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NASA NTRS2018

Environmental Requirements and Verification for NASA’s Planned Europa Clipper

NASA’s Jet Propulsion Laboratory (JPL) and its partner are planning a mission to explore an icy moon of Jupiter, Europa. The objective of the planned Europa Clipper mission is to gain insight into the key ingredients for this potentially habitable world. This mission will conduct investigations using a suite of instruments that includes a set of five remote sensing instruments, four in-situ fields and particles instruments, and a two-channel ice-penetrating radar. Among its science objectives are to produce high-resolution images of Europa's surface, determine its composition, look for signs of recent or ongoing activity, measure the thickness of the ice shell, search for subsurface lakes, and determine the depth and salinity of Europa's ocean. Europa Clipper is expected to encounter a very challenging environment, particularly in radiation. This paper provides a comprehensive description of the challenging environments and the environmental requirements that are levied onto the Europa Clipper system design. The mitigations activities being conducted or planned to ensure compliance with the severe environments will be described. Also discussed will be how the environmental requirements are verified and at which level of integration to ensure mission success.

Man, Kin F.↗

NASA NTRS2017

Europa Clipper Proposed Mission - an Update

Europa, the fourth largest moon of Jupiter, is believed to be one of the best places in the solar system to look for extant life beyond Earth. The 2011 Planetary Decadal Survey, Vision and Voyages, states: “Because of this ocean’s potential suitability for life, Europa is one of the most important targets in all of planetary science.” Exploring Europa to investigate its habitability is the goal of the planned Europa Mission. This exploration is intimately tied to understanding the three “ingredients” for life: liquid water, chemistry, and energy. The Europa Mission would investigate these ingredients by comprehensively exploring Europa’s ice shell and liquid ocean interface, surface geology and surface composition to glean insight into the inner workings of this fascinating moon. In addition, a lander mission is seen as a possible future step, but current data about the Jovian radiation environment and about potential landing site hazards and potential safe landing zones is insufficient. Therefore an additional goal of the mission would be to characterize the radiation environment near Europa and investigate scientifically compelling sites for hazards, to inform a potential future landed mission.

Kirby, Karen↗

NASA NTRS2022

Science Objectives for Flagship-Class Mission Concepts for the Search for Evidence of Life at Enceladus

Cassini revealed that Saturn's Moon Enceladus hosts a subsurface ocean that meets the accepted criteria for habitability with bio-essential elements and compounds, liquid water, and energy sources available in the environment. Whether these conditions are sufficiently abundant and collocated to support life remains unknown and cannot be determined from Cassini data. However, thanks to the plume of oceanic material emanating from Enceladus’ south pole, a new mission to Enceladus could search for evidence of life without having to descend through kilometers of ice. In this article, we outline the science motivations for such a successor to Cassini, choosing the primary science goal to be determining whether Enceladus is inhabited and assuming a resource level equivalent to NASA's Flagship-class missions. We selected a set of potential biosignature measurements that are complementary and orthogonal to build a robust case for any life detection result. This result would be further informed by quantifications of the habitability of the environment through geochemical and geophysical investigations into the ocean and ice shell crust. This study demonstrates that Enceladus’ plume offers an unparalleled opportunity for in situ exploration of an Ocean World and that the planetary science and astrobiology community is well equipped to take full advantage of it in the coming decades.

Enceladus↗

NASA NTRS2020

Environmental Verification of NASA’s Europa Clipper Mission

NASA’s Jet Propulsion Laboratory (JPL) and its partner are planning a mission to explore an icy moon of Jupiter, Europa. The objective of the planned Europa Clipper mission is to gain insight into the key ingredients for this potentially habitable world. This mission will conduct investigations using a suite of remote sensing and in-situ fields and particles instruments, and a two-channel ice-penetrating radar. Among its science objectives are to produce high-resolution images of Europa's surface, determine its composition, look for signs of recent or ongoing activity, measure the thickness of the ice shell, search for subsurface lakes, and determine the depth and salinity of Europa's ocean. Europa Clipper is expected to encounter very challenging environments. These environments include radiation, dynamics, thermal, and electromagnetics, which have been translated into a set of environmental requirements that are levied onto the Europa Clipper flight system design. This paper describes how the environmental requirements are specified and verified for each flight component and at which level of integration. A couple of examples will be provided to illustrate the process by which a comprehensive set of verification activities is specified and performed for individual subsystems and instruments to ensure mission success.

