The Exospheres of Europa, Ganymede, and Callisto
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The Exospheres of Europa,
Ganymede, and Callisto
Peter Wurz, Audrey Vorburger, André Galli, Marek Tulej,
Yann Alibert, and Nicolas Thomas
Physikalisches Institut and Center for Space and Habitability, Universität
Bern, 3012 Bern, Switzerland (peter.wurz@space.unibe.ch, 41 31 631
44 26)
Olivier Mousis
Observatoire des Sciences de l'Univers THETA de Franche-Comté,
Besançon, FRANCE
Stas Barabash, Martin Wieser,
Swedish Institute of Space Physics, S-981 28 Kiruna, Sweden
Helmut Lammer
Austrian Academy of Sciences, A-8042 Graz, Austria
Planet formation in the core-accretion
model !
1. Cloud collapse and star-disk
formation
2. Dust growth and
planetesimal formation
3. Planetary embryo growth
4. Gas capture and gap
opening
5. Long term gravitational
evolution
Exo-Climes III, March 2014!
Formation of regular satellites:
4. Gas capture and gap opening
subnebula
Image: LPI http://www.lpi.usra.edu/education/timeline/
> Subnebula: birthplace of regular satellites
— fed by gas and planetesimals coming
from the solar nebula
— chemical evolution/condensation/volatile
loss composition of satellites
— accretion of planetesimals/migration
Exo-Climes III, March 2014!
Composition of satellites
!
Ogihara & Ida 2012 Mousis & Alibert 2006
condensation chemistry
formation
evolution
JUICE / ESA
Exo-Climes III, March 2014!
Initial gas phase conditions in the solar
nebula!
Disk’s gas phase conditions can vary between oxidising and
reducing states:
q Oxidising conditions: most of C in CO and most of N in N2
Two gas phase compositions investigated:
- C/O = 0.5 (solar)
- C/O = 1 (O depleted)
q Reducing conditions: all C in CH4 and most of N in NH3
Two gas phase compositions investigated:
- C/O = 0.5 (solar)
- C/O = 1 (O depleted)
Johnson et al. 2012, ApJ 757, 192; Mousis et al. 2012, ApJLExo-Climes
751, L7 III, March 2014!
Planetesimal volatile composition
Oxidising
conditions
Reducing
conditions
Mousis et al. (2012, ApJL 751, L7; 2014, PSS, in prep) Exo-Climes III, March 2014!
Chemical Composition of Jupiter‘s Icy
Moons!
MOLE FRACTIONS
C/O = 0.55 Oxidising Reducing
Oxidising Case: H2O/TOT 0.597666208 0.731283454
CH4/TOT 0.005728505 0.196739225
CO:CO2:CH3OH:CH4 = 70:10:2:1 CH3OH/TOT 0.016141017 0
N2:NH3 = 10 CO/TOT 0.260669522 0
CO2/TOT 0.068451276 0
Reducing Case N2/TOT 0.027201595 0.002147177
NH3/TOT 0.007034856 0.061644189
All C in CH4
H2S/TOT 0.014459801 0.006919254
N2:NH3 = 0.1 Xe/TOT 3.20E-07 1.15E-07
Kr/TOT 2.05E-06 9.81E-07
Ar/TOT 0.00212381 0.001016278
PH3/TOT 0.00052104 0.000249326
Total 1.000 1.000
Exo-Climes III, March 2014!
JUICE Science Goals
http://sci.esa.int/juice/!
For Europa, Callisto and Ganymede, the JUICE science
objectives are:
> Characterise and determine the extent of sub-surface
oceans and their relations to the deeper interior.
> Characterise the ice shells and any subsurface water,
including the heterogeneity of the ice, and the nature of
surface-ice-ocean exchange.
> Characterise the deep internal structure, differentiation
history, and (for Ganymede) the intrinsic magnetic field.
> Compare the atmospheres, plasma environments, and
magnetospheric interactions.
> Determine global surface compositions and chemistry,
especially as related to habitability.
> Understand the formation of surface features, including
sites of recent or current activity, and identify and
characterise candidate sites for future in situExo-Climes
exploration. !!
III, March 2014
Particle Environment Package (PEP)!
> PEP combines remote global imaging with
in-situ measurements
— PI: Stas Barabash, IRF, Kiruna, Sweden
— Co-PI: Peter Wurz, Uni Bern, Switzerland
> PEP consists of three units with modular
design hosting sensors and electronics, and
well-defined, minimal interfaces to the
spacecraft
— Zenith Unit
— Nadir Unit
— JENI
> PEP sensors are
— JoEE: Energetic electrons
— JEI: Plasma electrons
— JENI: Energetic ENA and energetic ions
— JNA: Plasma ENA
— JDC: Plasma positive ions
— NIM: Neutral gas and Ion Mass spectrometer
Exo-Climes III, March 2014!
PEP / NIM
Neutral and Ion Mass Spectrometer!
