A Mercury Orbiter Mission

D.N. Baker

1. Introduction

The Mariner 10 flybys of Mercury in 1974 and 1975 resulted in the discovery of a strong planetary magnetic field and an active magnetosphere similar in many ways to that of Earth. Based upon the small size of the planet, Mercury’s interior was expected to have cooled and solidified long ago. The presence of an intrinsic magnetic field, however, implied an internal dynamo in a fluid core, posing numerous, unresolved questions concerning the origin, composition, and thermal history of Mercury. The Mariner 10 spacecraft also detected intense particle bursts and magnetic field disturbances, indicating that phenomena analogous to terrestrial magnetospheric substorms occur at Mercury. The Mariner 10 images revealed a number of surface features unique to Mercury, including large-scale thrust faults apparently associated with crustal compression as the planet cooled and contracted.

The magnetospheric and planetary physics rationale for a Mercury orbiter mission has been reported upon previously in several NASA [JPL, 1977, 1990] and National Academy of Sciences [NAS, 1978, 1985, 1988] reports. The primary space physics science objectives identified in these reports are: 1) to map in three dimensions the magnetic structure and plasma environment of the "miniature" Mercury magnetosphere; 2) to study in detail the principal physical processes taking place during Hermean magnetospheric substorms with an emphasis on differences from Earth due to Mercury’s lack of a highly conducting ionosphere; 3) to assess the role of interplanetary conditions in determining the rate at which the Hermean magnetosphere draws energy from the solar wind and the manner in which it is later dissipated; 4) to investigate heliospheric structure and dynamics inside of 0.5 AU; and 5) to utilize the proximity of Mercury to the Sun to achieve fundamental solar physics objectives by measuring neutrons and charged particles emanating from solar active regions. The primary planetology science objectives are: 1) to complete the global surface mapping initiated by Mariner 10; 2) to obtain global geochemical terrain maps of the occurrence of such elements as Fe, Th, K, Ti, Al, Mg, and Si; 3) to measure the intrinsic magnetic field in sufficient detail to allow for the detection of magnetic anomalies; and 4) to map Mercury’s gravitational field and associated anomalies.

Previously, Mercury missions have been studied with the goal of addressing both planetary science and space physics goals. Such missions were major NASA programs, inconsistent with the present focus on cheaper and faster missions. As NASA reconsiders missions to Mercury, very critical space physics goals could be lost in favor of more limited or focused goals of studying planetary materials. It is therefore critical to define new missions which demonstrate that multidisciplinary goals, including space physics goals, can be achieved at Mercury in the present political climate of cheaper, faster, and better.

Recent advances in spacecraft miniaturization and innovative propulsion systems suggest the value of re-examining a mission to Mercury. Such a mission would provide unique particles and fields measurements which are unobtainable at other planets due to the constraints of orbital mechanics and the large dimensions of other magnetospheres relative to their planetary bodies. A Mercury Orbiter would be a mission of exploration and discovery that would provide the essential data necessary to formulate the next generation of theories and models for terrestrial-type magnetospheric structure and dynamics. Such a mission would also return critical measurements necessary for the understanding of not just the surface history and internal structure of Mercury, but the formation and chemical differentiation of the Solar System as a whole.

Siscoe et al. [1975] and Ogilvie et al. [1977] showed that the energetic particle burst periods in Figure 2 tended to be times of substorm-like behavior. Mariner 10 entered the near-tail plasma sheet on the duskside and the sheath field was northward on tail entry while the field inside the magnetopause was very tail-like and relatively quiet. |B| then increased with time in approaching the planet, and according to Siscoe et al., the higher-energy plasma electrons decreased in intensity. Shortly after closest approach, |B| decreased rapidly, and the field inclination increased markedly. This indicated a transition from a tail-like to a dipole-like field orientation and in the terrestrial case [Baker et al., 1984, 1997] this would be a classic signature of substorm expansive phase onset. In fact, between 2047 and 2054 UT there were numerous large changes in B and these occurred in the same time period as did the large energetic particle bursts B, B’, and C. The magnetic field was strongly southward upon Mariner’s outbound exit.

