«S. ALAN STERN University of Colorado and FAITH VILAS NASA Johnson Space Center The continued and expanded study of Mercury is important to several ...»
FUTURE OBSERVATIONS OF AND MISSIONS TO MERCURY
S. ALAN STERN
University of Colorado
NASA Johnson Space Center
The continued and expanded study of Mercury is important to several aspects of
planetary science. Wejrst review the broad scientijc objectives of such explora-
tion and describe the methods by which such scientijc objectives may be ad-
dressed. Groundbased optical, infrared and radar astronomy are discussed$rst, followed by Earth-orbital observations and in situ missions to Mercury. Several planned NASA missions, including the ASTRO Spacelab payload and the Hub- ble Space Telescope, have the potential for making important contributions to the study of Mercury. Sounding rockets can obtain ultraviolet spectroscopy when spacecraft lack such technical capabilities as extensive optical bafling. There are dzjicult performance requirements for getting spacecraft to Mercury, al- though technical solutions have been proposed to overcome these dtjiculties. A method that offers immediate potential to mount substantial Mercury missions with current launch vehicle inventory is the use of the multiple gravity-assist trajectories recently discovered by Yen. We discuss potential pay10,ads for Mer- cury orbiters and the importance of eventually landing on the surface.
I. INTRODUCTIONMotivations for continued observation and exploration of Mercury are made clear by various authors in this book: Mercury's atmosphere displays unique characteristics important to the understanding of satellite and cometary 25
FUTURE OBSERVATIONS AND MISSIONSatmospheres. Mercury's geology is relevant to the general processes which affect terrestrial and satellite bodies. Mercury's magnetosphere is complex and enigmatic: it represents the sole known example of a substantially magne- tized small body and of a magnetosphere existing in the absence of an ionosphere. Additionally, Mercury's evolution represents an end-member case because it formed-or at least is now located-at a temperature extreme in the solar system.
Although Mercury is an important object for study, its location near the Sun makes it a difficult object either to reach or observe. Fielding orbiter and lander missions is difficult because of the tremendous launch and orbit inser- tion energy requirements imposed by Mercury's orbit close to the Sun. Addi- tionally, the insolation environment imposes considerable constraints on spacecraft design, and hence mission cost. Mercury's physical proximity to the Sun also makes the planet difficult to observe from the Earth, since solar elongationangles in excess of 28 degrees are never obtained.
Due in part to the intrinsic difficulty of reaching Mercury, potential missions have received low priority in planetary exploration plans. Neither NASA's Solar System Exploration Committee nor the NRC's Space Science Board have formally recommended high priority new missions to Mercury.
Faced with combined technical and programmatic difficulties, researchers interested in Mercury may for some time be faced with the prospect of adapting instruments and spacecraft designed for other purposes to observe Mercury remotely. Here, we examine the near-term avenues presently available for observing Mercury, both from the Earth and from space. We also discuss the scientific rationale and technical concepts for missions to Mercury. Before addressing these subjects, however, we first review the broad scientific questions which future observalions must address.
11. KEY SCIENTIFIC QUESTIONS AND EXPERIMENTSi i As described in this book, our knowledge of Mercury today stands at a point somewhat similar to our state of knowledge of Mars prior to Mariner 9.
Although Mariner 10's three successful flybys of Mercury provided sufficient information to develop pointed scientific questions, we still lack the data necessary to characterize the basic compositional make-up, phenomenological processes, and evolutionary history. Many of our questions require synoptic observations.
Simply put, the key scientific questions which must be addressed by the
next generation of Mercury observations are:
1. What does the still unimaged hemisphere of Mercury look like and what are the inferred geomorphological and tectonic processes?
2. What is the chemical and mineralogical composition of the surface? What S. A. STERN AND F. VILAS 26 are the textural properties of the surface? How do these vary among geologic units?
3. What is the full chemical composition of the atmosphere? How do the composition and pressure vary with location on Mercury and orbital phase?
