To describe how to model the exosphere of small objects

1. Ceres, Vesta, and comet 67p
LIN TU

2. Outline
 Dawn Mission
 Ceres & Vesta
 Exospheric & Thermal Model
 67p & Rosseta Mission

3. Dawn Mission

4. Dawn Mission
Vesta
 Launch : 9. 27, 2007
 Vesta arrival (initial orbit)
 7. 16, 2011
 Vesta lowest orbit
 Vesta departure
 12. 13, 2011
Ceres
 Arrival : c. February 2015

5. Vesta & Ceres

6. Exosphere

7. The mechanisms of exosphere
production

8. Herschel detected water vapor on Ceres.
Ceres’ exosphere
Küppers et al., 2014
 Largest main belt asteroid
 Wet
 Escape velocity
~ 0.52 km/s
 Thermal velocity
~ 0.48km/s by 200K

9. Exospheric model
 Monte Carlo Method
 Ballistic motion
 Maxwellian distribution
 Sticking effect (T<100K)
lifetime (~453 sec) lifetime (~106 sec)

10. Ceres’ exosphere
High production rate case
~1025 molecules/s
Low production rate case
~1024 molecules/s

11. Vesta’s Exosphere(?)

12. Shape Model of Vesta

13. Ray tracing
We use Monte Carlo Method to do ray tracing.
500 points on each facet

14. Thermal Model
 Diffusion equation:
 k
𝜕2T
𝜕z2 = ρcp
𝜕T
𝜕t
 Lower boundary condition:

𝜕T
𝜕z z=−2
= 0
 Upper boundary condition
 S 1 − A cosθ cos∅ = εσT4 + k
dT
dz z=0
+ LM

15. Solstice Equinox
Temperature map of Vesta

16. exosphere of Vesta(?)

17. Comet 67p
Cometary jets
from Hartley 2

18. Shape Model of 67p

19. Upper boundary condition
Surface energy balance
J. J. Cowan & M. F. A’Hearn (1979)
Washburn (1928)
Lebofsky (1975)
latent heat of Water
sublimation
Effect of
shadow
Upper Boundary Condition

20. Comet 67P

21. Thank you for attention!