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The Surface Composition of Europa: Implications for Landed Missions

J.B. Dalton Jet Propulsion Laboratory



Remote Sensing from Galileo NIMS

Trailing

Leading


Surface Modification Processes

1. Emplacement
Endogenic material placed at the surface by geologic processes


Surface Modification Processes

2. Implantation
Ions (H, Na, K, Cl, S, O... Mg?) implanted into the surface ice


Surface Modification Processes

3. Radiolysis and photolysis
Radiation-driven chemistry alters surface composition


Surface Modification Processes

4. Impact Gardening
Micrometeoritic bombardment mixes the upper ~1-2 m


Surface Modification Processes

5. Frost (re-)Deposition
Linea brighten over time as water ice is vapor-deposited on surface


Surface Modification Processes
Material emplaced at the surface has been modified by radiation, impact gardening, and the re-deposition of sputtered and/or excavated frost


Europa Surface Character
· Highly variable surface topography Typical ~100-300m Up to > 1250 m · Many scarps and vertical cliffs · Ridges of ~30-35 degree slope · Exposed faces · Lag deposits

(Titanic Courtesy of Patricio Figueredo)




Surface Composition of Europa
Known: · Water Ice · Carbon Dioxide Ice · Sulfur Dioxide Ice · Hydrogen Peroxide · Hydronium Ion (H3O+) Expected: · Hydrogen sulfide (H2S) · Formaldehyde (H2CO) · Hydrochloric Acid (HCl) · Carbon Monoxide (CO) · Oxygen (O2), Ozone (O3)


Surface Composition of Europa
"Non-Ice" Material: · Sulfuric Acid Hydrate · Hydrated Sulfate Salts · Irradiated Material · Organic Material? · Clathrate Hydrates?


Hydrated Sulfur Compounds
Magnesium Sulfate Hydrates
MgSO MgSO MgSO MgSO MgSO MgSO MgSO
4 4 4 4 4 4 4

Sodium Sulfate Hydrates
Na2SO4 · 10H2O Na2Mg(SO4)2 · 4H2O Mirabilite Bloedite

· · · · · · ·

1H2O 2H2O 4H2O 5H2O 6H2O 7H2O 11H2O

Kieserite Sanderite Starkeyite Pentahydrite Hexahydrite Epsomite Undecahydrite

Other Sulfur Compounds
Na2S · 9H2O H2SO4 · 8H2O MgSO4 + H2O(l) Sodium Sulfide Nonahydrate Sulfuric Acid Hydrate Magnesium Sulfate Brine


Galileo NIMS spectroscopy of Europan surface units
0.8 0.7 0.6

Scaled Reflectance

0.5

Dark terrain
0.4 0.3 0.2 0.1 0 0.8 1.2 1.6 2 2.4

Icy Plains

Wavelength (µm)


Spectral Effects: Water of Hydration
MgSO4
1.2

.

nH2O

3

n 0

Europa 1

2.5

0.8

2

1 1.5

Reflectance (plus offset)

Reflectance

0.6 Water Ice

1.5

2 3

0.4

1

4
0.5

5 6 7

0.2

0 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 Wavelength (microns)

0 1 1.5 2 2.5

Wavelength (microns)


Europa Spectrum and Hydrated Materials
1.2

1

Dark terrain Scaled Reflectance
0.8

0.6

Icy Plains
0.4

0.2

Water Ice

0 0.8 1.2 1.6 2 2.4

1

Wavelength (µm)

Hydrated sulfates all exhibit Europa-like spectral characteristics


Spectral Effects: Water of Hydration
1.2

MgSO4 (dried)
1

MgSO4 "anhydrous"

Reflectance (plus offset)

0.8

Water (liquid)
0.6

0.4

0.2

0 1 1.5 2 2.5

Wavelength (microns)

These absorptions are not intrinsic to the host molecule!


Temperature-dependent Effects
Hexahydrite (MgSO4 · 6H2O) and Bloedite (MgNa2(SO4)2 · 4H2O) suggested to comprise 80% of surface material (McCord et al., 1999)
2

Separation of Water Features in Hydrates
1

Bloedite 300K

Scaled Reflectance

0.8

Hexahydrite
1.5 300K

0.6

Scaled Reflectance

0.4

120K 1

0.2
300K 0.5

Bloedite 100K

0 1.45
120K

1.5

1.55

1.6

1.65

1.7

1.75

Wavelength (um)

Bloedite
0 1 1.5 2 2.5

Wavelength (microns)

BUT... Spectral behavior at cryogenic temperatures may differ markedly from that at room temperature (Dalton and Clark, 1999; Dalton, 2000, 2003, 2005)