4. By what processes are the Mercurian atmosphere generated?
5. By what process is the Mercurian magnetosphere generated and how does this magnetosphere interact with the time-dependent atmosphere and variable solar wind?
6. Does Mercury have a present-day liquid core and attendant dynamo? If so, how much of the core is molten?
7. What are the global geophysical properties of Mercury (gravity field, heat flow, and seismicity)?
8. What is the chronology of internal and external processes that have modified Mercury over time? Are there clues about how the planet formed?
Beyond these questions, Mercury observations also offer promise toward the general understanding of planetary magnetospheric, cratering and exospheric processes. Further still, ~ e r c u provides a unique location for tests r~ pertaining to general relativity and for the study of solar physics.
In order to address the scientific goals of future Mercury studies, multiple observational techniques must be employed: Question (1) requires high-quality imaging and altimetry of the entire planet. Question (2) requires multispectral imaging, orbital X-ray and gamma-ray fluorescence measurements, and surface geochemistry experiments. Questions (3) and (4) cannot be answered definitively without high-resolution synoptic spectroscopy (particularly in the ultraviolet), as well as in situ charged-particle measurements, and in situ mass spectroscopy. Questions (5) and (6) await in situ magnetic field and charged-particle environment observations extending over time scales at least as long as the Mercurian year (88 days). Complete answers to Question (7) require surface exploration. Question (8) involves a synthesis, and will therefore rely upon information obtained from all of the aforementioned techniques, and others.
This recounting of the key scientific questions and the methods by which these questions can be resolved provides two insights. First, without future spacecraft missions to Mercury, even our first-order questions cannot be fully answered. Second, however, synoptic Earth- and space-based remote sensing can clearly still contribute important findings, particularly in terms of improved imaging, altimetry and spectroscopy. In the remainder of this chapter, the specific projects and programs which can improve our knowledge of Mercury are reviewed.
111. FUTURE GROUNDBASED STUDIES Mercury must always be observed from the Earth at small solar elongation angles. At its maximum, Mercury never strays farther than 28" from the
FUTURE OBSERVATIONS AND MISSIONSSun. Such close proximity makes observations difficult at best. However, as evidenced by the recent discovery of sodium and potassium in Mercury's atmosphere by Potter and Morgan (1985a, 1986a), groundbased work can still make important contributions.
Among the priorities for future groundbased work are: (1) expanded atmospheric composition and abundance studies, particularly as a function of Mercury's orbital position; (2) surface composition studies by the technique of spectrophotometry; and (3) radar observations. In addition, related activities such as radar and spectrophotometricobservations of asteroids, satellites, and the Moon as well as laboratory studies of meteorites will contribute to our understanding of Mercury by placing it in the larger context of planetary studies.
Several key projects remain concerning Mercury's atmosphere. Most important is the study of spatial and temporal variations in the atmospheric abundance of sodium, potassium and any other as-yet-undiscovered constituents of the Mercurian atmosphere. The measurement of such variations over Mercury's orbital and rotational periods is vital for understanding the mechanism(~) responsible for the generation of this tenuous atmosphere (cf. Chapter by Hunten et al., and references therein). Temporal variations in Mercury's atmosphere are particularly important to understand in light of the well-known convolution of true abundance and observed abundance caused by the high radial velocities of the planet in its elliptical orbit. In addition to spectroscopy, stellar occultation opportunities (Mink 1987) could potentially provide measurements of the vertical composition, pressure, and temperature structure of Mercury's atmosphere.
Groundbased spectrophotometric observations of Mercury's surface should unlock significant information about Mercury's formation conditions.
Resolving the question of the presence of Fe2+ in the surface mineralogy (Chapter by Vilas) would provide information about volatiles in the surface materials, and by extension, address questions concerning the extent of the feeding zone which Mercury sampled during its formation. Mercury's silicate composition could be probed with a search for the Restrahlen bands at thermal infrared wavelengths.
