Transcript Dt. Standardfoliensatz

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
Dokumentname
Dokumentname> >23.11.2004B
23.11.2004
Outline
1.
2.
3.
4.
Introduction
Activity of comets
Thermal model for activity
Conclusion
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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
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Key observations to understand activity
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)
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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)
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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 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)
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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
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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)
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2. Thermal modeling of comets
Problem:
Capria (2002)
•
•
•
•
•
•
K=3 W/mK
wrong spin axis
trapped CO is set
free
extended source
water curve failed
CO > 10 m below
surface
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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
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Equations
Heat conduction equ.
Upper boundary cond.
(energy conservation)
Lower bound. cond.
Initial condition
dx 
Z (T )

dt
Stefan equation
bulk sublimation and
gas diffusion
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Stefan problem (ablation)
velocity of erosion
Surface x1(t)
Surface x1(t+Δt)
Ve 
H2O
+
dust
dx ZH 2O (T )

dt
1
Ve ~ 3 mm/h
Vp ~ 100 mm/h @ K=1
Vp ~ 3 mm/h @ K=0.001
Interface x2(t)
Interface x2(t + Δt) H O +
2
CO +
dust
velocity of heat wave
Ve 
dx ZCO (T )

dt
2
Z: sublimation rate
T: temperature
ρ: density
K: heat conductivity
τ: spin period
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Results HB (for calibration of the model)
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Water production rates
CO production rates
K = 0.01 W/Km
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Results CG
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Water production rates
CO production rates
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3. Conclusions
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
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Depth of CO
T-profile at perihelion
k1 = 0.001 W/mK
k2 = 0.01 W/Km
k3 = 0.1 W/Km
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