Man, Kin Fung↗

NASA NTRS2022

A Novel High-Performance Mission-Enabling Multi-Purpose Radioisotope Heat Source

Recent studies indicate science mission concepts targeting access to the sub-surface oceans of icy moons require ice-penetrating cryobots powered by advanced Radioisotope Power Systems (RPS). These systems would deliver waste heat for ice-melting in the range of 10 kW. Minimizing the transit time through the kilometers-thick ice shells to just a few years requires these RPS to utilize heat sources having a higher thermal energy volumetric density than the existing flight-qualified General-Purpose Heat Source (GPHS). A Compact Heat Source (CPHS)has been conceptualized in which the graphite impact shells(GIS) of the existing GPHS are rearranged in a hexagonal aeroshell containing seven GIS per module, as opposed to the standard two per module; offering a thermal energy density of 0.57 W/cm3versus 0.29 W/cm3 offered by the GPHS simply from the repackaging of Technology Readiness Level (TRL)9 subassemblies. Preliminary thermal modeling of the CPHS integrated into a notional radioisotope thermoelectric generator(RTG) structure further suggests that centerline temperatures are well within allowable limits during nominal operation. Given the need for the CPHS for a subset of missions, it is worth exploring the applicability of the CPHS for more general RTG purposes. We discuss herein how the CPHS may be implemented with either heritage or in-development thermoelectric converter technologies into a Next-Generation RTG concept. Due to a higher energy density, the legacy heat rejection fin arrangement must be modified to permit a sufficiently low cold-side temperature. Preliminary finite element analysis suggests fin-root temperatures can be kept as low as 520 K while allowing the generator to fit within the usable dimensions of currently available United States Department of Energy shipping containers. Such temperatures would certainly be compatible with the use of high temperature thermoelectric converter technologies.. A prime candidate is the heritage silicon-germanium (SiGe) unicouple, whose design could be adapted by approximately halving the leg-length, but without changes in hot and cold junction interfaces, which are features critical to the proven performance and reliability of these devices. The estimated Beginning of Life power for a SiGe-based CPHS-RTG using 12 CPHS for a thermal inventory of 10.5 kW is greater than 600 W under deep space operating conditions. Using higher performance segmented couples currently in development that are based on skutterudite,La3−xTe4and 14-1-11 Zintl thermoelectric materials in lieu of the SiGe unicouples would increase the power level to more than1 kW. The high specific power (We/kg) attribute of CPHS-RTGs found in this study could potentially enable Radioisotope Electric Propulsion (REP) mission concepts. Past NASA REP mission concept studies identified specific power needs in excess of 6to 8 We/kg. Based on a GPHS-RTG-like system configuration, we show that at fin root temperatures between 530 K and 570K (deep space environment), specific powers exceeding 10 We/kg are achievable using high performance segmented thermoelectric converters. The compact sizing and power density of the CPHS-RTG would constitute a significant step upgrade in specific power when compared to heritage GPHS-RTG (approximately5.1 We/kg) and off-the-shelf Multi-Mission RTG (approximately 2.6 We/kg).

