Ocean Worlds Exploration
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Author’s Accepted Manuscript Ocean Worlds Exploration Jonathan I. Lunine www.elsevier.com/locate/actaastro PII: S0094-5765(16)31151-1 DOI: http://dx.doi.org/10.1016/j.actaastro.2016.11.017 Reference: AA6081 To appear in: Acta Astronautica Received date: 4 November 2016 Accepted date: 7 November 2016 Cite this article as: Jonathan I. Lunine, Ocean Worlds Exploration, Acta Astronautica, http://dx.doi.org/10.1016/j.actaastro.2016.11.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. 67th International Astronautical Congress (IAC), Guadalajara, Mexico, 26-30 September 2016. Copyright ©2016 by the International Astronautical Federation (IAF). All rights reserved. Ocean Worlds Exploration Jonathan I. Luninea* aDepartment of Astronomy and Carl Sagan Institute, Cornell University, 122 Sciences Drive, Ithaca NY 14853, USA. *Corresponding Author. Abstract Ocean worlds is the label given to objects in the solar system that host stable, globe-girdling bodies of liquid water—“oceans”. Of these, the Earth is the only one to support its oceans on the surface, making it a model for habitable planets around other stars but not for habitable worlds elsewhere in the solar system. Elsewhere in the solar system, three objects—Jupiter’s moon Europa, and Saturn’s moons Enceladus and Titan—have subsurface oceans whose existence has been detected or inferred by two independent spacecraft techniques. A host of other bodies in the outer solar system are inferred by a single type of observation or by theoretical modeling to have subsurface oceans. This paper focusses on the three best-documented water oceans beyond Earth: those within Europa, Titan and Enceladus. Of these, Europa’s is closest to the surface (less than 10 km and possibly less than 1 km in places), and hence potentially best suited for eventual direct exploration. Enceladus’ ocean is deeper—5-40 km below its surface—but fractures beneath the south pole of this moon allow ice and gas from the ocean to escape to space where it has been sampled by mass spectrometers aboard the Cassini Saturn Orbiter. Titan’s ocean is the deepest—perhaps 50-100 km—and no evidence for plumes or ice volcanism exist on the surface. In terms of the search for evidence of life within these oceans, the plume of ice and gas emanating from Enceladus makes this the moon of choice for a fast-track program to search for life. If plumes exist on Europa—yet to be confirmed—or places can be located where ocean water is extruded onto the surface, then the search for life on this lunar-sized body can also be accomplished quickly by the standards of outer solar system exploration. Keywords: planetary exploration, moons, Saturn, Jupiter, exobiology 1. Introduction: The meaning of “Ocean Worlds” The post-Renaissance perception of the solar system’s planets (and our Moon) as abodes for life was driven first in the West by the Copernican concept that Earth is not unique in any astronomical sense and then by 20th century science fiction aided by marginal and sometimes misinterpreted telescopic observations [1], [2]. This perception has largely been dashed by the past half-century of increasingly detailed astronomical observations and growing sophistication of planetary exploration. In particular, Mars has been largely relegated to a long-past abode of life, with speculations of remnant microbial organisms eking out an existence today deep within its rocky crust [3]. Exploration of the rocky and icy moons of the giant planets beginning in 1979 with Voyagers 1 and 2 at Jupiter changed this perception of a geologically and biologically inactive solar system by showing that such bodies— previously imagined to be cold, dead and geologically uninteresting—possess diverse levels of geological activity up to and including Io, the most volcanically active world in the solar system [4]. What was not understood until 1979 was the importance of tidal heating—the transformation of potential energy of an eccentric orbit into frictional heating within the body—in the multi-moon systems of the giant planets [5]. Subsequent discoveries by the Voyagers 1 and 2 in the 1980’s[6], the Galileo Jupiter Orbiter in the 1990’s [7], the Cassini Saturn Orbiter over the past 12 years and its Huygens Probe which landed on Titan in 2005 [8], the New Horizons Pluto flyby in 2015 [9], and the Dawn rendezvous with the asteroid Ceres [10] have provided evidence for liquid water oceans beneath the surfaces of multiple bodies in the solar system—from Mars, to the asteroid belt and the vast realm beyond. In a few cases the evidence comes from multiple observations