1. 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

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

3. OBSERVATIONAL GEOMETRY

4. Observational Geometry
Burger et al., 2014

5. MESSENGER MASCS Ca DATA

6. 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

7. Calcium Source Rate at Mercury

8. 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

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

10. 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

12. Impact Vaporization for Dust Disks With varying inclinations

13. Two disk Model low inclination disk plus high inclination disk

14. Comet Encke Dust Trail Spitzer image

15. Mercury’s Orbit and Comet Encke

16. Comet Encke Orbit vs. Mercury

17. 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

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

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

20. 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

21. 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)

23. 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, /j.icarus , 2014.]

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