Nesmith, Bill J.↗

NASA NTRS2025

Large Impact Features on Ganymede and Callisto as Revealed By Geological Mapping and Morphometry

The icy Galilean satellites are host to a broad range of impact feature morphologies. Hypotheses seeking to explain the diversity of these impact features consider the effects of impact melt, the physical state of the subsurface at the time of impact, and the impactor characteristics. As part of a larger effort to assess the role of these factors in the formation and evolution of these impact features, we have performed topographic and geological mapping of 19 large impact features on Ganymede and Callisto. These are divided into two main morphological groups: craters (subdivided into pit, dome, and anomalous dome craters), and penepalimpsests/palimpsests. The transitions from pit, dome, to anomalous dome craters appear to be size-dependent up to diameters of ∼170 km. The morphologies of pit and dome craters appear to be independent of their age or geologic context. The impacts that formed them only affected a cold, rigid ice layer, with the development of pits and raised annuli on their floors possibly stemming from the evolution of a pocket of impact melt. The subdued rims and floors of anomalous dome craters indicate the increasing effect of a weak, warm ice layer on impact feature morphology with increasing size, but their prominent annuli and pits indicate that mobilization of impact melt is also a factor. The very low topographic relief of older penepalimpsests and palimpsests indicates that their impacts penetrated the ice shell to mobilize very large volumes of pre-existing liquid from a subsurface layer, with little contribution to the final feature morphology from impact melt. Penepalimpsests are distinguished from palimpsests by the higher frequency of concentric ridges within their interiors, indicating a generally more robust state of the subsurface that could better support the rotation and uplift of solid material during impact, even if a crater-like depression could not be supported. A few impact features seem to be transitional between anomalous dome craters and penepalimpsests, and the overlap of anomalous dome craters, penepalimpsests, and palimpsests in terms of diameter as well as age indicates that impactor size and subsurface properties over time are major factors in determining which of these morphologies emerges.

Oliver L White↗

NASA NTRS2005

Convection in Ice I with Composite Newtonian/Non-Newtonian Rheology: Application to the Icy Galilean Satellites

Ice I exhibits a complex rheology at temperature and pressure conditions appropriate for the interiors of the outer ice I shells of Europa, Ganymede, and Callisto. We use numerical methods to determine the conditions required to trigger convection in an ice I shell with a stress-, temperature-, and grain-size-dependent rheology measured in laboratory experiments by Goldsby and Kohlstedt [2001] (henceforth GK2001). Triggering convection from an initially conductive ice shell with a non-Newtonian rheology for ice I requires that a finite-amplitude temperature perturbation be issued to the ice shell [2]. Here, we characterize the amplitude and wavelength of temperature perturbation required to initiate convection in the outer ice I shells of Europa, Ganymede, and Callisto using the GK2001 rheology for a range of ice grain sizes.

Barr, Amy C.↗

NASA NTRS2004

Onset of Convection in Ice I with Composite Newtonian and Non-Newtonian Rheology: Application to the Icy Galilean Satellites

Ice I exhibits a complex rheology at temperature and pressure conditions appropriate for the interiors of the ice I shells of Europa, Ganymede, and Callisto. We use numerical methods and existing parameterizations of the critical Rayleigh number to determine the conditions required to trigger convection in an ice I shell with the stress-, temperature- and grain size- dependent rheology measured in laboratory experiments by Goldsby and Kohlstedt [2001]. The critical Rayleigh number depends on the ice grain size and the amplitude and wavelength of temperature perturbation issued to an initially conductive ice I shell. If the shells have an assumed uniform grain size less than 0.4 mm, deformation during initial plume growth is accommodated by Newtonian volume diffusion. If the ice grain size is between 0.4 mm and 3 cm, deformation during plume growth is accommodated by weakly non-Newtonian grain boundary sliding, where the critical ice shell thickness for convection depends on the amplitude of temperature perturbation to the _0.5 power. If the ice grain size exceeds 2 cm, convection can not occur in the ice I shells of the Galilean satellites regardless of the amplitude or wavelength of temperature perturbation. If the grain size in a convecting ice I shell evolves to effective values greater than 2 cm, convection will cease. If the ice shell has a grain size large enough to permit flow by dislocation creep, the ice is too stiff to permit convection, even in the thickest possible ice I shell. Consideration of the composite rheology implies that estimates of the grain size in the satellites and knowledge of their initial thermal states are required when judging the convective instability of their ice I shells.

