Kevin P. Hand

Energy, Chemical Disequilibrium, and Geological Constraints on Europa

Europa is a prime target for astrobiology. The presence of a global subsurface liquid water ocean and a composition likely to contain a suite of biogenic elements make it a compelling world in the search for a second origin of life. Critical to these factors, however, may be the availability of energy for biological processes on Europa. We have examined the production and availability of oxidants and carbon-containing reductants on Europa to better understand the habitability of the subsurface ocean. Data from the Galileo Near-Infrared Mapping Spectrometer were used to constrain the surface abundance of CO(2) to 0.036% by number relative to water. Laboratory results indicate that radiolytically processed CO(2)-rich ices yield CO and H(2)CO(3); the reductants H(2)CO, CH(3)OH, and CH(4) are at most minor species. We analyzed chemical sources and sinks and concluded that the radiolytically processed surface of Europa could serve to maintain an oxidized ocean even if the surface oxidants (O(2), H(2)O(2), CO(2), SO(2), and SO(4) (2)) are delivered only once every approximately 0.5 Gyr. If delivery periods are comparable to the observed surface age (30-70 Myr), then Europas ocean could reach O(2) concentrations comparable to those found in terrestrial surface waters, even if approximately 10(9) moles yr(1) of hydrothermally delivered reductants consume most of the oxidant flux. Such an ocean would be energetically hospitable for terrestrial marine macrofauna. The availability of reductants could be the limiting factor for biologically useful chemical energy on Europa.

Salts and Radiation Products on the Surface of Europa

The surface of Europa could contain the compositional imprint of an underlying interior ocean, but competing hypotheses differ over whether spectral observations from the Galileo spacecraft show the signature of ocean evaporates or simply surface radiation products unrelated to the interior. Using adaptive optics at the W. M. Keck Observatory, we have obtained spatially resolved spectra of most of the disk of Europa at a spectral resolution ~40 times higher than seen by the Galileo spacecraft. These spectra show a previously undetected distinct signature of magnesium sulfate salts on Europa, but the magnesium sulfate is confined to the trailing hemisphere and spatially correlated with the presence of radiation products like sulfuric acid and SO_2. On the leading, less irradiated, hemisphere, our observations rule out the presence of many of the proposed sulfate salts, but do show the presence of distorted water ice bands. Based on the association of the potential MgSO_4 detection on the trailing side with other radiation products, we conclude that MgSO_4 is also a radiation product, rather than a constituent of a Europa ocean brine. Based on ocean chemistry models, we hypothesize that, prior to irradiation, magnesium is primarily in the form of MgCl_2, and we predict that NaCl and KCl are even more abundant, and, in fact, dominate the non-ice component of the leading hemisphere. We propose observational tests of this new hypothesis.

Science potential from a Europa Lander

The prospect of a future soft landing on the surface of Europa is enticing, as it would create science opportunities that could not be achieved through flyby or orbital remote sensing, with direct relevance to Europas potential habitability. Here, we summarize the science of a Europa lander concept, as developed by our NASA-commissioned Science Definition Team. The science concept concentrates on observations that can best be achieved by in situ examination of Europa from its surface. We discuss the suggested science objectives and investigations for a Europa lander mission, along with a model planning payload of instruments that could address these objectives. The highest priority is active sampling of Europas non-ice material from at least two different depths (0.5-2 cm and 5-10 cm) to understand its detailed composition and chemistry and the specific nature of salts, any organic materials, and other contaminants. A secondary focus is geophysical prospecting of Europa, through seismology and magnetometry, to probe the satellites ice shell and ocean. Finally, the surface geology can be characterized in situ at a human scale. A Europa lander could take advantage of the complex radiation environment of the satellite, landing where modeling suggests that radiation is about an order of magnitude less intense than in other regions. However, to choose a landing site that is safe and would yield the maximum science return, thorough reconnaissance of Europa would be required prior to selecting a scientifically optimized landing site.