> The atmospheres of Europa, Ganymede and
Callisto are largely unknown
— Result of evaporation / sublimation and
exogenic processes (sputtering)
— Atmospheric species are directly related to the
surface
> NIM / PEP will perform first-ever gas mass
spectroscopy at the icy moons
> 2 Europa flybys, 20 Callisto flybys, 11
Ganymede flybys, and orbit phase
> With NIM / PEP we will characterise these
atmospheres
— Chemical composition of volatiles
— Contribution from non-ice material on the
surface
— Isotopic composition of major species
> With PEP plasma instruments we will
— Characterise plasma interaction with moons’
surfaces
— Infer radiolysis in the surface material Exo-Climes III, March 2014!Exosphere Modelling!
> Exosphere model!
— Wurz & Lammer, Icarus 2003; Wurz et al., PSS 2007, PSS 2010!
> For surface-bound exosphere!
— Provides exospheric densities from surface composition!
> Includes 4 release processes!
— sublimation, photon-stimulated desorption, sputtering, micro-meteorite
impact vaporisation!
> Model includes!
— Temperature field on surface!
— Gravitational escape!
— Photo-ionisation, electron-ionisation!
— Fragmentation!
> Results!
— Density profiles, column densities, transversal column densities
— Loss fractions, escape fluxes, ionisation, ...
— Velocity distributions, ....
Exo-Climes III, March 2014!Europa!
Surface
composi.on
•Bright Areas (ice-rich regions)
–H2O, CO2,
–SO2, Sx,
–H2O2, ...
•Dark Areas (ice-poor regions)
–MgSO4 • xH20
–Na2SO4 • xH20
Atmosphere
composi.on
–Na2CO3 • xH20
–H2SO4 • xH20
•Possible extremophile bacteria
–Cyanidium
–Deinococcus radiodurans
–Sulfolobus shibatae
–Escherichia coli
The surface composition of the dark areas is not
well constrained by infra-red (IR) spectroscopy,
even with high spectral resolution.
Exo-Climes III, March 2014!Europa Atmosphere Model!
> 2 Europa flybys by JUICE
> NIM flyby operations
— Full mass spectra at 5-sec cadence
— Detection threshold 30 cm–3, in
Europa’s radiation environment
— dynamic range of > 105 at Europa
> All species known in Europa‘s
exosphere can be detected by NIM/
PEP during the JUICE flybys
— O, O2, H2, H2O, Na, SO2, SO, CO2,
CO
> Expected exospheric species from
non-ice surface
— Detection if surface concentration is
>= 10–3
— Mg, MgO, NaO, Ca, CaO, Al,
AlO, ...
> Isotopes:
— With a threshold of 30 cm–3 and a
dynamic range of > 105 the D/H
ratios can be resolved in the
thermal component of H2
— 18O/16O from the O2 and H2O in the
sputtered signal
Exo-Climes III, March 2014!PEP / NIM Mass spectra during nominal
Europa flyby
100‘000 km 1‘000 km
Europa Closest Approach
400 km 100 km
Exo-Climes III, March 2014!Ganymede!
Ganymede Exosphere Measurements
> Molecular oxygen (O2): Spencer et al. JGR 1996
— Leading / trailing side differences
— Oxygen trapped in surface
> Oxygen atoms: Hall et al. ApJ, 1998
— Inferred vertical O2 column densities are in the
range NC = (1–10)·1014 cm2
— Localised emission regions near north and south
pole
> Oxygen atoms: Feldman et al. ApJ, 2000
— Correlation of oxygen emissions with magnetic
field topology
> Ozone detection: Noll et al. Science 1996
Exosphere modelling — Ozone gas trapped in ice
— Radiolytic ozone
> M.L. Marconi, Icarus, 2007
— H2O, O2, H2 Ionosphere Measurements
— Surface densities up to 10 cm (~ 10
9 –2 –7
> Eviatar et al. PSS, 2001
mbar)
— Bound ionosphere
— Inferred exosphere molecular oxygen in polar
regions, NC = 7.4·1012 cm–2
— Atomic oxygen at low latitudes, NC = 3·1014 cm–2
— Corona of hot oxygen atoms
Exo-Climes III, March 2014!Ganymede‘s Exosphere Model!
> 11 Ganymede flybys by JUICE
elliptic orbit > Ganymede orbit phase
> 10’000 x 200 km & 5000 km (150 days)
200 km orbit > 500 km circular (102 days)
> 200 km circular (30 days)
500 km orbit
> NIM flyby operations
— Full mass spectra at 5-sec cadence
— Detection threshold ≈ 1 cm–3,
— dynamic range of > 106
Exo-Climes III, March 2014!Callisto!
