FEATURE Mercury redux

In January 2008, 33 years after Mariner 10 fl ew past the solar system’s innermost planet, MESSENGER crossed Mercury’s magnetosphere. Ancient volcanoes, contractional faults, and a rich soup of exospheric ions give clues to Mercury’s structure and dynamical evolution.

Th e Mercury fl yby of the MESSENGER two have not been ruled out, but for those (Mercury surface, space environment, mechanisms shorter-wavelength magnetic geochemistry and ranging) probe was the features would be expected, which were not fi rst of three braking manoeuvres for the observed during the MESSENGER fl yby1. spacecraft , in preparation for its insertion Recent libration observations that require into a polar orbit in 2011. Th e probe a partially molten core11, and the limited achieved the closest approach (201 km) of contraction of Mercury, which implies a Mercury’s surface yet, and took a variety largely molten core, favour a convective of measurements in the magnetosphere, dynamo origin for Mercury’s magnetic fi eld. exosphere and on Mercury’s surface. Some Although Mercury’s magnetosphere of the fi rst results of the MESSENGER looks like a miniature version of Earth’s, mission1–6 reveal Mercury as a planet with Mercury’s relatively weak magnetic richly interconnected dynamics, from fi eld implies that its dynamo must work the dynamo in its molten outer core, a diff erently from that of the Earth. Th e crust and surface with great lobate faults geodynamo, which gives the Earth its and relatively young volcanoes, to a strong magnetic fi eld, is thought to operate magnetosphere that interacts with the core in a magnetostrophic regime in which the dynamo and the interplanetary solar wind. Figure 1 Four images of Mercury. The more colourful Coriolis force, due to the Earth’s rotation, Until the mid-1970s, it was thought image in the northeast shows the Caloris basin, a large roughly balances the magnetic Lorentz that Earth was the only planet inside the impact crater. Bright gold-coloured features near the force. Such a balance may also operate asteroid belt with an internally generated Caloris basin’s rim are interpreted to be volcanos6. Red in Mercury’s core. Indeed, several recent global magnetosphere. Scientists boxes: One volcano with a central vent is surrounded modelling eff orts have shown that there are were astonished when the Mariner 10 by a smooth dome. Green boxes: A major lobate many ways for dynamos, even ones with spacecraft sent evidence of a global contractional fault about 650 km long called Beagle strong internal fi elds, to produce relatively magnetic fi eld enveloping Mercury. Rupes, in a region of Mercury imaged for the fi rst time weak external (that is, measurable) Space missions to the outer planets have by MESSENGER’s Mercury Dual Imaging System, is magnetic fi elds12–16. revealed global magnetism on Jupiter, shown in the equatorial West near the terminator. Images One way to produce a weak external Saturn, Uranus and Neptune, and even a courtesy of NASA/Johns Hopkins University Applied magnetic fi eld is with a slow planetary few planetary satellites. Of the terrestrial Physics Laboratory/Carnegie Institution of Washington. rotation rate. Numerical simulations have bodies, only Mercury and Earth have shown that when rotational forces are too signifi cant internally generated magnetic weak to maintain large scale convection fi elds. It is not known whether Venus and vortices, the magnetic fi eld structure, which the Moon had intrinsic fi elds in the past. January 2008 was equatorial, and new tends to follow the fl ow fi eld, becomes Th e Moon has patches of magnetized information about the high latitudes must small-scale. Th is eff ect leads to a multipolar crust, and evidence of a past venusian therefore wait until orbital insertion in dynamo with a sharply reduced dipolar dynamo may have been wiped out because 2011. Nevertheless, MESSENGER largely component. Assuming that the dynamos its surface temperature of 730 Kelvin confi rmed the Mariner 10 observations and of Earth and Mercury have driving forces is near the Curie point, above which provided additional constraints on the fi eld that scale similarly, Mercury’s slow rotation remanent magnetism cannot persist. morphology. Th e modelled observations (its sidereal day is 58.6 Earth days) means Strong magnetic anomalies indicate an yield a surface fi eld of 230–290 nT that that its dynamo is expected to operate in the ancient global fi eld on Mars, but it is is primarily dipolar. However, additional multipolar regime12. Given the uncertainty thought that the martian dynamo ceased higher multipolar contributions could in the relative contributions of dipolar and to operate early in the planet’s evolution, account for up to half the magnetic fi eld multipolar components, and that the higher perhaps over 4 billion years ago. strength1. Th is magnetic fi eld, although multipolar components decay more rapidly suffi cient to form a global magnetosphere, outward from the outer core source region, MAGNETIC FIELD AND CORE DYNAMICS has a surface strength only roughly 1/100 the MESSENGER observations indicating a that of the Earth. dipolar external fi eld could still be consistent Mercury’s magnetic fi eld, as observed by Th ree types of mechanisms are currently with a multipolar core dynamo. the two fl ybys of the Mariner 10 spacecraft considered plausible candidates for the For a diff erent class of models, that penetrated the magnetosphere generation of Mercury’s intrinsic magnetic Mercury’s weak global fi eld can be (one of which was a polar pass), had fi eld: coherently distributed remanent produced by a dipolar or multipolar a magnitude of roughly 300 nT at the magnetization of the crust8, a thermoelectric dynamo that is constrained to a small surface7. Th e fl yby of MESSENGER in dynamo9 or a convective dynamo10. Th e fi rst volume of its large core. Th is can be