Tubular Compression Fossils from the Ediacaran Nama Group, Namibia

Abstract Abundant tubular macrofossils occur in finely laminated siltstones and shales of the 548–542 Ma Schwarzrand Subgroup, Nama Group, Namibia. The Nama tubes occur in both the Vingerbreek and Feldschuhhorn members commonly in dense populations and always in fine-grained, lower shore-face lithologies deposited below fair-weather wave base. The tubes are preserved mostly as compressed casts and molds that range in width from 0.6 to 2.1 mm; apparently incomplete specimens reach lengths up to 10 cm. All specimens show sinuous bending and occasional brittle fracture, indicating an original construction of strong but flexible organic matter. Feldschuhhorn specimens preserve fine longitudinal pleats or folds that record pliant organic walls, but the older Vingerbreek populations do not. Similarly, some specimens in the Feldschuhhorn Member display branching, while Vingerbreek tubes do not. The abundant Feldschuhhorn tubes are assigned to the widespread Ediacaran problematicum Vendotaenia antiqua; however, the distinctive Vingerbreek population remains in open nomenclature. The most abundant fossils in Nama rocks, these tubes resemble populations in Ediacaran successions from Russia, China, Spain, and elsewhere. Beyond their local importance, then, such tubes may turn out to be the most abundant record of Ediacaran life.

Europa's surface color suggests an ocean rich with sodium chloride

The composition of Europas surface may be representative of the subsurface ocean; however, considerable debate persists regarding the endogenous or exogenous nature of a hydrated sulfate feature on Europa. Direct evidence of oceanic salts on Europas surface has been largely inconclusive. We show that the observed color within geologically young features on Europas surface can be explained by sodium chloride delivered from the ocean below. We find that sodium chloride, when exposed to Europa surface conditions, accumulates electrons in F and M centers, yielding a yellow-brown discoloration comparable to Europas surface. Irradiation of sodium chloride from Europas ocean thus provides a simple and elegant solution to the color of the non-ice material observed on Europa. This evidence for endogenous salts suggests that Europas ocean is interacting with a silicate seafloor, a critical consideration for assessing habitability.

Fourier transform infrared spectroscopy for Mars science

Presented here is a Fourier transform infrared spectrometer (FTIR) for field studies that serves as a prototype for future Mars science applications. Infrared spectroscopy provides chemical information that is relevant to a number of Mars science questions. This includes mineralogical analysis, nitrogen compound recognition, truth testing of remote sensing measurements, and the ability to detect organic compounds. The challenges and scientific opportunities are given for the in situ FTIR analysis of Mars soil and rock samples. Various FTIR sampling techniques are assessed and compared to other analytical instrumentation. The prototype instrument presented is capable of providing field analysis in a Mars analog Antarctic environment. FTIR analysis of endolithic microbial communities in Antarctic rocks and a Mars meteor are given as analytical examples.

Active Cryovolcanism on Europa

Evidence for plumes of water on Europa has previously been found using the Hubble Space Telescope (HST) using two different observing techniques. Roth et al. (2014) found line emission from the dissociation products of water. Sparks et al. (2016) found evidence for off-limb continuum absorption as Europa transited Jupiter. Here, we present a new transit observation of Europa that shows a second event at the same location as a previous plume candidate from Sparks et al. (2016), raising the possibility of a consistently active source of erupting material on Europa. This conclusion is bolstered by comparison with a nighttime thermal image from the Galileo Photopolarimeter-Radiometer (PPR) which shows a thermal anomaly at the same location, within the uncertainties (Spencer et al. 1999). The anomaly has the highest observed brightness temperature on the Europa nightside. If heat flow from a subsurface liquid water reservoir causes the thermal anomaly, its depth is ~1.8-2 km, under simple modeling assumptions, consistent with scenarios in which a liquid water reservoir has formed within a thick ice shell. Models that favor thin regions within the ice shell that connect directly to the ocean, however, cannot be excluded, nor modifications to surface thermal inertia by subsurface activity. Alternatively, vapor deposition surrounding an active vent could increase the thermal inertia of the surface and cause the thermal anomaly. This candidate plume region may offer a promising location for an initial characterization of Europas internal water and ice and for seeking evidence of Europas habitability. ~

