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Space Studies Board Search: Jump to Top NewsJump to Science in the Subscribe to our FREE e- Headlines newsletter! NATIONAL ACADEMY OF SCIENCES NATIONAL ACADEMY OF ENGINEERING INSTITUTE OF MEDICINE NATIONAL RESEARCH COUNCIL June 18, 2004 Current Operating Status 1990 Update to Strategy for Exploration of the Inner Planets 3 Mercury PROGRESS Since the writing of the 1978 report, new discoveries have been made concerning Mercury's composition, atmosphere, and geology, and new insights have been gained into the planet's origin and evolution. One significant change in our understanding has resulted from our discovery of a variable tenuous atmosphere of Na and K by Earth-based, high-resolution spectroscopy. A component of this atmosphere may be derived from the vaporization of infalling meteoritic material, as well as, from Mercury's regolith. It is potentially significant that Mercury's surface rocks may contain these relatively volatile elements. A reinterpretation of Mariner 10 ultraviolet (UV) spectrometer measurements indicates amounts of atmospheric oxygen comparable to that of Na. Photoionization and injection of Na ions into the magnetosphere may mean that Na is an important magnetospheric component. The Na atmosphere also has at significant electrical conductivity, and so magnetospheric currents can close in the Na atmosphere, allowing it to influence magnetospheric dynamics as does Earth's ionosphere. This coupling to the Na atmosphere has fundamental implications for magnetospheric processes, such as magnetic substorms and flux transfer events. Improved understanding of surface composition has come from thermal emission measurements in the 7.2- to 11.2- m region. The globally averaged data so far indicate an intermediate silicate composition. New reflectance spectra in the visible and near-infrared (IR) show no Fe2+ absorption band at 0.9 m. The implication is that whatever basalts may exist on Mercury are extremely iron-poor, or that shock processing by impact masks this and other spectral absorptions. Earth-based radar altimeter measurements show large scarps and regional elevation differences, confirming and quantifying the significant past tectonic activity revealed by Mariner 10. Radio maps of the planet's surface can now (1 of 4) [6/18/2004 10:06:36 AM]

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Space Studies Board determine regolith temperatures and, with sufficient analysis, regolith properties, such as dielectric constant and density averaged over broad areas. Progress toward resolution of the question of volcanism on Mercury has been made as well. The global extent, depositional settings, and age relations of Mercurian smooth plains collectively indicate they are predominantly of volcanic origin. The plains within and surrounding the Caloris basin are, in particular, younger than Caloris, consistent with a volcanic origin. The plains outside are depressed in elevation, consistent with down-warping and the hypothesis that these plains create a gravity anomaly similar to that associated with lunar mascons. Systematic differences in crater shape and morphology suggest that signatures of both impact velocity and gravity exist, thereby providing an important comparison with other planets. From a more theoretical standpoint, it is now recognized that tidal heating may play an important role for Mercury. Tidal heating could occur in a large solid inner core, supplying power to the planet's magnetic dynamo and prolonging the core's thermal evolution. Mercury's relatively large core may itself be due to a catastrophic mantle-stripping impact. SCIENTIFIC OBJECTIVES The primary objectives in the 1978 report were to determine the chemical composition of the planet's surface on both a global and regional scale, to determine, the structure and state of the interior, and to extend the coverage and improve the resolution of orbital imaging. These objectives should be retained, but in addition, determination of the structure and time variation of the magnetic field should be elevated from a secondary to a primary objective. Surface Chemistry Determination of the major chemical components of Mercury's surface and characterization of regional variation in chemistry remain primary objectives. The composition of various surface and exposed deeper units remains unknown. This knowledge will also allow surface composition to be related to atmospheric composition. The bulk chemical analysis of the surface will provide a critical test of new cosmochemical and accretion theories by assessing the range of solar nebula conditions sampled by the innermost planet. Such an analysis will also show if the planet was chemically fractionated in a giant impact. Internal Structure and State Determining the extent and state of a dense central core on Mercury remains a primary objective. .The extent of solid and/or liquid regions within the core is of particular interest to a more complete understanding of Mercury's interior chemistry, thermal evolution, spin-down history, and magnetic field generation. The composition of the core may vary from pure iron-nickel, and this too bears on (2 of 4) [6/18/2004 10:06:36 AM]

