Joint Europa Mission (JEM): a multi-scale study of Europa to characterize its habitability and search for extant life

Europa is the closest and probably the most promising target to search for extant life in the Solar System, based on complementary evidence that it may fulfil the key criteria for habitability: the Galileo discovery of a sub-surface ocean; the many indications that the ice shell is active and may be partly permeable to transfer of chemical species, biomolecules and elementary forms of life; the identification of candidate thermal and chemical energy sources necessary to drive a metabolic activity near the ocean floor. In this article we are proposing that ESA collaborates with NASA to design and fly jointly an ambitious and exciting planetary mission, which we call the Joint Europa Mission (JEM), to reach two objectives: perform a full characterization of Europa's habitability with the capabilities of a Europa orbiter, and search for bio-signatures in the environment of Europa (surface, subsurface and exosphere) by the combination of an orbiter and a lander. JEM can build on the advanced understanding of this system which the missions preceding JEM will provide: Juno, JUICE and Europa Clipper, and on the Europa lander concept currently designed by NASA (Maize, report to OPAG, 2019). We propose the following overarching goals for our Joint Europa Mission (JEM): Understand Europa as a complex system responding to Jupiter system forcing, characterize the habitability of its potential biosphere, and search for life at its surface and in its sub-surface and exosphere. We address these goals by a combination of five Priority Scientific Objectives, each with focused measurement objectives providing detailed constraints on the science payloads and on the platforms used by the mission. The JEM observation strategy will combine three types of scientific measurement sequences: measurements on a high-latitude, low-altitude Europan orbit; in-situ measurements to be performed at the surface, using a soft lander; and measurements during the final descent to Europa's surface. The implementation of these three observation sequences will rest on the combination of two science platforms: a soft lander to perform all scientific measurements at the surface and sub-surface at a selected landing site, and an orbiter to perform the orbital survey and descent sequences. We describe a science payload for the lander and orbiter that will meet our science objectives. We propose an innovative distribution of roles for NASA and ESA; while NASA would provide an SLS launcher, the lander stack and most of the mission operations, ESA would provide the carrier-orbiter-relay platform and a stand-alone astrobiology module for the characterization of life at Europa's surface: the Astrobiology Wet Laboratory (AWL). Following this approach, JEM will be a major exciting joint venture to the outer Solar System of NASA and ESA, working together toward one of the most exciting scientific endeavours of the 21st century: to search for life beyond our own planet

Evolution of the Southwest Indian Ridge from 55°45′E to 62°E : changes in plate-boundary geometry since 26 Ma

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 8 (2007): Q06022, doi:10.1029/2006GC001559.From 55°45′E to 58°45′E and from 60°30′E to 62°00′E, the ultraslow-spreading Southwest Indian Ridge (SWIR) consists of magmatic spreading segments separated by oblique amagmatic spreading segments, transform faults, and nontransform discontinuities. Off-axis magnetic and multibeam bathymetric data permit investigation of the evolution of this part of the SWIR. Individual magmatic segments show varying magnitudes and directions of asymmetric spreading, which requires that the shape of the plate boundary has changed significantly over time. In particular, since 26 Ma the Atlantis II transform fault grew by 90 km to reach 199 km, while a 45-km-long transform fault at 56°30′E shrank to become an 11 km offset nontransform discontinuity. Conversely, an oblique amagmatic segment at the center of a first-order spreading segment shows little change in orientation with time. These changes are consistent with the clockwise rotation of two ~450-km-wide first-order spreading segments between the Gallieni and Melville transform faults (52–60°E) to become more orthogonal to spreading. We suggest that suborthogonal first-order spreading segments reflect a stable configuration for mid-ocean ridges that maximizes upwelling rates in the asthenospheric mantle and results in a hotter and weaker ridge-axis that can more easily accommodate seafloor spreading.Funding for this work came from a JOI-Schlanger Fellowship to Baines and NSF grant 0352054 to Cheadle and John

Tidally Heated Convection and the Occurrence of Melting in Icy Satellites: Application to Europa

International audienceObservations of icy satellites have revealed widespread marks of cryovolcanism. Because aqueous cryomagmas are negatively buoyant, two processes are required to explain these observations: one mechanism to generate melt close enough to the surface and another one to transport this melt to the surface. Here, we investigate the generation of melting in a systematic way, using a set of 85 numerical simulations where we vary the viscosity contrast, Rayleigh number, and tidal heating rate. Applied to Europa, considering a hydrosphere composed of pure water, our simulations suggest that isolated melt pockets can be generated close to the surface (∼5 km) as long as the ice layer thickness (d *) remains modest (15 ≤ d * ≤ 35 km). However, the generation of melting becomes increasingly difficult as the amount of antifreeze compounds in the subsurface ocean increases. Furthermore, the proportion of melting increases very sharply with increasing tidal heating rate. In particular, when the tidal heating rate exceeds a threshold, an asymptotic regime is reached where the surface heat flux remains constant indicating that the tidal heat generated above this threshold is only used for melting the ice shell. In that regime, we found a direct relationship between the surface heat flux and d *. Finally, we provide a new assessment of Europa's thermal state. Based on available constraints, we propose that the ice shell thickness should exceed 15 km. However, d * ∼15-35 km implies a tidal power (>2 TW) much larger than expected. An extrapolation of the trends suggested by our results indicates that a more reasonable tidal power (∼1 TW) would involve d * ∼50-90 km. Plain Language Summary Eruptions of cryomagmas have been inferred in many icy satellites. In many cases, the mechanisms producing these eruptions are not yet unambiguously identified. Here, we focus on one part of the problem, which is how to generate melting within the ice shell of icy satellites. To do so, we conducte a large number of numerical simulations in 3-D. When applied to Europa, our results indicate that melt reservoirs may only be generated for a moderately thick ice shell. We also find that, if a large amount of melting is generated, the surface heat flux is a good proxy for the thickness of the ice shell. Finally, we provide a new assessment of Europa's thermal state. An extrapolation of our results, combined to properties generally assumed for Europa, suggests a thick ice shell (∼50-90 km)