KECK II OBSERVATIONS OF HEMISPHERICAL DIFFERENCES IN H2O2 ON EUROPA

We present results from Keck II observations of Europa over four consecutive nights using the near-infrared spectrograph. Spectra were collected in the 3.14-4.0 μm range, enabling detection and monitoring of the 3.5 μm feature due to hydrogen peroxide. Galileo Near-Infrared Mapping Spectrometer results first revealed hydrogen peroxide on Europa in the anti-Jovian region of the leading hemisphere at a percent by number abundance of 0.13% ± 0.07% relative to water. We find comparable results for the two nights over which we observed the leading hemisphere. Significantly, we observed a small amount of hydrogen peroxide (~0.04%) during observations of Europas anti-Jovian and sub-Jovian hemispheres. Almost no hydrogen peroxide was detected during observations of just the trailing hemisphere. We conclude that the Galileo observations likely represent the maximum hydrogen peroxide concentration, the exception potentially being the cold water ice regions of the poles, which are not readily observable from the ground. Our mapping of the peroxide abundance across Europa requires revisions to previous estimates for Europas global surface abundance of oxidants and leads to a reduction in the total oxidant delivery expected for the subsurface ocean if an exchange of surface material with the ocean occurs.

DEEP-WATER INCISED VALLEY DEPOSITS AT THE EDIACARAN-CAMBRIAN BOUNDARY IN SOUTHERN NAMIBIA CONTAIN ABUNDANT TREPTICHNUS PEDUM

Abstract Valley-filling deposits of the Nama Group, southern Namibia, record two episodes of erosional downcutting and backfill, developed close together in time near the Ediacaran-Cambrian boundary. Geochronological constraints indicate that the older valley fill began 539.4 ± 1 Ma or later; the younger of these deposits contains unusually well-preserved populations of the basal Cambrian trace fossil Treptichnus pedum. Facies analysis shows that T. pedum is closely linked to a nearshore sandstone deposit, indicating a close environmental or taphonomic connection to very shallow, mud-draped sandy seafloor swept by tidal currents. Facies restriction may limit the biostratigraphic potential of T. pedum in Namibia and elsewhere, but it also illuminates functional and ecological interpretation. The T. pedum tracemaker was a motile bilaterian animal that lived below the sediment-water interface—propelling itself forward in upward-curving projections that breached the sediment surface. The T. pedum animal, therefore, lived infaunally, perhaps to avoid predation, surfacing regularly to feed and take in oxygen. Alternatively, the T. pedum animal may have been a deposit feeder that surfaced largely for purposes of gas exchange, an interpretation that has some support in the observed association of T. pedum with mud drapes. Treptichnus pedum provides our oldest record of animals that combined anatomical and behavioral complexity. Insights from comparative biology suggest that basal Cambrian T. pedum animals already possessed the anatomical, neurological, and genetic complexity needed to enable the body plan and behavioral diversification recorded by younger Cambrian fossils.

Spatially Resolved Spectroscopy of Europa: The Distinct Spectrum of Large-scale Chaos

We present a comprehensive analysis of spatially resolved moderate spectral resolution near infrared spectra obtained with the adaptive optics system at the Keck Observatory. We identify three compositionally distinct end member regions: the trailing hemisphere bullseye, the leading hemisphere upper latitudes, and a third component associated with leading hemisphere chaos units. We interpret the composition of the three end member regions to be dominated by irradiation products, water ice, and evaporite deposits or salt brines, respectively. The third component is associated with geological features and distinct from the geography of irradiation, suggesting an endogenous identity. Identifying the endogenous composition is of particular interest for revealing the subsurface composition. However, its spectrum is not consistent with linear mixtures of the salt minerals previously considered relevant to Europa. The spectrum of this component is distinguished by distorted hydration features rather than distinct spectral features, indicating hydrated minerals but making unique identification difficult. In particular, it lacks features common to hydrated sulfate minerals, challenging the traditional view of an endogenous salty component dominated by Mg-sulfates. Chloride evaporite deposits are one possible alternative.