New groundbased instrumentation allows telescopic observations to be made across extended spectral ranges, and facilitates compensation of the effects of high airmass and bright sky background. The correlation of compositional variations with geologic units identified in Mariner 10 images, and the extension of such research to the as-yet-unimaged hemisphere of Mercury, is of particular interest.
Groundbased observations are also useful for studying surface properties and composition. However, as with atmospheric studies, such observations are complicated by Mercury's small size and its proximity to the Sun. However, spectrophotometry (Chapter by Vilas) and polarimetry (Gehrels et al.
1987) each continue to be profitable. Of particular interest would be the correlation of compositional variations with the specific geologic units mapped 28 S. A. STERN AND F. VILAS by Mariner 10 (Rava and Hapke 1987), and the extension of such research to the prediction of the as yet unmapped portions of Mercury.
Radar and radio observations can provide information about topographic slope and altitude, surface electric properties, surface thermal properties, spin-axis orientation, and the ephemerides of Mercury (R. Landau, personal
communication). Indeed, it was by radar techniques that Mercury's 2:3
orbital-to-rotational resonance was discovered.
Radar work has demonstrated that Mercury's surface may be less lunarlike than once suspected. In particular, some radar data have indicated that Mercury is comparatively smooth from an rms-slope standpoint, but anomalously rough on small scales (1-10 cm). Studies by a variety of groups (see, e.g., Chapters by Clark et al. and Harmon and Campbell) have demonstrated that radar-derived roughness and slope data appear to correlate with terrain units imaged by Mariner 10. Over the next decade, radar system improvements at both Goldstone and Arecibo, as well as increased latitudinal coverage will significantly improve the radar data base on Mercury, and provide our only means of actively probing the planet without mounting a spabe mission.
Radio and infrared observations can also contribute to studies of the thermal, mechanical and electric properties of the surface regolith.
IV. OBSERVING MERCURY REMOTELY FROM EARTH ORBIT
AND DEEP SPACERemote observations of Mercury from space offer certain advantages over Earth-based studies. They can achieve broader spectral coverage and diffraction-limited resolution and also can circumvent solar-elongation difficulties with appropriate baffling systems. In this section we demonstrate the desirability of adapting future satellite platforms for observations of Mercury from Earth orbit.
It is important to recognize that a few "proof-of-concept" observations of Mercury have already been carried out from Earth orbit. Included among these are serendipitous observations by the Skylab ATM and Solar Maximum Mission coronagraphs. Additionally, one simple Space Shuttle payload (CHAMPS) had planned to observe Mercury during orbital twilight periods to obtain low-resolution ultraviolet spectra; CHAMPS was destroyed with the Orbiter Challenger on its tragic last mission.
While thermal and telescope baffling constraints have precluded observations of Mercury by recent orbiting observatories, including Copernicus, IRAS and IUE, several possibilities exist for near-term space-based observations of Mercury. Among the possibilities, however, only one spacecraft is actually planning to make observations-the Hubble Space Telescope (HST).
While HST was not originally believed capable of observing at small solar elongation angles, a study (LMSC 1984) conducted by the manufacturer (Lockheed Missiles and Space Company) has demonstrated that imaging obFUTURE OBSERVATIONS AND MISSIONS 29 servations can indeed be made during orbital twilight periods when the Sun is occulted by the disk of the Earth.
It is estimated that HST will be able to achieve 30 to 60 km resolution at Mercury. This will permit the as yet unmapped hemisphere to be imaged, thereby revealing the full pattern of global geology for the first time. At a spatial resolution of 30 to 60 krn, basins, large craters, scarps and other largescale constructs may be recognizable. The question of hemispheric geologic asymmetries may also be addressed. Sufficient observations are planned to map the entire planet (except for certain polar regions) at 1:150,000 scale.
Because HST's imaging systems include extensive filter wheels, multispectral imaging will also be possible.