Varied inclination, heliocentric radial variation Vertical distribution of dust due to 3 different reservoirs ASCENDING NODE ASSUMED FREE PARAMETER

Interplanetary Dust vs. Ecliptic Latitude cometary dust: green Halley-type comets: magenta asteroids and JFCs: blue

Dust Disk Model and Variations Ascending Node of Disk was free parameter Inclination of Dust Disk was free parameter Varied the heliocentric radial dependence of the dust density

Impact Vaporization for Dust Disks With varying inclinations

Two disk Model low inclination disk plus high inclination disk

Comet Encke Dust Trail Spitzer image

Mercury’s Orbit and Comet Encke

Comet Encke Orbit vs. Mercury

Nodal Footprint of Impacts from Encke 1 mm spheres Blue: Integrated from 20,000 years ago Red: Integrated from 10,000 years ago Red particles cross Mercury plane near 0 – 20 TAA and near 140 TAA

Flux at Mercury from Meteor Streams red x = Encke dust blue x= Halley dust

Low inclination disk plus comet (meteor shower) ↵ Encke Dust Aretids or Encke dust 

CONCLUSIONS Mercury’s Calcium Exosphere has a repeatable pattern every Mercury Year The pattern can be modeled using impact vaporization as Mercury traverses the interplanetary dust disk An enhancement at 25±5° true anomaly angle cannot be reproduced by the nominal or a high inclination dust disk The enhancement is likely due to a meteoroid stream, likely due to Comet Encke, crossing Mercury’s orbital plane

Further Considerations Model Variations in Comet Encke’s Orbit with Apostolos Christou (Armagh, N. Ireland) Employ maps of surface composition for surface source (Weider et al., 2014) cannot be the source because we would see the enhancement every other year, not every year Compare with Mg data from MASCS (Sarantos et al. in preparation) (inconclusive so far)

MESSENGER Data Suggests Meteor Shower on Mercury The closest planet to the sun appears to get hit by a periodic meteor shower, possibly due to a comet that produces multiple events annually on Earth, 2P/Encke.   Encke Encke Dust Trail Mercury The clues pointing to Mercury’s shower were discovered in the very thin halo of gases that make up the planet’s exosphere, which is under study as part of NASA’s MESSENGER mission. [Published in Icarus by R.M. Killen and J.M. Hahn, Impact Vaporization as a Possible Source of Mercury's Calcium Exosphere, Icarus, 10.1016/j.icarus.2014.11.035, 2014.]

Thanks to MESSENGER MASCS Team: Matthew Burger, Menelaos Sarantos, Timothy Cassidy, Aimee Merkel, Bill McClintock, Ronald Vervack Apostolos Christou (Armagh Observatory)