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RPC Braunschweig 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
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Dokumentname > 23.11.2004Dokumentname > 23.11.2004B
Recent Results of Comet Activity Modeling Recent Results of Comet Activity Modeling as input for RPC Plasma Simulationsas input for RPC Plasma Simulations
E. Kührt, N. Gortsas, DLR Berlin U. Motschmann, H. U. Keller, TU Braunschweig
RPC Braunschweig 7.9. 2010
Outline
1. Introduction2. Activity of comets3. Thermal model for activity4. Conclusion
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 windThe picture of cometary activity has changed in the last decade with new knowledge from observations, space missions and lab experimentsWe apply a new model (Gortsas: Thesis 2010) to derive the gas production as an important input for plasma simulations
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 surfaceactivity is localized: strong CO jet near 20° n.l. (Bockelée-Morvan et al. 2009)
Key observations to understand activity
RPC Braunschweig 7.9. 2010
2. Lab experimentsamorphous 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 mixturenew experiments are needed (Blum)
RPC Braunschweig 7.9. 2010
3. Space missions
Deep Impact at Tempel-1K < 0.005 W/Km (Groussin et al. 2007)K >1 W/mK (Davidsson 2009) different source areas of H2O and CO2 (Feaga 2007)below 1 m depth original composition low density = 400 kg/m3
From IR spectroscopy: only 0.03 km2 of the surface is water ice, but: this is much too less to explain the observed activity (Sunshine 2006)
RPC Braunschweig 7.9. 2010
Stardust at Wild-2
dust mostly of solar system origin, only some stardust
was a very surprising resultsome 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
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)
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
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 accountobservational 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 parametersstrict control of energy conservation and numerical stability
RPC Braunschweig 7.9. 2010
dtTZdx)(
Heat conduction equ.
Upper boundary cond.(energy conservation)
Lower bound. cond.
Initial condition
Stefan equation
bulk sublimation and gas diffusion
Equations
RPC Braunschweig 7.9. 2010
Stefan problem (ablation)
Z: sublimation rateT: temperatureρ: densityK: heat conductivityτ: spin period
1
2 )(TZ
dtdxV OH
e
H2O +
dust
H2O +CO +dust
Interface x2(t)
Surface x1(t)Surface x1(t+Δt)
Interface x2(t + Δt)
2
)(TZ
dtdxV CO
e
velocity of erosionvelocity of heat wave
Ve ~ 3 mm/h Vp ~ 100 mm/h @ K=1Vp ~ 3 mm/h @ K=0.001
RPC Braunschweig 7.9. 2010
Results HB (for calibration of the model)
RPC Braunschweig 7.9. 2010
Water production rates CO production rates
K = 0.01 W/Km
RPC Braunschweig 7.9. 2010
Results CG
RPC Braunschweig 7.9. 2010
Water production rates CO production rates
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
RPC Braunschweig 7.9. 2010
Depth of CO T-profile at perihelion
k1 = 0.001 W/mKk2 = 0.01 W/Kmk3 = 0.1 W/Km