Energy Options for Future Humans on Titan

We review the possibilities for in situ energy resources on Titan for use by future humans, including chemical, nuclear, wind, solar, geothermal and hydropower. All of these options, with the possible exception of geothermal, represent effective sources of power. Combustion of methane (after electrolysis of the native water), in combination with another source of power such as nuclear, is a viable option; another chemical source of energy is the hydrogenation of acetylene. The large seas Kraken and Ligeia potentially represent effective sources of hydropower. Wind power, particularly at altitudes ~40 km, is expected to be productive. Despite the distance from the sun and the absorbing atmosphere, solar power is (as on Earth) an extremely efficient source of power on Titan

Rough Surfaces: Is the Dark Stuff Just Shadow?: "Who knows what evil lurks in the hearts of men? The shadow knows!"

Remote observations of the surfaces of airless planetary objects are fundamental to inferring the physical structure and compositional makeup of the surface material. A number of forward models have been developed to reproduce the photometric behavior of these surfaces, based on specific, assumed structural properties such as macroscopic roughness and associated shadowing. Most work of this type is applied to geometric albedos, which are affected by complicated effects near zero phase angle that represent only a tiny fraction of the net energy reflected by the object. Other applications include parameter fits to resolved portions of some planetary surface as viewed over a range of geometries. The spherical albedo of the entire object (when it can be determined) captures the net energy balance of the particle more robustly than the geometric albedo. In most treatments involving spherical albedos, spherical albedos and particle phase functions are often treated as if they are independent, neglecting the effects of roughness. In this paper we take a different approach. We note that whatever function captures the phase angle dependence of the brightness of a realistic rough, shadowed, flat surface element relative to that of a smooth granular surface of the same material, it is manifested directly in both the integral phase function and the spherical albedo of the object. We suggest that, where broad phase angle coverage is possible, spherical albedos may be easily corrected for the effects of shadowing using observed (or assumed) phase functions, and then modeled more robustly using smooth-surface regolith radiative transfer models without further imposed (forward-modeled) shadowing corrections. Our approach attributes observed "power law" phase functions of various slope (and "linear" ranges of magnitude-vs.-phase angle) to shadowing, as have others, and goes on to suggest that regolith-model-based inferences of composition based on shadow-uncorrected spherical albedos overestimate the amount of absorbing material contained in the regolith

The composition of Saturn's rings

The origin and evolution of Saturn's rings is critical to understanding the Saturnian system as a whole. Here, we discuss the physical and chemical composition of the rings, as a foundation for evolutionary models described in subsequent chapters. We review the physical characteristics of the main rings, and summarize current constraints on their chemical composition. Radial trends are observed in temperature and to a limited extent in particle size distribution, with the C ring exhibiting higher temperatures and a larger population of small particles. The C ring also shows evidence for the greatest abundance of silicate material, perhaps indicative of formation from a rocky body. The C ring and Cassini Division have lower optical depths than the A and B rings, which contributes to the higher abundance of the exogenous neutral absorber in these regions. Overall, the main ring composition is strongly dominated by water ice, with minor silicate, UV absorber, and neutral absorber components. Sampling of the innermost D ring during Cassini's Grand Finale provides a new set of in situ constraints on the ring composition, and we explore ongoing work to understand the linkages between the main rings and the D ring. The D ring material is organic- and silicate-rich and water-poor relative to the main rings, with a large population of small grains. This composition may be explained in part by volatile losses in the D ring, and current constraints suggest some degree of fractionation rather than sampling of the bulk D ring material.Comment: Submitted to SSR for publication in the collection "New Vision of the Saturnian System in the Context of a Highly Dissipative Saturn

The Science Case for a Return to Enceladus

The plume of Enceladus is unique in the solar system in providing direct access to fresh material from an extraterrestrial subsurface ocean. The Cassini Mission, though not specifically designed for it, was able to take advantage of the plume to conduct the best characterization to date of an extraterrestrial ocean. Evidence gathered from multiple instruments points to a global, subsurface liquid water ocean rich in salts and organic compounds, with water-rock interactions occurring presumably in hydrothermal systems at or below the moon’s sea floor. Meeting the criteria of “extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy source(s) to sustain metabolism,” the ocean of Enceladus can therefore be considered habitable. It is also the only confirmed place beyond the Earth where we can easily sample fresh material from a demonstrably habitable environment without the complications of digging or drilling. The next step is to investigate whether Enceladus’ ocean is actually inhabited. Here, we summarize the evidence for Enceladus’ ocean and its habitability, identify constraints and outstanding questions on the detectability of life within its ocean, and recommend a return to Enceladus with a dedicated search-for-life mission (or missions)

Mid-ultraviolet Hubble Observations of Europa and the Global Surface Distribution of SO₂