Barr, A. C.↗

NASA NTRS2004

Numerical Implementation of Ice Rheology for Europa's Shell

We present a discussion of approximations to the temperature dependent part of the rheology of ice. We have constructed deformation maps using the superplastic rheology of Goldsby & Kohlstedt and find that the rheologies that control convective flow in the Europa's are likely grain boundary sliding and basal slip for a range of grain sizes 0.1 mm < d < 1 cm. We compare the relative merits of two different approximations to the temperature dependence of viscosity and argue that for temperature ranges appropriate to Europa, implementing the non-Newtonian, lab-derived flow law directly is required to accurately judge the onset of convection in the ice shell and temperature gradient in the near-surface ice.

Barr, A. C.↗

NASA NTRS2004

Convective Instability in Ice I: Application to Callisto and Ganymede

Laboratory experiments measuring ice rheology suggest that it deforms under the influence of several nonNewtonian creep mechanisms, where the viscosity depends on both strain rate and temperature [Goldsby & Kohlstedt, 2001]. Whether or not a fluid with a purely temperature-dependent viscosity convects can be determined by comparing the Rayleigh number of the system to the critical Rayleigh number (Racr), which depends on rheological, thermal, and physical parameters of the fluid layer. However, in a nonNewtonian fluid where viscosity depends on the strain rate (i.e. velocity), convection can only occur if a temperature or velocity perturbation is issued to the system to lower the viscosity and permit fluid motions. Therefore, whether convection occurs in an ice I layer depends on initial conditions, in addition to rheological, thermal, and physical properties of the layer. We show new results for a scaling between the critical Rayleigh number and perturbation amplitude for grain boundary sliding rheology. This scaling can be used to determine the conditions required to initiate convection in the ice I shell of a generic icy satellite. We use this scaling to judge the convective instability of Ganymede and Callisto's ice shells in the absence of tidal dissipation.

Barr, A. C.↗

NASA NTRS1985

Three-layer Generic Ganymedes: Structure and Evolution

Accurate, three layered structural models were generated for Ganymede and Callisto. Three layered satellites consist of a rock core, a region of mixed rock and ice, and an outer shell of pure ice. This structure would result from either accretional melting of the outermost region of the satellite or differentiation subsequent to localized melting. A completely undifferentiated ice rock satellite possesses only the single, top boundary layer and, in this case, melting is also relatively difficult to initiate; this suggests that, if accretional melting results in a small degree of differentiation initially, subsequent melting may readily occur as the satellite warms due to radiogenic heating. Alternatively, if accretion is not accompanied by a small degree of differentiation, it may prove too difficult to initiate nonaccretional melting and the satellite might remain undifferentiated indefinitely.

Mueller, S. W.↗

NASA NTRS1998

Europa's differentiated internal structure: inferences from four Galileo encounters

Radio Doppler data from four encounters of the Galileo spacecraft with the jovian moon Europa have been used to refine models of Europa's interior. Europa is most likely differentiated into a metallic core surrounded by a rock mantle and a water ice-liquid outer shell, but the data cannot eliminate the possibility of a uniform mixture of dense silicate and metal beneath the water ice-liquid shell. The size of a metallic core is uncertain because of its unknown composition, but it could be as large as about 50 percent of Europa's radius. The thickness of Europa's outer shell of water ice-liquid must lie in the range of about 80 to 170 kilometers.

unmanned↗

NASA NTRS2009

Liquid Water on Enceladus from Observations of Ammonia and Ar-40 in the Plume

Jets of water ice from surface fractures near the south pole of Saturn's icy moon Enceladus produce a plume of gas and particles. The source of the jets may be a liquid water region under the ice shell-as suggested most recently by the discovery of salts in E-ring particles derived from the plume-or warm ice that is heated, causing dissociation of clathrate hydrates. Here we report that ammonia is present in the plume, along with various organic compounds, deuterium and, very probably, Ar-40. The presence of ammonia provides strong evidence for the existence of at least some liquid water, given that temperatures in excess of 180 K have been measured near the fractures from which the jets emanate. We conclude, from the overall composition of the material, that the plume derives from both a liquid reservoir (or from ice that in recent geological time has been in contact with such a reservoir) as well as from degassing, volatile-charged ice. As part of a general comprehensive review of the midsize saturnian satellites at the conclusion of the prime Cassini mission, PI McKinnon and co-I Barr contributed to three review chapters.