564 nature geoscience | VOL 1 | SEPTEMBER 2008 | www.nature.com/naturegeoscience © 2008 Macmillan Publishers Limited. All rights reserved. FEATURE accomplished in several ways, including Magnetotail fl ow in a thick liquid shell, in which strong convection is regionalized near a small inner core13, or alternatively through convection in a thin shell, which tends to Magnetopause generate magnetic fl ux more effi ciently at low latitudes14. Further reductions in Core magnetic fi eld strength occur if the upper part of the liquid outer core is stably stratifi ed. In that case the magnetic fi eld, Mantle Regolith produced by compositional convection and confi ned near the inner-core boundary, is strongly attenuated by the stratifi ed 15 layer above . Finally, if solidifi cation Bow shock Cusp occurs away from the inner-core boundary, convection could be restricted to a thin layer above the horizon of iron precipitation, which ‘snows’ down to the deeper core16. From the available data it is not yet possible to distinguish between MESSENGER orbit these various possibilities.

CONTRACTION AND INNER-CORE GROWTH

Because Mercury’s contraction places constraints on the fraction of melt in Figure 2 Modelling perspective of dynamical processes of Mercury’s magnetosphere and core. Purple lines its core, the planet’s surface tectonics, are orbit trajectories planned for MESSENGER when it arrives in 2011. Magnetosphere dynamics taken from a cratering and history of volcanism are magnetohydrodynamical simulation of the interaction of the solar wind magnetic fi eld with a dipolar intrinsic intimately related to inferences about the magnetic fi eld19 are visualized. Magnetic fi eld lines are shown in white. Plasma pressure is shown in meridional growth of the inner core, the operation plane through the magnetosphere: red and blue indicate high and low pressure respectively. Core dynamics of the dynamo and the planet’s thermal are taken from a numerical dynamo simulation that produces a dipolar magnetic fi eld12. Isosurfaces of the fl uid evolution. In its January 2008 fl yby, vorticity component parallel to the planetary rotation axis are shown in the outer core. Red and blue vortices MESSENGER imaged 21% of Mercury’s indicate prograde and retrograde convective fl ow. The inner core is shown as a silver sphere. Artistic visualization surface that had never before been and model rendering by Chris Want. observed by a spacecraft . Moreover, in areas previously imaged by Mariner 10, diff erent lighting conditions allowed MESSENGER to reveal new relationships between volcanic fl ows, have been observed in the Given the large uncertainties in core contractional faults and volcanic plains. Mariner 10 and MESSENGER images. formation, and additional uncertainties Th e discovery of additional contractional Some images also show younger craters in how much observed contraction was faults, such as the contractional lobate overprinting contractional scarps. Th e due to mantle and lithosphere cooling, it fault called Beagle Rupes (Fig. 1), results new MESSENGER images now reveal is diffi cult to estimate the total amount of in estimates of global surface contraction more detailed features, including the core solidifi cation precisely. Nevertheless, that are about a third higher than those embayment of lobate faults by volcanic the observationally inferred contraction based on the Mariner 10 observations2. fl ows2. Th ese observations show that of less than 3 km is small compared to the At present the total radial contraction contractional faulting commenced before 17 km of contraction estimated to result estimated from lobate faults is less than many of the volcanic plains were emplaced if the entire core were solidifi ed17. Th is 3 km. However, the increased contraction and continued aft er the