exploring the structure and composition of io's atmosphere ... · comparison with volcanic...
TRANSCRIPT
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Exploring the structure and composition of Io's atmosphere with (sub)mm observations
Arielle Moullet, Jansky Fellow, NRAO
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Lava plains
Volcanic plumes
Volcanic deposits
Galileo surface mapping (1995-2002)
Image Credit JPL/NASA
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Lava plains
Volcanic plumes
Volcanic deposits
Atmosphere
Galileo surface mapping (1995-2002)
Image Credit JPL/NASA
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First detection of vibrational band of SO
2
by Voyager over Loki
Permanent, gravitationally-bound gas layer : atmosphere
Pearl et al., 1979
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Different processes (sources and sinks) to explain the observed variety
0.1-10 nbar SO
2
traces
<1 pbar Ar
Exosphere, O, Na
1.5 bar CO
2
< 10 pbar CO
2, O
2
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Remarquable features on Io :
- Active volcanism
- SO2 frost-covered surface
- Plasma torus feeding
Morabito et al., 1979
Schneider and Bagenal, 2007
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Frost sublimation Gas condensation
Thermal escape
Torus stripping ~ 1ton/s
Surface sputtering
Volcanic outgassing
Photochemistry
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I) Column density
II) Spatial Distribution
III) Composition
IV) Dynamics
(sub)mm observations of Io's atmosphere allow to determine its main physical characteristics
And help to understand the roles of atmospheric sources
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Spatially unresolved observations→ averaged SO
2 column
Compare to sources efficiencies
- sublimation (pbar-mbar)
- sputtering
- volcanic outgassing (a few tons/s/plume)
I) Column density
Lellouch, 1996
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SO2 absorption bands
Contribution of frost/gas : assumption on ground albedo
HST results :5-10. e16 cm-2 column, inhomogeneous coverage
UV measurements
Ballester et al., 1994
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SO2 vibrational bands
Non LTE conditions : modeling highly T-dependant
IRTF results (19 microns) :
Column 1-15e16 cm-2
high spatial variations,T<140 K
IR measurements
Spencer et al., 2005
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Strong SO2 rotational lines in emission (contrasts 20-80 K)
Sound the bulk atmosphere (1st scale height)
LTE conditions : easier radiative transfer modeling
High spectral res. : line profile analysis (~1km/s width)
(sub)mm observations
First rotational SO2 line
detection,Lellouch et al., 1992
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Observations possible at max. elongation to avoid Jupiter's contribution in the beam
Quick source : up to 15“/h
High changes in velocity (+/- 15 km/s) : online Doppler-tracking or off line velocity correction
Max elongation East (leading hemisphere)
Max elongation West (trailing hemisphere)
1.7 days-long synchronous revolution
(sub)mm observations
~45” Ø
120”
Io's Ø : 1.2”max
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Observations possible at max. elongation to avoid Jupiter's contribution in the beam
Quick source : up to 15“/h
High changes in velocity (+/- 15 km/s) : online Doppler-tracking or off line velocity correction
Max elongation East (leading hemisphere)
Max elongation West (trailing hemisphere)
1.7 days-long synchronous revolution
(sub)mm observations
~45” Ø
120”
Io's Ø : 1.2”max
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Observations possible at max. elongation to avoid Jupiter's contribution in the beam
Quick source : up to 15“/h
High changes in velocity (+/- 15 km/s) : online Doppler-tracking or off line velocity correction
Max elongation East (leading hemisphere)
Max elongation West (trailing hemisphere)
1.7 days-long synchronous revolution
(sub)mm observations
~45” Ø
120”
Io's Ø : 1.2”max
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Single-dish observations at IRAM-30m, CSO, APEX
Up to 40 kHz spectral resolution
On-off mode, continuum emission subtraction
(sub)mm observations
IRAM, 1990, 221 GHz
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IRAM, 1994, 221 GHz
(sub)mm observations
Single-dish observations at IRAM-30m, CSO, APEX
Up to 40 kHz spectral resolution
On-off mode, continuum emission subtraction
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IRAM, 1999, 221 GHz
(sub)mm observations
Single-dish observations at IRAM-30m, CSO, APEX
Up to 40 kHz spectral resolution
On-off mode, continuum emission subtraction
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(sub)mm observations
Single-dish observations at IRAM-30m, CSO, APEX
Up to 40 kHz spectral resolution
On-off mode, continuum emission subtraction
APEX, 2010, 346 GHz
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Plume models integrated in radiative transfer model Instantaneous condensation assumed
Different types : Pele (800 km radius, rare)Prometheus (300 km radius)
Volcanic modeling
Zhang et al., 2000
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Observed emission could be produced by : >40 Prometheus active plumes >5 Pele active plumes
More than the number of plumes detected by Galileo (16)
Volcanic modeling
Emission expected from a Pele-type plume at different viewing angles
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Hydrostatic, isothermal assumption
Homogenous or localized atmospheric distribution
Simultaneous fitting of column density, temperature, fractional coverage
Hydrostatic modeling
Width (T,d,f)
Contrast (T,d,f)
Relative contrast (d)
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Possible interpretations go from :
- very localized (<20%), hot (~500 K), dense (6e17cm-2)
