Dokumentname > 23.11.2004Dokumentname > 23.11.2004B Recent Results of Comet Activity Modeling as...

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


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


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


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)


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)


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


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)


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


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


dtTZdx)(

Heat conduction equ.

Upper boundary cond.(energy conservation)

Lower bound. cond.

Initial condition

Stefan equation

bulk sublimation and gas diffusion

Equations


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


Results HB (for calibration of the model)


Water production rates CO production rates

K = 0.01 W/Km


Results CG


Water production rates CO production rates


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


Depth of CO T-profile at perihelion

k1 = 0.001 W/mKk2 = 0.01 W/Kmk3 = 0.1 W/Km

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