Report Series: Committee on Astrobiology and Planetary Science Options for the Fifth New Frontiers Announcement of Opportunity (2020) / Chapter Skim
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2 Advances Since the Publication of Vision and Voyages
2 Advances Since the Publication of Vision and Voyages
Pages 6-14
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From page 6... ...
Even asteroid Ceres shows evidence for briny eruptions -- a tantalizing continuum of ocean worlds that may be or may once have been habitable.3 In the coming decade, NASA is investing in the further exploration of ocean worlds through missions -- Europa Clipper and its contribution to the European Space Agency's JUICE,4 to Europa and Ganymede, respectively -- and major technology investments and studies of cryogenic sample return and curation -- through, for example, the Scientific Exploration Subsurface Access Mechanism for Europa (SESAME) , Instrument Concepts for Europa Exploration (ICEE)
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From page 7... ...
The recently initiated planetary science and astrobiology decadal survey will comprehensively evaluate the merits of ocean worlds exploration endeavors at different bodies and for a variety of mission classes. Nevertheless, the Ocean Worlds mission theme presents a certain level of ambiguity in regard to programmatic decisions within the New Frontiers program.
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From page 8... ...
Enceladus Enceladus is the only world with confirmed cryovolcanic plumes.15 Relative to other ocean worlds of the solar system, Enceladus is special -- not only do data clearly show the existence of a present-day subsurface ocean, but that ocean and its chemistry are presently accessible to exploration without having to penetrate the ice shell.16 Consequently, Enceladus is uniquely compelling, or nearly so,17 for understanding the processes that generate ocean worlds, as well as for assessing habitability and searching for extant life. At the time of V&V, it was known that water vapor and ice particles emanated from the "tiger stripe" fracture systems at Enceladus's southern pole, although it was unknown whether the plume material came from a global subsurface ocean of liquid water.
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From page 9... ...
Recent studies of the rings of Saturn have led to the proposal that the inner midsize moons may be geologically young, and tidal flexure of a porous core has been proposed as a means to sustain long-term heating that supports an ocean.27 Modern processes are also of interest, as solar occultation studies and maps of plume fallout on the Enceladus surface elucidate the means of their emplacement.28,29 Ewald and Ingersoll have found a longterm periodicity to plume water-ice flux so far unexplained by models of tidal flexure.30 Key outstanding questions include the deep structure of Enceladus and the interaction with tidal processes to enable this unique exposure of the waters from an interior ocean. The potential exploration of Enceladus has also benefited from substantial technology investment, both from the Enceladus-focused missions proposed (albeit unsuccessfully)
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From page 10... ...
Future Enceladus exploration can also leverage technologies under development for a possible Europa lander.31 TROJAN TOUR AND RENDEZVOUS Knowledge of the Trojan asteroids has increased considerably since the completion of V&V, in terms of the quality and completeness of their astronomical characterization; new kinds of telescopic observations, including in the thermal-infrared and mid-ultraviolet; and the discoveries of additional binary systems, for which we can determine masses and bulk densities, and dynamical families indicative of disruptions. We have a more complete view of their great diversity in size, shape, and composition.32 Additionally, significant advancements in understanding solar system dynamics have provided powerful constraints on the migration of Jupiter and other giant planets after their formation, and the possible origin of the Trojan swarm.
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From page 11... ...
As described below, a key scientific difference between Lucy and the Trojan Tour and Rendezvous New Frontiers mission envisioned in V&V is the lack of a final rendezvous encounter and the associated instrumentation that would measure elemental composition of near-surface materials. IO OBSERVER Driven by its orbital resonance with Europa and Ganymede, Io exhibits extraordinary characteristics that are unprecedented in the solar system -- inferred extreme flexure and tidal heating, high heat flow, extensive ongoing volcanism, striking tectonic mountains, an unusual and dynamic atmosphere dominated by sulfur dioxide gas, and an outsized influence on the jovian magnetosphere.36 Io's heat flux, as directly measured in the infrared from Earth, is at least 30 times that of Earth's average value today (per unit area)
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From page 12... ...
These include deep crustal porosity, fractures and other tectonic structures, mascons, lava tubes and other volcanic landforms, impact basin rings, and the shape and size of complex to peak-ring lunar craters.43 For example, GRAIL gravity gradient data over the Procellarum KREEP (potassium, rare earth elements, and phosphorus) Terrane revealed a surrounding dike system, suggesting that it may be a magmatictectonic feature overlying the nearside "magma plumbing system" that supplied the mare with their basaltic infills.44 LRO data revealed subtle tectonic features across the lunar surface, including thrust faults that have been proposed to be the cause of the high-magnitude shallow moonquakes, although a definitive linkage cannot be made without a seismic network.45 40 See, for example, K
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From page 13... ...
Laser ranging data suggest a liquid core, although combined gravity, topography, and laser ranging data predict a solid inner core, along with a total core size similar to the core modeled using Apollo seismic data.47 Paleomagnetic studies of Apollo samples have demonstrated that the Moon had surface magnetic fields of ~30-100 μT lasting from at least 4.2 Ga until 3.56 Ga, interpreted as evidence for the existence of an ancient core dynamo.48 When the dynamo initiated and ended is uncertain, however, as is the mechanism or mechanisms that allowed it to operate. Substantial efforts have been made to mature potential lunar geophysical instruments over the last decade, including investments by NASA, and to increase understanding of the scattering properties of the lunar regolith through the application of terrestrial seismic data-analysis techniques to Apollo data.
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From page 14... ...
-- which will focus on resource prospecting, particularly the search for water ice in the lunar regolith, and is unrelated to the objectives of the LGN. Within STMD, the Lunar Surface Innovation Initiative (LSII)
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