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Space Studies Board Mercury's accretional history. The 1978 report emphasized seismology. This remains a viable option, with penetrators providing a possible delivery system. The committee notes that the thermal environment on Mercury's surface is a severe challenge to the design of any instrumentation, and steps should be taken to address the technological issue. Deployment of seismometers would create an opportunity to also deploy other geophysical instruments, such as magnetometers and sensors for measuring the global heat flow, which is an important geophysical goal. Consideration should also be given to the landing and tracking of passive radio beacons or other. technologies that allow the obliquity and spin-axis libration to be determined, These values (along with knowledge of the second-degree gravity harmonics) could determine the size of a possible liquid core. This is somewhat analogous to the analysis of the Moon's librations using laser ranging data, which has yielded indirect evidence.for a lunar liquid core. History of the Surface As recommended in the 1978 report, imaging should be extended to include the 50 percent of the planet not observed by Mariner 10. Complete imaging, at a resolution of at least 0.5 km, is necessary to understand Mercury's tectonic and possible volcanic evolution, to further characterize the impact cratering record in the inner solar system, and to broaden our understanding of the planet's overall geologic history. It would also provide an essential geologic context for interpreting surface chemistry measurements. In the 1978 report, determinations of the global gravity and topography were stated as important, but secondary, scientific objectives. Such measurements would be of great use in understanding Mercury's geologic and geophysical history. Magnetic Field Characterization of the multipolar structure of the internal magnetic field has direct implications for the size and physical state of Mercury's core and for our understanding of the physical processes involved in the dynamo generation of the magnetic field. High-order multipole coefficients of the internal magnetic field and the temporal variability of these coefficients should be determined. The magnetospheric plasmas, energetic particles, and fields should be adequately characterized as well. This is important in its own right but also is necessary to provide the basis for separation of the internally generated magnetic field from the field generated by magnetopause and magnetospheric currents, or remanent surface fields. UPDATED RECOMMENDATIONS The updated strategy for exploration of Mercury differs from the strategy in the (3 of 4) [6/18/2004 10:06:36 AM]

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Space Studies Board 1978 report in a number of important aspects. The 1978 report concluded that insertion of an appropriately instrumented planetological payload into a relatively low-altitude, circular orbit around Mercury required development of a low-thrust propulsion system. It has recently been demonstrated, however, that such a mission is feasible with conventional rocket launches and gravity assists at Venus and Mercury, thereby removing the perceived impediments to near-term orbital exploration of Mercury. Therefore, the 1978 report's recommendation that investigation of Mercury "be included . . . later in the decade . . .with the proviso that initiating such a mission does not inhibit or detrimentally affect the primary emphasis on the triad Earth-Mars-Venus" needs revision in two respects. A Mercury mission is a possible near-term activity, and justification of such a mission should rest on the important role of Mercury in understanding the origin and evolution of all the terrestrial planets. The 1978 report relegated exploration of Mercury's magnetic field and magnetosphere to a secondary objective. Because of the direct connections between Mercury's magnetic field and the size and physical state of its core, determination of the multipole structure of the planet's magnetic field should be a primary scientific objective along with surface chemistry, internal structure, and imaging. Understanding how the Earth's geodynamo works is one of the major unsolved problems in geophysics; characterization of Mercury's magnetic field will provide crucial insights and constraints on dynamo theories. COMPLEX therefore recommends that characterization of Mercury's magnetic field be a primary objective for exploration of that planet. Last update 9/26/00 at 2:03 pm Site managed by Anne Simmons, Space Studies Board Site managed by the SSB Web Group. To comment on this Web page or report an error, please send feedback to the Space Studies Board. Subscribe to e-newsletters | Feedback | Back to Top Copyright © 2004. National Academy of Sciences. All rights reserved. 500 Fifth St. N.W., Washington, D.C. 20001. Terms of Use and Privacy Statement (4 of 4) [6/18/2004 10:06:36 AM]