3. Space Plasma Science at Mercury

Mercury has a tenuous, neutral atmosphere whose constituents are poorly known. It is more properly termed an exosphere because the atmosphere is collisionless and the exobase is at the surface; i.e., an atmospheric neutral will typically fall back to the surface of Mercury before colliding with another neutral. The five known species in Mercury’s exosphere — H, He, O, Na, and K — are also thought to be important constituents of the lunar atmosphere. Mariner 10 ultraviolet spectrometer observations detected H, He, and O at Mercury, while Na and K were later discovered by ground-based optical spectrophotometry (see Table 1 for surface density estimates). The mechanisms responsible for maintaining an atmosphere at Mercury, despite its high dayside surface temperature and low surface gravity, are not well understood. Atmospheric neutrals must continually fall to the surface and be re-ejected from it [Morgan and Killen, 1997]. Surface interactions are, therefore, critical in determining the atmospheric temperature, composition, and geographic distribution. Magnetospheric processes, including ion precipitation onto Mercury’s surface and the pickup of photo-ions, may be extremely important for both atmospheric sources and losses (see Plate 1).

Table 1. Mercury exospheric number densities

The absence of a collisional ionosphere has important consequences for global electric currents and plasma circulation patterns at Mercury. At Earth, high-latitude magnetospheric current systems close by flowing through the ionosphere. At Mercury, these currents cannot close through a collisional ionosphere (since none is present) or through the surface (because it is expected to be a good insulator). Closure of magnetospheric current systems through a resistive regolith or partially through an ionized exosphere rather than a collisional ionosphere would have important implications, both for the global current systems and magnetospheric convection as well as for dynamical processes such as substorms and flux transfer events. The heated and energized substorm particles could impact directly onto the cold regolith surface of Mercury, thereby creating a heated "auroral" band (see Figure 3) at the surface [Baker et al., 1987]. Some theories hold that the timescale for the substorm growth phase at Earth is determined by ionospheric line-tying which limits the rate of magnetic flux return to the dayside following enhanced reconnection at the dayside magnetopause. Also, there are theories of the substorm expansion phase at Earth that consider active feedback between the magnetosphere and ionosphere (specifically enhanced conductivities in the auroral zones) as the essential ingredient for substorms. Such feedback is presumably absent at Mercury. Observations in Mercury’s magnetosphere may determine whether ionospheric coupling, in the form of current closure through a resistive medium, is a necessary and central element in substorm-like energy conversion processes.

It is essential at Mercury to learn how magnetospheric structure evolves at relatively large distances and how the magnetotail responds to changes in the interplanetary medium. Does the magnetotail have a coherent structure; i.e., an identifiable plasma sheet and lobes, which extends to large distances? Structure and motion of the tail should be related to solar wind and near-planet magnetospheric changes. Many believe that during substorms in Earth’s magnetosphere, the plasma sheet is severed by magnetic reconnection quite close to Earth and flows rapidly down the tail as a magnetically confined structure (a plasmoid). Some theories predict that this is the primary way that solar wind plasma and energy, earlier acquired by the magnetosphere, is dissipated and a portion returned to the solar wind. Thus, plasmoids may be of fundamental importance to magnetospheric physics. Observations of plasmoids in Mercury’s magnetotail would provide important confirmation that magnetic reconnection and plasmoid formation are basic features of the process by which stored energy is released within planetary magnetospheres.

Solar wind energy coupling into Earth’s magnetosphere is known to be strongly influenced by the polarity of the IMF. Southward IMF leads to strong coupling, through reconnection with the northward geomagnetic field at the surface of the magnetosphere. The occurrence of substorms, the basic mechanism for stored energy release and dissipation, clearly relates on a statistical basis to the occurrence of southward IMF. However, IMF direction typically varies on a time scale of a few minutes, i.e., much shorter than the time scale of energy storage and substorm occurrence at the Earth which is about an hour. Accordingly, detailed cause and effect relationships are very difficult to discern. In the case of Mercury, where the magnetospheric response time is believed, on the basis of Mariner 10 data, to be only a minute or so, relations between the IMF and internal magnetospheric processes could be studied with great benefit. For example, it is not uncommon for the IMF to remain southward and constant for ten minutes. At Mercury this time span is long compared to the substorm cycle time and it would be possible to see whether the magnetosphere responded to this situation by repeated substorms and plasmoid releases as some substorm theories predict (see Table 2 for estimates of energy coupling strengths at Mercury).