Temperature Dependence of Spectral Properties
Hexahydrite (MgSO4 · 6H2O) and Bloedite (MgNa2(SO4)2 · 4H2O) suggested to comprise >70% of surface material (McCord et al., 1999) based on available room temperature spectra Subsequent cryogenic spectra (eg., Dalton et al., 2000, 2003, 2005) demonstrated strong temperature dependence of spectral absorption ba strengths, shapes, positions Spectral models utilizing cryogenic spectroscopy thus have the potential to provide realistic constraints on surface composition of icy satellites


Magnesium Sulfate Undecahydrate
As temperature is reduced below 200K, individual absorption features separate and narrow, producing fine structure that can be used to discriminate between materials
1.4 250K 1.2 200K 1

MgSO4·11HO O MgSO4·12H2 2

150K

Scaled Reflectance

0.8

100K

50K 0.6

0.4

0.2

0 0.8 1.2 1.6 2 2.4

Dalton et al., 2005

Wavelength (microns)


The Search for Life on Europa....


Psychrophiles at bottom edge of sea ice core near Barrow, Alaska


Halophiles and salt crust at San Francico Bay (salinity >300 ppt)


Spectra of extremophiles at 120 K
1

0.8

Scaled Reflectance

Escherichia coli 0.6

· Life contains many things · Life contains hydrates

0.4

Sulfolobus shibatae

· Life contains amides

0.2 Deinococcus radiodurans 0 0.8 1.2 1.6 2 2.4

Wavelength (µm)


Some Popular Hydrated Materials
2.5

2

H2SO4 · 8H2O Bloedite

Sulfolobus Shibatae

Reflectance

1.5 Hexahydrite 1 Europa Dark 0.5

0

0.8

1.2

1.6 Wavelength (µm)

2

2.4


Spatial Considerations for Interpretation of Surface Composition
Discrimination of surface constituents requires high spectral AND high spatial resolution Surface heterogeneous at small spatial scales Spectra are mixtures of adjacent surface units large footprint reduces detectability of constituents
Identification of surface materials requires spectral imaging of individual units

Spatial Resolution: Surface heterogeneous at 25 - 100 m scales

Galileo SSI image of unique crater with apparent subsurface material flowing onto surface from radial fractures.

500 m

Signal to Noise: Sampling statistics and radiation noise require multipl pixels across contiguous surface units

Galileo SSI image of ridged plains at 6 m/pixel. Linear troughs containing dark material are about 100 m wide.


Fit to Europa "Non-Icy" Spectrum


Fit to Europa "Non-Icy" Spectrum
1
Mixture of Room Temperature Salts (figure 12c, McCord et al., 1999): 59% Bloedite 24% Mirabilite 14% Hexahydrite 3% Epsomite

0.8

Reflectance

0.6
Europa Non-Icy

0.4
0% 14% 0% 11% 12% 0% 62% 0%

0.2

Mixture of Main Contenders: Water Ice Hexahydrite Epsomite Bloedite Mirabilite Dodecahydrate Sulfuric Acid Octahydrate MgSO4, NaSO4, NaHCO3 Brines

0 0.5 1 1.5 Wavelength (um) 2 2.5


Fit to Europa "Non-Icy" Spectrum
1

0.8

Reflectance

0.6
Europa Non-Icy No H2SO4

0.4
46% 0% 29% 25% 0% 0% 0%

0.2

Best Fit Mixture w/o H2SO4: Water Ice Hexahydrite Epsomite Bloedite Mirabilite Dodecahydrate Sulfuric Acid Octahydrate MgSO4, NaSO4, NaHCO3 Brines

0 0.5 1 1.5 Wavelength (um) 2 2.5


Galileo SSI Observation at 9 m/pixel

Future investigations such as a Europa Flagship Mission may shed light on these mysteries...given sufficient spatial and spectral resolution, and the availability of relevant laboratory data.



Conclusions:
· Highly hydrated sulfate salts exhibit more Europa-like spectral behavior than those of lower hydration states. · Best spectral models include BOTH hydrated salts and hydrated sulfuric acid. · Fine structure at low temperatures can be exploited to discriminate between candidate materials. · Lab spectra are needed for all candidate surface compounds.


Conclusions:
· Detritus of emplaced organisms is consistent with the observed spectral signature. · Microbes are capable of surviving the low surface temperatures at Europa. · Consideration should be given to a landed package that can confirm the presence of sulfate salts and acids, as well as search for evidence of biologically-derived material in the near subsurface. · It is important to "look before we leap" in determining landing sites.


Recommendations:
· The extreme surface roughness, and character of the upper surface layers need to be considered in developing a lander concept. · Unraveling surface composition requires a concerted, collaborative effort that includes laboratory simulations. · A lander must be prepared to encounter sulfuric acid, hydrogen peroxide, and a variety of salts. · Access to the subsurface has the potential to acquire direct evidence of past or even recent biological activity.