- homogeneous, cold (~140 K), low density (~1e16 cm-2) Evidence for temporal/spatial variationVariation with heliocentric distance
Hydrostatic modeling
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Column density measurements span almost 2 orders of magnitude
Temperature, surface coverage hardly constrained
Different interpretations support either volcanic/sublimation source
I) Column density
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Spatially resolved measurement help to constrain the link to sublimation :
- correlation to ices distribution
- diurnal and latitudinal variation
Doute et al., 2002
II) Spatial distribution
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Proofs of link to volcanism :
- correlation to volcanic center mapping
- presence of gas in cold regions
II) Spatial distribution
Geissler et al., 2007
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HST mapping (~150km res)
Decrease of column density with latitude, low variation with local time
Enhanced density near volcanic centers
Jessup et al., 2004
HST maps
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First atmospheric map :
- restricted to an equatorial band
- higher densities on anti-jovian hemisphere
HST maps
Feaga et al., 2009
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Interferometry necessary to resolve source (~1”)
- Continuum (~100 K, 9Jy@346 GHz), spectral maps
- Use of phase self calibration with a continuum model
- Analysis in the Fourier plane / image plane
HST maps (sub)mm maps
Moullet et al., 2008 :IRAM observations
Synthesized beam
Io's disk
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IRAM-PdBI (2005) : SO2 line at 216 GHz, 0.5” max
resolution, 55 m/s spectral resolution
SMA (2006, 2008) : two SO2 lines at 345 GHz,
0.6” max resolution, 170 m/s spectral resolution
(sub)mm maps
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- SO2 emission spatially extended (> 50% of surface)
- restricted in local hour
- concentrated on the anti-jovian hemisphere
IRAM maps
Moullet et al., 2008
Leading hemisphere Trailing hemisphere
Jupiter direction
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SMA maps
Moullet et al., 2010
Trailing hemisphere Leading hemisphere
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SMA maps
Moullet et al., 2010
Trailing hemisphere Leading hemisphere
Maps globally coherent with IRAM results
Less local-time restricted
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Global agreement with IR and UV observations : - compatible distribution- similar column densities (factor 0.3-3)
Evidencing local-hour restricted emission
Moullet et al., 2010
Distribution models
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Leading hemisphere
Volcanic models
Comparison with known volcanic plumes distribution :
- insufficient emitted flux (<20% of total)- emission more localized
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Comparison with volcanic plumes distribution determinedBy Galileo :- insufficient emitted flux (<20% total)- also concentrated on the anti-jovian hemisphere
Trailing hemisphere
Comparison with known volcanic plumes distribution :
- insufficient emitted flux (<20% of total)- emission more localized
Volcanic models
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Observations converge towards :
- extended atmosphere (>50% coverage),- ~1.e16 cm-2 SO
2
- evidence of latitude and local hour dependance- concentration on the anti-jovian hemisphere
Globally coherent with sublimation-sustained bulk atmosphere,volcanic contribution can only be minor
II) Spatial distribution
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Observations converge towards :
- extended atmosphere (>50% coverage),- ~1.e16 cm-2 SO
2
- evidence of latitude and local hour dependance- concentration on the anti-jovian hemisphere
Globally coherent with sublimation-sustained bulk atmosphere,volcanic contribution can only be minor
II) Spatial distribution
Geissler et al., 2004
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Non/less-volatile species not controlled by sublimation
Constraints on other sources : photochemistry, sputtering, volcanism
Detected : SO, NaCl, S2
(Smith et al., 1979)
III) Composition
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Composition depends on vent temperature, conduit pressure, atomic ratios : constraints on volcanic regimes
Zolotov et al., 1998 Schaefer et al., 2004
III) Composition
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Zolotov et al., 1998 Schaefer et al., 2004
Composition depends on vent temperature, conduit pressure, atomic ratios : constraints on volcanic regimes
III) Composition
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- unknown condensability
- expected volcanic product (SO/SO
2 1-10%)
- photochemistry product of SO
2
Mm-measurements : abundance < 10%
First SO detection at IRAM-30m (Lellouch et al., 1996)
SO
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Mapping of forbbiden rovibronic SO line (Keck)
Sensitive to hot gas (> 600 K)
Very localized, good correlation to volcanic centers/ hot spots
(De Pater et al., 2007)
SO
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SMA maps of 346 GHz line :
-less extended emission than SO
2
-concentrated on anti-jovian hemisphere
-possibly linked to Zamama plume eruption
(Moullet et al., 2010)
SOTrailing hemisphere Leading hemisphere
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Volcanic models (immediate condensation)- can reproduce spatial distribution- only 40% max of the total emission
Volcanic models (no condensation)- cannot reproduce spatial distribution
(Moullet et al., 2010)
SO
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Photolysis models :- can reproduce data with lifetime ~hours
Results in favor of coexistence of both volcanic source and photodissociation, at comparable contributions.