Callisto Exosphere Measurements
> Carlson, Science, 1999
— Thin CO2 atmosphere with CO2 density of 4·108 cm3
> Cunningham et al., 2013
— O detected, from dissociation of O2
Ionosphere Measurements
> Only at trailing side illuminated by the Sun (Gurnett
et al., 2000; Kliore et al., 2002)
— Ionisation of CO2 atmosphere not enough
— Postulated O2 exosphere N0 ≈ 1010 cm–3
Callisto Surface Composition Exosphere modelling
> Kliore et al., 2002
> About half ice, other half rocks
— Sputtered H2O to produce O2 atmosphere of 1010 cm3
—Ice: water ice, CO2, SO2, ammonia
> Strobel et al., 2002
—Rock: L/LL chondritic composition
— upper limits for the abundances of O2 and CO to be
1017 cm2
— Upper limits and atomic carbon and atomic oxygen to
be 1013 and 2.5·1013 cm2
> M.-C. Liang et al. JGR, 2005
— H2O, OH, H2
— Surface densities up to 109 cm–2 (~ 10 –7 mbar)
Exo-Climes III, March 2014!Callisto’s Exosphere Model! > 20 Callisto flybys by JUICE
> NIM orbit operations
— Closest approach at ~ 199 km
— Detection threshold ~1 cm–3
(instrument background)
> Sublimated particles dominate close
to surface
> Sublimated density profiles drop off
quickly except for H and H2
> Sputtered particles dominate heavy
thermal and sublimated particles
above ~1000 km
> Both sputtered particles from the ice
and from the rocky surfaces will be
measured
> 128 density profiles implemented
— 18 sublimated
— 6 thermally released
— 1 directly sputtered (H2O (x3)
— 2 directly sp. or with H2O (CO2 &
CH4) (x3)
— 15 ice_sputtered (x3)
— 16 mineral_sputtered (x3)
– x3 = sputtered by H+, On+ or Sn+
Exo-Climes III, March 2014!Europa plumes!
Visible images of the observed hemispheres (A
to C) with sub-observer longitudes listed and
combined STIS images of the hydrogen and
oxygen emissions (D to O). The Lyman-α
morphology [(D) to (F)] reveals an anti-correlation
with the brightness in the visible (15). (G) to (O)
Same Lyman-α images, and OI130.4 nm and
OI135.6 nm images with solar disk-reflectance
subtracted. 3 x 3 pixels are binned and the STIS
images are smoothed to enhance visibility of the
significant features. The dotted light blue circles
indicate the multiplet lines (15). The colour scale is
normalised to the respective brightness and the
scale maximum (corresponding to 1.0 on the scale)
is listed in each image. Oversaturated pixels with
intensities above maximum are white. The
contours show signal-to-noise (SNR) ratios of the
binned pixels [and contours for SNR = 1 are
omitted here in (D) to (F) and (M) to (O)].
Roth et al., Science, 2013 Exo-Climes III, March 2014!Europa Plume Model!
> Water plume modelled to
match Roth et al., Science
2013, observations!
— Scale height ~ 100 km!
— Tangential column density
NCH2O ≈ 1020 m–2!
> Plume chemical
composition adapted from
icy moon accretion
modelling (Mousis et al.,
2014)!
> Signal a factor 100 – 1000
larger than regular
atmosphere!
Exo-Climes III, March 2014!Europa Plume Measurements: PEP / NIM!
> Direct sampling of liquid body, perhaps liquid pocket in ice, perhaps the
ocean!
— No inversion from exosphere to surface composition! > Chemical inventory of
— No removal of chemistry arising from radiolysis! liquid body!
— CO, CO2, N2, NH3, PH3, ..!
— Organics, e.g. CH3CO!
— Noble gases can be
detected: Ne, Ar, Kr, Xe!
> Densities are high enough
for isotope measurements,
e.g.:!
— D/H in water!
— 32S/34S
in H2S !
— 12C/13C in CO and CO2, !
— 14N/15N in N2 and NH3 !
> Assess habitability of
ocean!
— Possible bio-markers !
Exo-Climes III, March 2014!Summary!
> JUICE mission of ESA will investigate the Jupiter system in detail!
— What are the conditions for planet formation and emergence of life? How does the Solar
System work?
— Emergence of habitable worlds around gas giants – Jupiter system as an archetype for gas
giants
> With the PEP experiment we will investigate!
— Jupiter’s magnetospheric plasma system!
— Plasma interaction with the surfaces of the icy moons!
> With NIM / PEP we will investigate!
— Exospheres of Europa, Ganymede, and Callisto!
— Europa !
– Composition measurements (ice and non-ice material)!
– Perhaps sample a liquid body of Europa (perhaps the global ocean)!
– Assess habitability!
— Ganymede !
– Detailed composition measurements (ice and non-ice material)!
— Callisto!
– Measure the ice and the rock component!
> By modelling we !
— Derive the chemical composition of the surface from the exosphere measurements!
— Derive the radio-chemistry in the surface applying the plasma measurements!
— Provide “undisturbed” chemical composition for formation models of icy moons !
Exo-Climes III, March 2014!Exo-Climes III, March 2014!
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