1. Dokumentname > 23.11.2004Dokumentname > 23.11.2004B Recent Results of Comet Activity Modeling as input for RPC Plasma Simulations Recent Results of Comet Activity Modeling as input for RPC Plasma Simulations E. Kührt, N. Gortsas, DLR Berlin U. Motschmann, H. U. Keller, TU Braunschweig

2. RPC Braunschweig 7.9. 2010 Outline 1.Introduction 2.Activity of comets 3.Thermal model for activity 4.Conclusion

3. RPC Braunschweig 7.9. 2010 1. Introduction Activity is the source of most cometary features (coma, tail) including the interaction of cometary ions with solar wind The picture of cometary activity has changed in the last decade with new knowledge from observations, space missions and lab experiments We apply a new model (Gortsas: Thesis 2010) to derive the gas production as an important input for plasma simulations

4. RPC Braunschweig 7.9. 2010 1. Hale-Bopp ground based observations activity of highly volatile ices (e.g. CO) scales nearly as the solar energy input (Biver et al. 2002), therefore one can conclude, that these volatiles are near the surface activity is localized: strong CO jet near 20° n.l. (Bockelée-Morvan et al. 2009) Key observations to understand activity

5. RPC Braunschweig 7.9. 2010 2. Lab experiments amorphous ice and trapping of gasses confirmed experimentally however, amorphous ice was never identified in the solar system KOSI (comet simulation): it is hard to keep activity alive in a dust-ice mixture new experiments are needed (Blum)

6. RPC Braunschweig 7.9. 2010 3. Space missions Deep Impact at Tempel-1 K < 0.005 W/Km (Groussin et al. 2007) K >1 W/mK (Davidsson 2009) different source areas of H 2 O and CO 2 (Feaga 2007) below 1 m depth original composition low density = 400 kg/m 3 From IR spectroscopy: only 0.03 km 2 of the surface is water ice, but: this is much too less to explain the observed activity (Sunshine 2006)

7. RPC Braunschweig 7.9. 2010 Stardust at Wild-2 dust mostly of solar system origin, only some stardust was a very surprising result some minerals require high temperature for formation (> 2000 K) cometary matter is composed by strong radial mixing through the solar system Organic components are present that have not previously been seen in other extraterrestrial materials

8. RPC Braunschweig 7.9. 2010 Update of main Puzzles to activity What is the nature of activity? What is the structural/compositional difference between more and less active areas? What is the degree of inhomogeneity? How is the heat conductivity (3 orders of magnitude range) Are there internal heat sources (phase transitions, chemical reactions?) What is the trigger for outbursts and splits? P/Holmes outburst 2007 (2 orders of magnitude higher production rate within days)

9. RPC Braunschweig 7.9. 2010 Problem: 2. Thermal modeling of comets Capria (2002) K=3 W/mK wrong spin axis trapped CO is set free extended source water curve failed CO > 10 m below surface

10. RPC Braunschweig 7.9. 2010 Our approach from observations we expect a low heat conductivity in the nucleus that requires an exact treatment as a Stefan problem (moving boundary problem) obliquity of spin axis is taken into account observational evidence that CO-activity of HB is mainly from northern hemisphere and near equator as simple as possible since we know too less about comets not too many free parameters strict control of energy conservation and numerical stability

11. RPC Braunschweig 7.9. 2010 Heat conduction equ. Upper boundary cond. (energy conservation) Lower bound. cond. Initial condition Stefan equation bulk sublimation and gas diffusion Equations

12. RPC Braunschweig 7.9. 2010 Stefan problem (ablation) Z: sublimation rate T : temperature ρ: density K: heat conductivity τ : spin period H 2 O + dust H 2 O + CO + dust Interface x 2 (t) Surface x 1 (t) Surface x 1 (t+Δt) Interface x 2 (t + Δt) velocity of erosionvelocity of heat wave V e ~ 3 mm/h V p ~ 100 mm/h @ K=1 V p ~ 3 mm/h @ K=0.001

13. RPC Braunschweig 7.9. 2010 Results HB (for calibration of the model)

14. RPC Braunschweig 7.9. 2010 Water production rates CO production rates K = 0.01 W/Km

15. RPC Braunschweig 7.9. 2010 Results CG

16. RPC Braunschweig 7.9. 2010 Water production rates CO production rates

17. RPC Braunschweig 7.9. 2010 Cometary activity is still puzzling, Rosetta should help to understand it Rigorous Stefan treatment is mandatory for low heat conductivity Exact Stefan solutions lead to important consequences: heat penetration is obscured temperature profiles are extremely steep near perihelion volatiles as CO can be close at the surface leads to other activity pattern Seasonal effects are important for activity Beyond ~3.5 AU CO becomes the dominating molecule Activity is anisotropic due to day/night effect and chemical inhomogeneities 3. Conclusions

18. RPC Braunschweig 7.9. 2010 Depth of COT-profile at perihelion k1 = 0.001 W/mK k2 = 0.01 W/Km k3 = 0.1 W/Km