Europa’s Surface Composition

Abstract

Europa is unique in the solar system, having a young, icy surface bombarded by high-energy radiation and possessing many possible sources of surface material. One possible source is Europa's putative subsurface ocean, from which material may be emplaced through cryovolcanic activity or effusive flows. Impact ejecta from Io and implantation of iogenic sulfur, oxygen, sodium, potassium, and chlorine ions on Europa's trailing hemisphere are likely sources, as well as direct meteoritic and cometary impacts and outer-satellite-derived impact ejecta that spiral toward Jupiter. While we cannot yet answer the central question of where the non-ice material on Europa's surface comes from, we can identify and quantify the species that are known or thought to be present: H 2 O, a hydrate, SO 2 , elemental sulfur, O 2 , H 2 O 2 , CO 2 , Na, and K. Europa, like many satellites, has a hemispherical dichotomy, in this case a reddish trailing hemisphere (in the sense of orbital motion) and a brighter, leading hemisphere. The purest H 2 O is found on the leading hemisphere while the trailing hemisphere contains the highest concentration of the next most prevalent species, a hydrated material of unknown composition. The H 2 O ice on the leading side is amorphous on the upper surface, with crystalline ice present at submillimeter depths. The trailing hemisphere contains ice plus a hydrated component that may be hydrated salts, derived from the ocean as brine, and/or hydrated sulfuric acid, the major equilibrium product from radiolysis of sulfurous material and H 2 O. The source of sulfurous material could be endogenic or from implantation of iogenic sulfur, or both. Sulfur dioxide and sulfur are thought to be present, mainly on the trailing hemisphere. This is consistent with ion implantation, but the sulfur distribution and that of the hydrate show correlations with geological features, so there must be some endogenic control of these constituents, either as a source or modification process. All the species in the ~1-m regolith are affected by radiation, but the archetypal radiolytic species, observed on both hemispheres, are molecular oxygen and hydrogen peroxide. These are certainly radiolytic products since continuous production is required, with O 2 being volatile and escaping easily, while H 2 O 2 is quickly destroyed by sunlight. Carbon dioxide is present and poses a mystery. It could be outgassing from the interior or a photolytic or radiolytic product of micrometeorite-derived carbonaceous material. Sodium and potassium atoms are found in the tenuous atmosphere and arise from sputtering of surface material. These atoms can be derived initially from the iogenic plasma and from endogenic salts, but the implantation flux rates are not known well enough to establish the source. *Average mass influx from Cooper et al. (2001). Relative composition from Anders and Grevasse (1989). † Sulfur flux value from Johnson et al. (2004) for trailing side apex and based on Bagenal (1994). No plasma deflection is included. ‡ Equatorial mass i...

Polymerization of Building Blocks of Life on Europa and Other Icy Moons

“…If the input of amino acids continues for 50 million years [a typical surface age based on the size distribution analysis of impact craters by Bierhaus et al, (2009)], the overall delivery of cometary amino acids is 5 · 10 9 kg. If this input is distributed evenly on Europa's surface and is mixed homogenously with a depth of 1 m of the ice shell [an average gardening depth of 10 million years old (Carlson et al, 2009)], the resultant surface concentration of amino acids is around 0.1 ppm. In contrast, localized impact events associated with refreezing of surface ice could afford higher local concentration of amino acids.…”

Radiolysis and Photolysis of Icy Satellite Surfaces: Experiments and Theory

“…Identification of specific radiolytic products in the UV is difficult because the bands are very broad and overlap, but H 2 O 2 has been detected via infrared absorption on Europa (e.g. Carlson et al 2009). H 2 O 2 is produced by the principal dissociation product OH in the absence of H (e.g., Laffon et al 2006).…”

“…In the following paragraphs we briefly describe some relevant sulfur compounds and their photolytic and radiolytic chemistry. Summaries of sulfur properties relevant for Io and Europa are given in Carlson et al ( , 2009.…”

“…Since the plasma so formed can diffuse throughout the magnetosphere, material from one body can be implanted in a neighboring body. Sulfur from Io populates the Jovian magnetosphere and may contribute to the sulfur signature on Europa's trailing hemisphere (Lane et al 1981;Carlson et al 2009). The description of such processes has led to considerable experimental and modeling efforts both in the planetary science and the radiation effects communities.…”

Subsurface Water Oceans on Icy Satellites: Chemical Composition and Exchange Processes

“…The composition of Europa's tenuous atmosphere and non-ice surface materials may imply the presence of MgSO 4 , Na 2 SO 4 , and H 2 SO 4 hydrates among other species (H 2 O 2 , O 2 , CO 2 , SO 3 , S compounds), hydrated salts and gases trapped in the satellite's icy shell (Carlson et al 2009). The seafloor rock and sediments could contain phyllosilicates, carbonates, Ca sulfates, Fe sulfides, and organic compounds (Zolotov and Kargel 2009).…”