Table 2. Magnetospheric Energy Budgets

Mercury is the best place to test and extend the understanding of magnetospheric physics acquired by studying the Earth’s magnetosphere. The major difference between Mercury and Earth, viz., the former’s lack of an ionosphere, is highly valuable in that it will allow us to ascertain the degree to which theories developed at Earth can be extended to general magnetospheric systems in the case when one of the critical features of the system is radically altered. The slow rotation of Mercury causes solar wind driven convection to dominate throughout the magnetosphere: It is the magnetosphere among all the planets where this dominance is most extreme. Mercury’s small magnetosphere may also solve the space-time ambiguity problem that has confounded efforts to perform synoptic studies of Earth’s magnetosphere. Approximately once per hour the solar wind conditions change significantly, and magnetospheres must change to accommodate these new conditions. An Earth satellite takes many hours to a day to traverse each of the magnetosphere’s structural units, which in the meantime is changing its shape and behavior. At Earth a satellite virtually never samples a complete structural unit before it changes its state. Hence, a statistical approach is necessary for synoptic studies of Earth’s magnetosphere. A satellite at Mercury crosses the entire magnetosphere in one-third of an hour or less. The solar wind typically will not change during this time. Thus, the changes a satellite records in a magnetospheric structure at Mercury characterize that structure while the magnetosphere is in a fixed state.

5. Comparative Planetology at Mercury

The composition, structure, and temporal behavior of the atmosphere must be determined. Because at least some of the neutral atmospheric species are derived from surface sputtering [Morgan and Killen, 1997], knowledge of atmospheric composition will be useful for inferring surface composition and investigating the long term weathering effects of the solar wind on the surface. In addition, detection and measurement of the amount of cold-trapped polar water ice is extremely important.

6. A Mercury Mission Scenario

Because of the large velocity changes required, it is a difficult task to place a spacecraft in orbit around Mercury under any circumstances, and particularly with a science payload capable of satisfying multidisciplinary aims. Thus, in recent times propulsion technologies alternative to chemical systems have been promoted as possibly providing much greater flexibility for Mercury mission scenarios. With the development of a new propulsion system for the "New Millennium" series of spacecraft, and with similar development under way in both Japan and Europe, Solar Electric Propulsion (SEP), utilizing electrostatic ion thrusters (or arc jets), appears to be a particularly promising alternative. By taking advantage of the extremely high specific impulses (thrust/mass) of the ion engines (2000-4000 s versus ~300 s for chemical systems), it is possible that much lower launch masses may be required relative to the instrument payload mass. However, at least at the present time, Solar Electric Propulsion cannot be considered a panacea. While the astronautics literature describes many electric propulsion scenarios for reaching several different planetary bodies, typically the true engineering complexities and hardware overheads are underestimated.

Another aspect of the chemical versus SEP trade evaluations is the mission trajectory. Figure 4, for example, provides an illustration of an SEP trajectory evaluation that was conducted for "Roadmap" studies. In this case, the spacecraft, launched by a Delta launch vehicle in December 2000, obtains a gravity assist from Venus, and with the ~5 kW solar electric propulsion system achieves orbit around Mercury with a total flight time of about 2.8 years. Such studies support the advertised capabilities of SEP, since for this approximate time frame there has not been found a reasonable chemical propulsion solution for getting to Mercury orbit (where "reasonable" means with fuel mass fractions that are consistent with standard engineering assumptions). Also, chemical solutions typically require substantially longer flight times. However, the inefficiencies are also quite significant. While chemical solutions require velocity changes of the order of ~3 km/s, SEP solutions require velocity changes of order ~11 km/s due to the inefficiencies of thrusting at non-optimum times (i.e., the very high specific impulses of SEP is squandered to a certain extent). These inefficiencies must be folded together with the substantial costs of including an SEP system on a proposed mission spacecraft.

Spinning a Mercury orbiter spacecraft effectively distributes the solar heat flux (and, in most instances, the planetary heat flux) around the perimeter. This reduces the peak surface temperatures of sensors, solar cells, and the sides of the spacecraft and helps distribute heat in the interior. Isothermalizing the interior of the spacecraft is important to reduce differences between the fuel and oxidizer temperatures and temperature gradients in each propellant. Thus, the "rotisserie" mode offers several potential operational benefits. In general, a spinning spacecraft also provides a simple, robust operational configuration and it aids in the complete and effective measurement of particles and fields. On the other hand, a spinning spacecraft is not as desirable as a 3-axis stabilized vehicle for imaging and other planetary science objectives. Thus, tradeoff studies between these options have been a crucial part of recent studies.