of diverse types, in others a single measurement, and in many cases only theoretical assertions can be made. But in at least one instance (Saturn’s moon Enceladus), there is strong evidence that ocean material has been directly sampled by mass spectrometry in a plume of gas and ice emanating from fractures in the south polar region of that moon. The planetary science community and NASA itself have therefore begun to recognize the existence of a class of objects—“Ocean Worlds”—which are solar system bodies that definitely, provisionally, or potentially host globe- girdling layers of liquid water within their interiors. Since liquid water is essential for life as we know it—life composed of various organic acids, sugars, and similar molecules—the presence of liquid water within a body makes it a candidate in the search for life. The present paper will summarize the evidence for oceans within each of the bodies currently included in the ocean worlds list as used by many planetary scientists, then focus on three objects--Jupiter’s Europa, Saturn’s Enceladus and Titan—where the evidence is strongest. It will describe what we know about these oceans and how we know it, and then move on to the prospects for determining more about their suitability for life (“habitability”) IAC-16-A.7.1.5.x35837 Page 1 of 13 67th International Astronautical Congress (IAC), Guadalajara, Mexico, 26-30 September 2016. Copyright ©2016 by the International Astronautical Federation (IAF). All rights reserved. and testing whether life is actually present. It will be argued that a robust search for life in the solar system beyond Earth requires a program that targets multiple bodies—but not too many—and a sustained determination to do so given the long (5-10 year) flight times. Before proceeding, the reader is cautioned about a potential confusion in terminology associated with the near- correspondence of the term “ocean worlds” to “ocean planet” and “water world”, both of which refer to exoplanets (planets orbiting other stars) with substantial mass fractions of water in their bulk compositions. While most of the solar system objects considered in the present paper are made up of comparable amounts of rock and water ice, and are therefore “like” extrasolar water worlds in this respect, one should be careful to refer to them as ocean worlds and reserve the other two terms for objects outside our solar system. 2. Ocean worlds: which bodies, and why There is as yet no official set of solar system objects considered to be “ocean worlds”. Table 1 lists the objects currently included by many planetary scientists; figure 1 in the paper by Sherwood et al. at this conference [11] provides a pictorial representation of these objects at their approximate relative sizes, and puts them in the following categories: “Ocean relics” (Mars, Ceres), “Jovian Moons” (Europa, Ganymede, Callisto), “Saturnian Moons” (Mimas, Enceladus, Dione, Titan), and “Kuiper Belt Objects” (Triton, Pluto, Charon). Earth is implictly on this list and we will discuss aspects of its ocean in the context of describing that of Enceladus. This section examines the evidence for each ocean in turn. 2.1 Ocean relics These are bodies for which evidence exists of oceans in the past that have now disappeared. On Mars, multiple orbiters and rovers have provided remote sensing and direct chemical evidence for standing bodies of liquid water on the surface in the ancient past [12]. That water is now gone, and occasional small amounts of meltwater notwithstanding [13], the surface of Mars is largely dry. Subsurface layers of ice or permafrost have been mapped from orbit [14], but any claim of liquid water beneath the Martian surface is based on theoretical calculations only [15]. The detection of methane in the Martian atmosphere from Earth [16] has recently been supported by Curiosity rover measurements [17], but its significance for biological activity beneath the surface is uncertain. Ceres is the largest asteroid and the first to be discovered; spectral observations from the Dawn mission indicate the presence of ammonia-bearing minerals and water of hydration [18], which suggest the presence of liquid water today or at one time in the interior. That water in the interior is not present today or is deep is only weakly suggested by a lack of jets or plumes, but overall the state of water in the interior of Ceres remains poorly constrained. Figure 1: Europa’s surface with crisscrossing fractures. Galileo image about 200 x 200 km. (NASA/JPL/University of Arizona). 2.2 Jovian moons Of the four large Galilean moons that orbit Jupiter, three show evidence for internal oceans of liquid water.