Exploring the structure and composition of Io's atmosphere ... · Comparison with volcanic...

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