The hostile thermal environment in the vicinity of Mercury is one of the key drivers of any spacecraft design. The solar irradiance at Mercury varies from 4.6 to 10.6 times that at Earth, and the normal planetary albedo is extremely low. As a result, Mercury’s surface absorbs 94% of the incident solar flux and re-emits this energy in the infrared waveband. Mercury rotates about its axis so slowly that the dayside surface temperature reaches a near-steady-state temperature of 700K at perihelion. Conversely the dark side temperature reaches as low as 100K. The direct solar flux on the spacecraft ranges from zero-solar constants (SC) during occultations to 1 SC at Earth to 10.6 SC at Mercury perihelion. Total solar flux depends on solar range, phase angle, altitude, and surface albedo in the vicinity of a spacecraft. Heat flux on a spacecraft reradiated from Mercury (in terms of equivalent solar constants) ranges from 0 SC when the spacecraft is far away or in occultation to ‰ 8.5 SC when a spacecraft is at periapsis above the subsolar point and Mercury is at perihelion. Other important considerations are that while the sun is small in the sky, Mercury reaches a large angular extent, and the energy from Mercury is predominantly at long wavelengths whereas that emitted by the sun is predominantly at short wavelengths.

7. Scientific Instruments

Recent spaceflight programs such as Clementine have demonstrated the remarkable potential of "miniaturized" space instrumentation [Rustan, 1994]. CCD cameras and particle detectors [e.g., Baker et al., 1995] weighing hundreds of grams have been flown. The pressures are now on to reduce scientific payload masses even more while maintaining good instrument performance and overall sensitivity. Small, capable instruments are critical to the success of any future Mercury mission. This is especially true if both space physics and planetary science goals are to be accommodated.

Spaceborne magnetometry is a very mature area with at least two technologies, ring core flux gates and optical pumping of helium, capable of delivering state-of-the-art measurements for only 1-2 kg and 1.5-2.5 watts. Magnetic cleanliness requirements and the probable need for a short boom upon which to mount the sensors have been included in recent studies.

To understand the origin, role, and fate of solar wind and planetary plasma species in the Hermean magnetosphere, high sensitivity, high time resolution, 3-D hot plasma composition measurements must be made. Such measurements provide information on the energy distribution, flow and density of species with drastically different sources and histories. For example, sputtered secondary ions or atmospheric photo-ions will be observed as pickup rings or shells, similar to those observed at comets. They provide information on electric fields far from the measurement points. Such measurements also provide information that is crucial to understanding global magnetospheric convection, entry of solar wind plasma (and exit of magnetospheric plasma) at the magnetopause and cusps, and heating, transport and loss of plasma sheet plasma. Because of the rapid timescales characteristic of the Hermean magnetosphere, high sensitivity, high time resolution measurements are crucial. The many species involved require high mass resolution and the relevant species range from photo-ions up to hot plasma sheet particles so an energy range from 1 eV to at least 30 keV is needed.

Electric field measurements are desirable to determine the global electric circuit for the unique plasma environment of Mercury. What is the electric potential drop across the planet and its magnetosphere (tens of kilovolts? more?) and how does this affect the resulting plasma convection and energetic particle acceleration? The understanding of how the electrical conductivity at the foot of magnetospheric field lines controls convection processes is central to all of magnetospheric physics. An electric field instrument flying on a orbiter mission at low altitudes across the polar cap of Mercury is the only way to directly answer this critical question.

Plasma wave instrumentation can detect and monitor emissions at Mercury that are associated with substorm activity (including local electron and ion acceleration and instability phenomena), as well as plasma waves that will be used as signatures of magnetospheric boundaries (such as the bow shock, the magnetopause, the cusp, auroral oval, etc.) that the spacecraft will encounter. Such instrumentation could also provide accurate plasma density measurements without the complication of spacecraft potential offsets suffered by thermal plasma instruments. The required instrument would measure the radio and plasma wave spectrum from 1 Hz to 300 kHz.

One of the most important planetary parameters is global elemental composition. By measuring the surface composition of a planet, and tying that composition to geologic features such as impact basins, highland deposits, ejecta blankets and volcanic features, one gains a more three-dimensional understanding of the planet's make-up. This is why gamma ray spectrometers (GRS) are being built for flight to Mars and the Moon. A GRS also doubles as a solar gamma ray detector for flare studies. Finally, a GRS is also sensitive to very high energy electrons (>1 MeV) arising from either magnetospheric or solar processes. This triple-duty nature of a GRS makes it a very valuable instrument for a Mercury mission.

For geochemical purposes the GRS detects gamma rays arising from the interaction of cosmic rays with surface planetary materials down to a depth of a few tens of cm. Gamma ray line emissions from radioactive decay, inelastic neutron scattering and neutron capture provide a detailed fingerprint of regional geochemistry for most mineralogically-important elements: Si, Mg, Fe, Al, Ti, K, Th, U and Ca all can be discerned with varying degrees of accuracy.

