1. Cometary activity - new data, new questions, new efforts - E. Kührt, N
Cometary activity - new data, new questions, new efforts - E. Kührt, N. Gortsas

2. San Antonio 2006, RPC meeting
What seems to be sure ?
Concept of localized activity; or better of inhomogeneous activity (Giotto, SD, DI)
Sublimation is on or near the surface (mm)
Activity is highly constant from orbit to orbit
Dominant volatile component is water ice
Surface temperatures is much higher than sublimation temperature (VEGA, DS1, DI)
San Antonio 2006, RPC meeting

3. San Antonio 2006, RPC meeting
What seems to be sure ?
Concept of localized activity; or better of inhomogeneous activity (Giotto, SD, DI)
Sublimation is on or near the surface (mm)
Porous structure (at least 30%)
Inhomogeneous surface pattern (rough and smooth areas, craters, highlands)
High diversity between comets
San Antonio 2006, RPC meeting

4. What seems to be probable?
High dust content (Halley, Tempel-1)
Liquid water was never present (Wild-2)
No regolith of small grains on surface (no strong increase in brightness at zero phase at Wild-2 => Duxbury 2004)
Low thermal conductivity to 0.1 W/mK (HB, Tempel-1)
San Antonio 2006, RPC meeting

5. San Antonio 2006, RPC meeting
Open questions related to activity?
How does cometary activity works?
What is the structural/compositional difference between more and less active areas?
Does activity correlate with landforms?
Are there internal heat sources (phase transitions, chemical reactions?)
Why is there no direct correlation in spatial distributions of H2O and dust at Tempel-1?
How pristine are comets (high temperature grains) ?
To what degree are gas and dust activity coupled (no direct correlation in spatial distributions of H2O and dust in DI images)?
What are the structure and strength of cometary nuclei?
San Antonio 2006, RPC meeting

6. San Antonio 2006, RPC meeting
Where are contradictions?
Sublimation on or near the surface measured high surface temperature (corresponds to black body equilibrium)
No or minor amount of water ice on surface was measured stable activity over numerous orbits (why no growing dust mantle?)
What are legends?
After recent cometary space missions we understand cometary activity and know basic physical properties
Comets represent pristine matter from presolar nebula
Thermal and activity modeling are on a good way
San Antonio 2006, RPC meeting

7. San Antonio 2006, RPC meeting
Minimum standard for thermal/activity modeling
Energy conservation is guaranteed by numerical procedure (should be checked in each step)
Sublimation takes place not only on a boundary but also in the volume
Moving boundaries and erosion of the surface must be taken into account (feedback to thermal processes)
Arbitrary initial conditions should not have an impact to the calculated results
Numerical results are stable against shorter time steps and smaller grid distances
San Antonio 2006, RPC meeting

8. San Antonio 2006, RPC meeting
New modeling efforts (Stefan problem):
Background:
comparison of erosion velocity ve and velocity of thermal heat wave vth:
ve = solar influx / H ρeis ~ m/s at perihelion (1 AU)
T (x,t) ~ , a=K/ρ c, for periodic solar flux
m/s, diurnal heat wave
vth = ω / k = 2 √ π K / τ ρ c =
m/s, orbital heat wave
San Antonio 2006, RPC meeting

9. San Antonio 2006, RPC meeting
Conclusions
The erosion velocity near perihelion is equal or larger than the phase velocity of the heat wave.
There is a strong feedback of erosion to the thermal state: Material erosion (ablation) takes away considerable internal energy from the nucleus (energy loss by mass loss!)
Penetration of heat to the interior is restricted!
San Antonio 2006, RPC meeting

10. San Antonio 2006, RPC meeting
Orbital skin depth ~ 0.7 m
Diurnal skin depth ~ 0.01 m
cometary surface
Erosion per Orbit ~ 2 m
2 m
1 m
San Antonio 2006, RPC meeting

11. San Antonio 2006, RPC meeting
t = t + dt
San Antonio 2006, RPC meeting

12. San Antonio 2006, RPC meeting

13. San Antonio 2006, RPC meeting

14. San Antonio 2006, RPC meeting
Next steps
Impact of solved Stefan problem on water activity
Consequences of reduced subsurface temperatures on sublimation of more volatile species (CO,…)
Evaluation of the effect of a thin dust cover
Simulation of depth of the sublimation front of embedded highly volatile ices
San Antonio 2006, RPC meeting