Sub-Millimeter Array
Harvard-Smithsonian Center for Astrophysics
Mapping of SO2, SO and NaCl
emission in Io's atmosphere
Arielle Moullet, Mark Gurwell
Io, Jupiter's volcanic moon
closest moon of Jupiter, size ~ Moon
● imaged by Voyager, Galileo, New Horizons
● most active body of Solar system:
volcanoes, lava flows, plumes, hotspots
● basalt/silicate surface + volcanic ring
deposits, temporal variations
●
Io's surface imaged by Galileo-SSI (Nasa Photo Gallery)
Dust plume Tvashtar
spotted by New HorizonsLORRI (Credits Nasa/
JHUAPL/ SRI)
Io's atmosphere
detected by UV and IR absorbtion bands,
mm/sub-mm lines
● composition: SO (90%), SO, NaCl, S ,...
2
2
● primary origin: interior outgassing
● extremely tenuous ~ 1 nanobar
● temporal variations
●
high escape: thermal + plama torus drag
●
Io's atmospheric near-IR glow
observed by Cassini-ISS
(Geissler et al., 2004)
Where is the atmosphere located?
SO2 distribution from UV observations
(Feaga et al., 2009)
SO2 distribution from IR observations
(Spencer et al., 2005)
Where is the atmosphere located?
SO2 distribution from UV observations
(Feaga et al., 2009)
Volcanic plumes and deposits distribution
(Geissler et al., 2004)
SO2 distribution from IR observations
(Spencer et al., 2005)
SO2 frost distribution (Doute et al., 2002)
Open questions on Io's atmosphere
Main sustainment mechanism: continuous volcanic
outgassing? SO2 frost sublimation equilibrium? Other
sources?
●
Dynamics: observed dynamics (Moullet et al., 2008)
different than subsolar-antisolar flow expected
●
●
Composition: volcanic regime characterisation
●
Surface/air/plasma interactions
Millimeter observations of Io's
atmosphere
Strong rotational lines (SO2, SO, NaCl)
in emission over surface thermal
emission continuum
●
High spectral resolution: line profile
resolved (access to column density,
temperature)
●
Single-dish observations: source
unresolved. Interferometric
observations: low spatial resolution
(source size 1-1.2'')
●
●
Line doppler-shift mapping: dynamics
SO2 line map
obtained in 2008 at SMA
SMA observations
2 tracks in 2006, 2 tracks in 2008. Both
hemispheres observed (leading/trailing)
●
Extended configuration: ~ 0.8''x0.8''
synthetized beam (Io :1.2'')
●
Line targets near 345 GHz: 2 SO2
lines, 1 SO line, 1 NaCl line
●
●
Good spectral resolution: 170-85 m/s
uv-plane coverage
obtained in 2006
SO2 mapping
Leading
hemisphere
Trailing
hemisphere
Line-integrated mapping
(1 MHz)
●
2006
Emission displacement
respect to moon's center:
SO2 concentrated on the
anti-jovian hemisphere
●
Line emission extent
smaller than continuum:
heterogen SO2distribution
●
2008
Does SO2 originate from plumes?
Comparison with plume models
from Zhang et al. (2003): rarefied
gas dynamics, surface and plasma
interactions, vent characteristics...
●
●
Location of all 16 plumes known
Synthetic emission maps:
concentrated on anti-jovian side,
unsufficient spatial extension
Column density distribution
model for 3 volcanic plumes
●
Hundreds of plumes needed to
produce the data emission:
unrealistic
Synthetic emission maps for volcanic models
●
Does SO2 originate from frost
sublimation ?
Comparison with hydrostatic models
with fixed SO2 gas distribution models:
mimicking sublimation sustained gas
●
Free parameters: column density, gas
temperature
●
Good coherence data/models:
emission map shape, column density.
Realistic temperature fit.
●
Low temporal variations of column
density (or related to Sun distance):
sublimation likely to be main SO2 source
●
Synthetic emission maps for
hydrostatic models
SO mapping
Leading
hemisphere
Trailing
hemisphere
Line-integrated maps
(1 MHz)
●
Concentrated in the antijovian hemisphere
●
2006
Less spatially extended
than SO2
●
Compatible with volcanic
models maps
●
2008
Other sources needed :
photodissociation
●
NaCl mapping
Leading
hemisphere
Low 3-sigma detection in 2008,
no detection in 2006
●
●
Trailing hemisphere
No information on spatial extent
Concentrated on the anti-jovian
hemisphere
●
Doppler-shift mapping : dynamic
detection
Mapping of the strongest SO2 line
Doppler-shifts
●
Detection of the expected day to
night flow on the trailing side, 2008
●
Right order of magnitude for winds
as predicted by Ingersoll, 1985:
~ 300 m/s at the terminator
●
Wind pattern coherent with
sublimation-sustained model
●
Doppler-shift contour map, in
m/s. Trailing side, 2008.
Doppler-shift mapping : dynamic
detection
On 3 other tracks, zonal wind
detection:
- prograde zonal flow on leading (2006)
about 200 m/s
- blue/redshift gradient in N/S direction
●
Origin not yet understood: Pressure
gradients due to frost distribution?
Sublimation speed? Plasma drag
transferred to low altitude?
●
Doppler-shift contour map, in
m/s. Trailing side, 2008.
Volcanic plume dynamic effects
excluded
●
Doppler-shift contour map, in
m/s. Leadng side, 2006.
Conclusions
Arguments in favor of mainly sublimation-sustained SO2 :
distribution, long-term stability, column densities
●
SO and NaCl presence may be mostly maintained by
volcanic plumes output
●
Dynamic patterns not understood: need for Doppler-shift
mapping with better spatial resolution
●
Future observing opportunities
APEX observations at 350 GHz
(expected summer 2009) : looking
for minor species (S2O, KCl, SiO)
and isotopes
●
ALMA (first call for proposal ~2012)
high-resolution mapping ~ 0.03''.
●
●
EJSM/Laplace mission ?
Simulation of ALMA imaging at 350 GHz
Surface thermal emission