(Moullet et al., 2010)
SO
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- provides Na to neutral clouds
- immediately condensible on ground
- expected volcanic product (0.01-4%)
- Disk-averaged mm-measurements : abundance < 0.5 %
First NaCl detection at IRAM-30m (Lellouch et al., 2003)
NaCl
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Low quality mapping suggests localized emission
Volcanism could be the only source with NaCl/SO2 0.6-2.5 %
Permanent detection : continuous volcanic activity ?
(Moullet et al., 2010)
NaCl
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First measurement of 34/32 S ~ 9%
Twice as much as in Earth, Sun, ISM
Output isotopic ratio or fractionation effect ?
First detection of 34SO2 lines
at APEX(Moullet et al., in prep)
34SO2
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KCl probably main source of K in the neutral cloud
Volcanic prediction : ~0.5%
Upper limit measured 0.09%
Tentative detection of KCl at APEX(Moullet et al., in prep)
KCl
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SiO undetected (upper limit 0.2%)Potential evidence of silicate-based volcanism
S2O undetected, condensates very quickly
Failed detection of SiO at APEX(Moullet et al., in prep)
more...
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(sub)mm observations start to bring unique clues on atmospheric composition and volcanism
ALMA Cycle 0 project accepted !
Band 7, extended configuration, resolution 0.4”
Search for SiO, CO, KCl, S2O,...
Expected increase in sensitivity > 10
III) Composition
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Io seen with ALMA band 9
ALMA can track Io
Oct 2010 CSV data
5 antennas, band 9
Beam 2”x0.8”
> 1 hour observation
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A coherent picture starts to emerge from combination of different observing methods :
- mostly sublimation-sustained bulk SO
2 atmosphere
- minor direct volcanic input
Less explored fields :- thermal structure (inversion layer?)- dynamics
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Winds driven by pressure gradients :
- planet-scale horizontal thermal structure
- geographic and diurnal pressure variations(nightside collapse)
Contribution from plume dynamics
III) Composition IV) Dynamics
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Modeling rarefied non-turbulent atmosphere dynamics :
Strong pressure gradient linked to sublimation
→ Expected supersonic day-to-night global flow
Walker et al., 2009
IV) Dynamics Models
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- all-over redshift, increased near limbs- could be a ~100 m/s day-night wind
Doppler-shift map observed at SMA, 2006,345 GHz SO
2 line
Trailing observations
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- blueshifts on the East (morning) limb- redshifts on the West (evening) limb
Doppler-shift map observed at IRAM-PdBI, 2005, 216 GHz SO
2 line
(Moullet et al., 2008)
Doppler-shift map observed at SMA, 2006345 GHz SO
2 line
Leading observations
vv
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Similar to 200 m/s prograde zonal wind (superrotation)
Physical origin of a zonal wind unknown : - upward wind from sublimation ? - plasma torus drag ? - geographic pressure gradients ?
Simulation of a 200 m/s superrotating atmosphere observed at IRAM-PdBI
v
Leading observations
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Re-interpreting single-dish data with super-rotation :
Reconciles (sub)mm with other results
IRAM-30m observation of SO
2 line @251 GHz
Leading observations
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(Sub)mm Doppler-shift mapping is a unique method to access Io's atmospheric dynamics
Need for better resolution to investigate the dynamic regimes
With full ALMA : spatial resolution <0.1”
Simulation of band 7 observations with a 3-km wide configuration(resolution 70 mas)
IV) Dynamics
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THANK YOU !
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THANK YOU !
Do I have more time ?
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Continuum brightness temperature maps :
- subsurface temperature distribution (thermal inertia, albedo) - emissivity distribution (roughness, refraction index, radio absorption)
Surface
NASA/JPL
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Fourier-plane studies :
- diagnostic of thermal emission shape and size
- measurement of limb darkening (Fresnel refraction, temperature decrease)
Very low limb darkening on Io: fluffy (snowy) surface ?
Continuum visibilities from SMA @346 GHz
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Brightness temperature Vs wavelength :
- differential sounding of the surface
- potential interest for calibration purposes
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Linear polarization measurements :
→ direct measurement of the soil refraction index
Failed linear polarization mappingon Ganymede, SMA @ 347 GHz