exploring the structure and composition of io's atmosphere ... · comparison with volcanic...
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Exploring the structure and composition of Io's atmosphere with (sub)mm observations
Arielle Moullet, Jansky Fellow, NRAO
Lava plains
Volcanic plumes
Volcanic deposits
Galileo surface mapping (1995-2002)
Image Credit JPL/NASA
Lava plains
Volcanic plumes
Volcanic deposits
Atmosphere
Galileo surface mapping (1995-2002)
Image Credit JPL/NASA
First detection of vibrational band of SO
2
by Voyager over Loki
Permanent, gravitationally-bound gas layer : atmosphere
Pearl et al., 1979
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
Remarquable features on Io :
- Active volcanism
- SO2 frost-covered surface
- Plasma torus feeding
Morabito et al., 1979
Schneider and Bagenal, 2007
Frost sublimation Gas condensation
Thermal escape
Torus stripping ~ 1ton/s
Surface sputtering
Volcanic outgassing
Photochemistry
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
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
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
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
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
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
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
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
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
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
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
(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
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
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
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)
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
Column density measurements span almost 2 orders of magnitude
Temperature, surface coverage hardly constrained
Different interpretations support either volcanic/sublimation source
I) Column density
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
Proofs of link to volcanism :
- correlation to volcanic center mapping
- presence of gas in cold regions
II) Spatial distribution
Geissler et al., 2007
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
First atmospheric map :
- restricted to an equatorial band
- higher densities on anti-jovian hemisphere
HST maps
Feaga et al., 2009
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
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
- 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
SMA maps
Moullet et al., 2010
Trailing hemisphere Leading hemisphere
SMA maps
Moullet et al., 2010
Trailing hemisphere Leading hemisphere
Maps globally coherent with IRAM results
Less local-time restricted
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
Leading hemisphere
Volcanic models
Comparison with known volcanic plumes distribution :
- insufficient emitted flux (<20% of total)- emission more localized
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
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
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
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
Composition depends on vent temperature, conduit pressure, atomic ratios : constraints on volcanic regimes
Zolotov et al., 1998 Schaefer et al., 2004
III) Composition
Zolotov et al., 1998 Schaefer et al., 2004
Composition depends on vent temperature, conduit pressure, atomic ratios : constraints on volcanic regimes
III) Composition
- 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
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
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
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
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
- 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
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
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
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
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...
(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
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
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
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
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
- 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
- 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
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
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
(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
THANK YOU !
THANK YOU !
Do I have more time ?
Continuum brightness temperature maps :
- subsurface temperature distribution (thermal inertia, albedo) - emissivity distribution (roughness, refraction index, radio absorption)
Surface
NASA/JPL
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
Brightness temperature Vs wavelength :
- differential sounding of the surface
- potential interest for calibration purposes
Linear polarization measurements :
→ direct measurement of the soil refraction index
Failed linear polarization mappingon Ganymede, SMA @ 347 GHz