We present spatially resolved reflectance spectra of Europa’s surface in the wavelength range of 210–315 nm obtained by the Hubble Space Telescope Imaging Spectrograph in 2018 and 2019. These data provide the first high-quality, near-global spectral observations of Europa from 210 to 240 nm. They show that the reflectance of Europa’s leading, trailing, anti-Jovian, and sub-Jovian hemispheres is ∼5% near 210 nm, with varying spectral slopes across the mid-UV. This low albedo, even on the more “pristine” leading hemisphere, indicates a lack of the signature far-UV spectral edge characteristic of water ice. We detected and mapped a strong absorption feature at 280 nm that is consistent with an S–O bond that has previously been attributed to SO2 on the surface, hypothesized to be formed through radiolytic processing of Iogenic sulfur ions that have been preferentially emplaced on Europa’s trailing hemisphere by Jupiter’s magnetic field. Our models show that small inclusions of SO2 (0.1%) within the water ice are sufficient to produce the 280 nm feature without producing a feature at 4.07 μm, which has not been observed in ground-based spectral observations of Europa. This data set is the first to produce a spatially resolved, near-global map of the assumed SO2 feature, which is primarily concentrated near the apex of the trailing hemisphere and correlated with large-scale darker regions in both the visible and the ultraviolet. This distribution is consistent with “cold” exogenic sulfur ion bombardment on Europa

The Saturn Ring Skimmer Mission Concept: The next step to explore Saturn's rings, atmosphere, interior, and inner magnetosphere

The innovative Saturn Ring Skimmer mission concept enables a wide range of investigations that address fundamental questions about Saturn and its rings, as well as giant planets and astrophysical disk systems in general. This mission would provide new insights into the dynamical processes that operate in astrophysical disk systems by observing individual particles in Saturn's rings for the first time. The Ring Skimmer would also constrain the origin, history, and fate of Saturn's rings by determining their compositional evolution and material transport rates. In addition, the Ring Skimmer would reveal how the rings, magnetosphere, and planet operate as an inter-connected system by making direct measurements of the ring's atmosphere, Saturn's inner magnetosphere and the material owing from the rings into the planet. At the same time, this mission would clarify the dynamical processes operating in the planet's visible atmosphere and deep interior by making extensive high-resolution observations of cloud features and repeated measurements of the planet's extremely dynamic gravitational field. Given the scientific potential of this basic mission concept, we advocate that it be studied in depth as a potential option for the New Frontiers program.Comment: White paper submitted to the Planetary Science and Astrobiology Decadal Survey (submission #420

The Saturn Ring Skimmer Mission Concept: The next step to explore Saturn's rings, atmosphere, interior and inner magnetosphere

The innovative Saturn Ring Skimmer mission concept enables a wide range of investigations that address fundamental questions about Saturn and its rings, as well as giant planets and astrophysical disk systems in general. This mission would provide new insights into the dynamical processes that operate in astrophysical disk systems by observing individual particles in Saturn's rings for the first time. The Ring Skimmer would also constrain the origin, history, and fate of Saturn's rings by determining their compositional evolution and material transport rates. In addition, the Ring Skimmer would reveal how the rings, magnetosphere, and planet operate as an inter-connected system by making direct measurements of the ring's atmosphere, Saturn's inner magnetosphere and the material owing from the rings into the planet. At the same time, this mission would clarify the dynamical processes operating in the planet's visible atmosphere and deep interior by making extensive high-resolution observations of cloud features and repeated measurements of the planet's extremely dynamic gravitational field. Given the scientific potential of this basic mission concept, we advocate that it be studied in depth as a potential option for the New Frontiers program

Asteroid Redirect Mission (ARM) Formulation Assessment and Support Team (FAST) Final Report

The Asteroid Redirect Mission (ARM) Formulation Assessment and Support Team (FAST) was a two-month effort, chartered by NASA, to provide timely inputs for mission requirement formulation in support of the Asteroid Redirect Robotic Mission (ARRM) Requirements Closure Technical Interchange Meeting held December 15-16, 2015, to assist in developing an initial list of potential mission investigations, and to provide input on potential hosted payloads and partnerships. The FAST explored several aspects of potential science benefits and knowledge gain from the ARM. Expertise from the science, engineering, and technology communities was represented in exploring lines of inquiry related to key characteristics of the ARRM reference target asteroid (2008 EV5) for engineering design purposes. Specific areas of interest included target origin, spatial distribution and size of boulders, surface geotechnical properties, boulder physical properties, and considerations for boulder handling, crew safety, and containment. In order to increase knowledge gain potential from the mission, opportunities for partnerships and accompanying payloads were also investigated. Potential investigations could be conducted to reduce mission risks and increase knowledge return in the areas of science, planetary defense, asteroid resources and in-situ resource utilization, and capability and technology demonstrations. This report represents the FAST"TM"s final product for the ARM