Waite, J. H., Jr.↗

NASA NTRS2004

Europa's Icy Shell: A Bridge Between Its Surface and Ocean

Europa, a Moon-sized, ice-covered satellite of Jupiter, is second only to Mars in its astrobiological potential. Beneath the icy surface, an ocean up to 150 km deep is thought to exist, providing a potential habitat for life,and a tempting target for future space missions. The Galileo mission to the Jovian system recently ended, but there are already long-range plans to send much more capable spacecraft,such as the proposed Jupiter Icy Moons Orbiter (JIMO), to take a closer look at Europa and her siblings, Ganymede and Callisto, some time in the next two decades. Europak outer icy shell is the only interface between this putative ocean and the surface, but many aspects of this shell are presently poorly understood; in particular, its composition, thickness, deformational history, and mechanical properties. To discuss the ice shell and our current understanding of it, 78 scientists from the terrestrial and planetary science communities in the United States and Europe gathered for a 3-day workshop hosted by the Lunar and Planetary Institute in Houston in February. A key goal was to bring researchers from disparate disciplines together to discuss the importance and limitations of available data on Europa with a post-Galileo perspective. The workshop featured 2 days of reviews and contributed talks on the composition, physical properties, stratigraphy, tectonics, and future exploration of the ice shell and underlying ocean. The final morning included an extended discussion period, moderated by a panel of noted experts, highlighting outstanding questions and areas requiring future research.

Schenk, Paul↗

NASA NTRS2003

An Examination of Issues Related to a Europa Subsurface Component for the JIMO Mission

The Galileo Europa data set served to revolutionize our view of Europa. In particular the strong evidence of a large, cold, salty Ocean beneath 5-30 km of ice has profoundly altered the significance of Europa in our thinking, especially of context of habitability in the solar system. While much remains to be learned from spacecraft observations of several sorts, there are significant questions answerable only by in-situ techniques; these relate to the formation of Europa, the nature of its ocean, and the prospects for life in its ocean, sediments, and ice. We feel that wide-ranging discussion of an in-situ subsurface mission to Europa, as part of JIMO, should proceed. The science objective of the mission is to characterize the icy shell of Europa to resolve its provenance, estimate the composition of brine of the Europa ocean, and search for evidence of Earth-like life. Probably anyone would agree that an in-situ mission to Europa would be of great value, but he or she would also immediately take the position that such a mission is utterly impractical. We take the position here of defining the least complex mission that can nonetheless justify its cost and to argue that such a mission is realistic enough that it should be seriously considered. Our mission thinking has been: 1) Soft landing. A soft lander is required on a site sufficiently flat to offer a stable platform; no further site selectivity is required. 2) Subsurface exploration. The Europa subsurface must be examined. Surficial processes on Europa arguably have exposed the upper 200 m of shell to chemical effects from the Jovian radiation belts as well as cometary infall, etc; to examine native ice we must descend below that point to, for discussion, 300 m. At that depth we argue that the ice is characteristic of ice at depth and possibly is effectively sea ice. 3) Science data. A few simple measurements at various depths and at 300 m constitute a scientifically successful mission. Measurements would include analysis of meltwater for a few inorganic ions and amino acids and an optical examination of the borehole wall. 4) Communication. Transmission of data to an orbiter is essential, but we will constrain the landed mission to a daily communication over a few days. 5) Subsurface access. Drilling to 300 m is a significant challenge; it can be addressed by several means: Thermal Probe (Cryobot) which permits water to refreeze above the vehicle. This is our tentative choice with plutonium as the fuel to generate thermal energy for drilling and electrical power for operations. Open Hole Drill, a thermal system in which the meltwater is removed for greater thermal efficiency. Meltwater removal on Europa is both a complexity and a risk, but analysis is improved. Mechanical Drilling in which cutting or grinding generates ice chips which are removed. This is too complex at Europa temperatures. The measurement objectives for the mission will be: Obj. 1: Determine the concentration of simple inorganic salts in the Europa Ice Shell and, by extrapolation, of the ocean. These data will also validate spaceborne sensors. Obj. 2: Determine the nature and abundance of amino acids in the ice such that cometary infall material in the upper ice can be compared to material at depth. Obj. 3: Optically examine the ice to resolve inclusion structure, particulate content, and stratification. Access to 300 m depth is a significant if not audacious plan; we are aware that this has not been done on any planetary body. Our approach is the use of a plutonium heat source; to overcome Europa's surface temperature and to melt ice a significant amount of plutonium is needed, and significant shielding and other protective steps will be required. The quantity of plutonium is a key concern. The mission will require subsurface collection and processing of samples for in situ analysis, calling for a miniature, high pressure micro-sampling system designed to meet needs of instruments that require low presses for operation. The inlet system itself collects a micro-sample in the external high pressure environment, then transfers it into a protected low pressure environment for analysis.