Neutron spectrometer (NS) measurements provide information on soil constituents with high neutron absorption cross-sections (such as Fe, Ti, Sm and Gd), assisting in the geochemical assay. But the most dramatic and important use of the NS is in the detection of water ice, the presence of which moderates the neutrons and very much changes the neutron energy spectrum. In particular, one finds that epithermal neutrons (energies between 0.3 eV and 500 keV) are very quickly moderated down to thermal energies in the presence of water. By measuring the ratios of fast (>500 keV) to epithermal to thermal fluxes, one can obtain an estimate of the water content in the relevant piece of planetary real estate.

Global imaging and spectroscopic observations of the surface and of the atmosphere are key measurements required to address the planetology and space plasma science goals of a Mercury orbiter mission. The primary imaging measurement objective would be to map the entire surface of Mercury with a resolution of 1 km and up to 25% of the surface with a resolution of 100 m to support geologic investigations. A secondary objective would be to obtain multispectral images of the surface for composition studies. This would determine atmospheric composition, structure, and temporal behavior. These measurements rely on limb scan observations with a vertical resolution of about 50 km (approximately 1 atmospheric scale height for Na). Spectral coverage in the range 0.115 to 0.6 microns with a resolution of 0.1 nm is sufficient to measure prominent emissions of known and candidate species including H, O, Na, K, S, S+, Si, Al, Ca, Ca+, Mg, Mg+, Fe, and OH [Morgan and Killen 1997]. These measurements would determine the atmosphere’s major sources and sinks (atmospheric processes) and investigate the interactions between the atmosphere, magnetosphere, and the solar wind. Measurement of atmospheric composition can also be used to infer surface composition. Except for the noble gases, hydrogen, and a few volatile species, atmospheric species are derived from the surface. By understanding the sources, sinks, and the gas surface interactions, and by measuring regolith derived elements (Ca, Mg, Na, K, and Fe) the relative ratios of these species in the surface rock can be determined. Recent advances in detector technology have led to the development of a new generation of low-mass, low-power remote sensing instruments (e.g., the visible imaging system for the Clementine mission and Ultraviolet Imaging Spectrographs [McClintock, 1996]).

8. Summary

Planetary exploration programs have revealed the benefits of comparative magnetospheric and planetological studies. The known intrinsic planetary magnetospheres of Mercury, Earth, Jupiter, Saturn, Uranus, and Neptune all have similarities of structure which allow the development of analogies between them. However, as perhaps an even more important test of present theoretical understanding, each planetary magnetosphere has significant differences from the other systems. This causes a substantial contrast from one planet to the next. Mercury’s most Earth-like of magnetospheres shows many familiar features such as energetic particle bursts and globally coherent dynamics. Thus, a pervasive feature of cosmic plasmas generally, and magnetospheres in particular, appears to be the rapid and efficient conversion of magnetic field free energy into the kinetic energy of suprathermal particle populations [e.g., Rosner et al., 1984]. A Mercury orbiter mission could revolutionize our understanding of Earth and the entire inner Solar System. New spacecraft designs and miniaturized instruments now place a comprehensive Mercury mission into our fiscal grasp.

Acknowledgments. This paper represents the thinking of the Space Physics "New Concepts" team as well as the Sun-Earth Connections "Roadmap" group. The author thanks the entire membership of these teams for useful discussions and valuable inputs. This work was supported by NASA.

D.N. Baker, Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309-0590.

Figure 1. Mariner 10 flyby geometries at Mercury in 1974 and 1975 [adapted from Ness, 1979].

Figure 2. Mariner 10 energetic particle, plasma, and magnetic field data on 29 March 1974 [from Eraker and Simpson, 1986].

Plate 1. A diagram showing processes possibly acting between the surface and atmosphere of Mercury [from Morgan and Killen, 1997].

Figure 3. A schematic of the Mercury magnetosphere showing likely region of substorm particle acceleration and possible auroral zones due to particle impacts [from Baker et al., 1987].

Figure 4. Possible Solar Electric (SEP) trajectory for Mercury mission launching in December 2000.

Figure 1. Mariner 10 flyby geometries at Mercury in 1974 and 1975 [adapted from Ness, 1979].

Figure 2. Mariner 10 energetic particle, plasma, and magnetic field data on 29 March 1974 [from Eraker and Simpson, 1986].

Plate 1. A diagram showing processes possibly acting between the surface and atmosphere of Mercury [from Morgan and Killen, 1997].

Figure 3. A schematic of the Mercury magnetosphere showing likely region of substorm particle acceleration and possible auroral zones due to particle impacts [from Baker et al., 1987].

Figure 5. Possible Solar Electric (SEP) trajectory for Mercury mission launching in December 2000.

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission

Baker: Mercury Orbiter Mission