NRC: Vision and Voyages for Planetary Science in the Decade 2013-2022 – Executive Summary
Executive Summary
In recent years, planetary science has seen a tremendous growth in new knowledge. Deposits of water ice exist at the Moon’s poles. Discoveries on the surface of Mars point to an early warm wet climate, and perhaps conditions under which life could have emerged. Liquid methane rain falls on Saturn’s moon Titan, creating rivers, lakes, and geologic landscapes with uncanny resemblances to Earth’s. Comets impact Jupiter, producing Earth-sized scars in the planet’s atmosphere. Saturn’s poles exhibit bizarre geometric cloud patterns and changes; its rings show processes that may help us understand the nature of planetary accretion. Venus may be volcanically active. Jupiter’s icy moons harbor oceans below their ice shells: conceivably Europa’s ocean could support life. Saturn’s tiny moon Enceladus has enough geothermal energy to drive plumes of ice and vapor from its south pole. Dust from comets shows the nature of the primitive materials from which the planets and life arose. And hundreds of new planets discovered around nearby stars have begun to reveal how our solar system fits into a vast collection of others.
This report was requested by NASA and the National Science Foundation (NSF) to review the status of planetary science in the United States and to develop a comprehensive strategy that will continue these advances in the coming decade. Drawing on extensive interactions with the broad planetary science community, the report presents a decadal program of science and exploration with the potential to yield revolutionary new discoveries. The program will achieve long-standing scientific goals with a suite of new missions across the solar system. It will provide fundamental new scientific knowledge, engage a broad segment of the planetary science community, and have wide appeal for the general public whose support enables the program.
A major accomplishment of the committee’s recommended program will be taking the first critical steps toward returning carefully selected samples from the surface of Mars. Mars is unique among the planets in having experienced processes comparable to those on Earth during its formation and evolution. Crucially, the martian surface preserves a record of earliest solar system history, on a planet with conditions that may have been similar to those on Earth when life emerged. It is now possible to select a site on Mars from which to collect samples that will address the question of whether the planet was ever an abode of life. The rocks from Mars that we have on Earth in the form of meteorites cannot provide an answer to this question. They are igneous rocks, whereas recent spacecraft observations have shown the occurrence on Mars of chemical sedimentary rocks of aqueous origin, and rocks that have been aqueously altered. It is these materials, none of which are found in meteorites, that provide the opportunity to study aqueous environments, potential prebiotic chemistry, and perhaps, the remains of early martian life.
If NASA’s planetary budget is augmented, then the program will also carry out the first in-depth exploration of Jupiter’s icy moon Europa. This moon, with its probable vast subsurface ocean sandwiched between a potentially active silicate interior and a highly dynamic surface ice shell, offers one of the most promising extraterrestrial habitable environments in our solar system and a plausible model for habitable environments outside it. The Jupiter system in which Europa resides hosts an astonishing diversity of phenomena, illuminating fundamental planetary processes. While Voyager and Galileo taught us much about Europa and the Jupiter system, the relatively primitive instrumentation of those missions, and the low data volumes returned, left many questions unanswered. Major discoveries surely remain to be made. The first step in understanding the potential of the outer solar system as an abode for life is a Europa mission with the goal of confirming the presence of an interior ocean, characterizing the satellite’s ice shell, and understanding its geological history.
The program will also break new ground deep in the outer solar system. The gas giants Jupiter and Saturn have been extensively studied by the Galileo and Cassini missions, respectively. But Uranus and Neptune represent a wholly distinct class of planet. While Jupiter and Saturn are made mostly of hydrogen, Uranus and Neptune have much smaller hydrogen envelopes. The bulk composition of these planets is dominated instead by heavier elements; oxygen, carbon, nitrogen, and sulfur are the likely candidates. What little we know about the internal structure and composition of these “ice giant” planets comes from the brief flybys of Voyager 2. So the ice giants are one of the great remaining unknowns in the solar system: the only class of planet that has never been explored in detail. The proposed program will fill this gap in our knowledge by initiating a mission to orbit Uranus and put a probe into the planet’s atmosphere. It is exploration in the truest sense, with the same potential for new discoveries as Galileo at Jupiter and Cassini at Saturn.
The program described in this report also vigorously continues NASA’s two programs of competed planetary missions: New Frontiers and Discovery. It includes seven candidate New Frontiers missions from which NASA will select two for flight in the coming decade. These New Frontiers candidates cover a vast sweep of exciting planetary science questions: The surface composition of Venus, the internal structure of the Moon, the composition of the lunar mantle, the nature of Trojan asteroids, the composition of comet nuclei, the geophysics of Jupiter’s volcanic moon Io, and the structure and detailed composition of Saturn’s atmosphere. And continuation of the highly successful Discovery program, which involves regular competitive selections, will provide a steady stream of scientific discoveries from small missions that draw on the full creativity of the science community.
Space exploration has become a worldwide venture, and international collaboration has the potential to enrich the program in ways that benefit all participants. The program therefore relies more strongly than ever before on international participation, presenting many opportunities for collaboration with other nations. Most notably, the ambitious and complex Mars Sample Return campaign is critically dependent on a long-term and enabling collaboration with the European Space Agency (ESA).
In order to assemble this program, four criteria were used to select and prioritize missions. The first and most important was science return per dollar. Science return was judged with respect to the key scientific questions identified by the planetary science community; costs were estimated via a careful and conservative procedure that is described in detail in the body of this report. The second was programmatic balance–striving to achieve an appropriate balance among mission targets across the solar system and an appropriate mix of small, medium, and large missions. The other two were technological readiness and availability of trajectory opportunities within the 2013-2022 time period.
In order to help develop its recommendations, the committee commissioned technical studies of many candidate missions that were selected for study on the basis of white papers submitted by the scientific community. A subset of these was chosen on the basis of the four prioritization criteria listed above for independent assessments of technical feasibility, as well as conservative estimates of costs. From these, the committee finalized a set of recommended missions intended to achieve the highest priority science identified by the community within the budget resources projected to be available. It consists of a balanced mix of small Discovery missions, medium-sized New Frontiers missions, and large “Flagship” missions, enabling both a steady stream of new discoveries and the capability to address major challenges. The mission recommendations assume full funding of all missions that are currently in development, and continuation of missions that are currently in flight, subject to approval via the appropriate review process.