Carsey, F. D.↗

NASA NTRS2018

Using the Hottest Particles in the Universe to Probe Icy Solar System Worlds

We present results of our Phase 1 NIAC Study to determine the feasibility of developing a competitive, low cost, low power, low mass passive instrument to measure ice depth on outer planet ice moons, such as Europa, Ganymede, Callisto, and Enceladus. Indirect measurements indicate that liquid water oceans are likely present beneath the icy shells of such moons (see e.g.,the JPL press release "The Solar System and Beyond is Awash in Water"), which has important astrobiological implications. Determining the thickness of these ice shells is challenging given spacecraft SWaP (Size, Weight and Power) resources. The current approach uses a suite of instruments, including a high power, massive ice penetrating radar. The instrument under study, called PRIDE (Passive Radio Ice Depth Experiment) exploits a remarkable confluence between methods from the high energy particle physics and the search for extraterrestrial life within the solar system. PRIDE is a passive receiver of a naturally occurring radio frequency (RF) signal generated by interactions of deep penetrating Extremely High Energy (> 10^18 eV) cosmic ray neutrinos. It could measure ice thickness directly, and at a significant savings to spacecraft resources. At RF frequencies the transparency of modeled Europan ice is up to many km, so an RF sensor in orbit can observe neutrino interactions to great depths, and thereby probe the thickness of the ice layer.

Exploration↗

NASA NTRS1997

Europa's differentiated internal structure: inferences from two Galileo encounters

Doppler data generated with the Galileo spacecraft's radio carrier wave during two Europa encounters on 19 December 1996 (E4) and 20 February 1997 (E6) were used to measure Europa's external gravitational field. The measurements indicate that Europa has a predominantly water ice-liquid outer shell about 100 to 200 kilometers thick and a deep interior with a density in excess of about 4000 kilograms per cubic meter. The deep interior could be a mixture of metal and rock or it could consist of a metal core with a radius about 40 percent of Europa's radius surrounded by a rock mantle with a density of 3000 to 3500 kilograms per cubic meter. The metallic core is favored if Europa has a magnetic field.

Flight Experiment↗

NASA NTRS2002

Continued evolution of Europa subsurface exploration technologies

The Galileo results convincingly indicate that Europa has a deep salty ocean covered by a shell of water ice a few tens of kilometers thick; this physical description gives rise to a host of thoughtful speculation as to the nature of the ocean, its seafloor, and the likelihood of microbial life within it. We argue that this situation points to the high desirability of a series of in-situ missions to examine the ice and, ultimately, the ocean.

subglacial Europa↗