eruption of the is consistent with a dynamo origin for inferred from MESSENGER’s observations younger volcanics. Mercury’s magnetic fi eld. Furthermore, must be considered a lower bound. Th e observations of contractional the small amount of contraction, which Future observations during the remaining faults help infer Mercury’s thermal scales with the volume of solidifi cation, two fl ybys and the orbital phase of evolution, interior composition and implies that the modern-day solid inner MESSENGER’s mission will no doubt the growth of the planet’s inner core. In core is relatively small, perhaps Earth-like reveal more observations of contractional standard thermal evolution models, global in relative scale. (Th e fraction of Earth’s faults, even in areas previously imaged, contraction is preceded by a phase of early inner core radius to the total core radius owing to increased coverage and planetary expansion, fuelled primarily by is 0.35.) Th e inference of a largely molten resolution as well as changes in lighting the mass distribution and gravitational outer core also puts constraints on the and perspective. energy release during diff erentiation dynamics of Mercury’s relatively thin Chronological relationships between into core and mantle. Th e timing of the rocky mantle, and on the composition of lobate thrust faults, cratering and volcanic onset of core solidifi cation and global the core. Th e operation of a convective plains reveal an extended period of global contraction is not known. Any evidence dynamo in the core requires thermal or contraction. Many instances of lobate of contraction that occurred before compositional buoyancy (and probably faults cutting across and deforming 3.8 Gyr ago would have been erased by both) generated by outward heat fl ux older craters and relatively young impact fl ux during the period of heavy and solidifi cation of the inner core. It is smooth plains, which are interpreted as impact bombardment2. likely that the mantle, through solid-state nature geoscience | VOL 1 | SEPTEMBER 2008 | www.nature.com/naturegeoscience 565 © 2008 Macmillan Publishers Limited. All rights reserved. FEATURE convection, has effi ciently transferred heat dynamic and electrodynamic effects) of observations of the solar system’s from the interior over much of Mercury’s of the impinging solar wind also has innermost planet. Th e next fl yby history. If that is the case then a light important consequences for the global is coming up in October 2008 and element like sulphur, which lowers the magnetospheric picture. In addition to comprehensive mapping of the planet melting temperature of iron alloy, must be causing bow shock and magnetosheath will begin in earnest when MESSENGER present at a relatively large concentration asymmetries, this also leads to a higher is placed in orbit in 2011. Th e planet is in the core17,18. efficiency of the magnetic reconnection also targeted by the European–Japanese process. For example, during the mission BepiColombo, scheduled to be MAGNETOSPHERE AND EXOSPHERE flyby, MESSENGER recorded several launched in 2013. Th at mission will have flux transfer events3. Combined with two orbiters, allowing the simultaneous Th e space environment of Mercury is the smaller spatial scale of Mercury’s monitoring of the solar wind and the determined by the interaction of the magnetosphere this efficiency leads to a magnetosphere interior. planet’s internal magnetic fi eld with more direct control of the magnetosphere Observations from the combination the magnetized solar wind, in the same by the solar wind and the interplanetary of the two missions will be over a process that defi nes the magnetospheres magnetic field than any other planetary suffi ciently long period to allow the of Earth, Jupiter, Saturn, Uranus and magnetosphere1,20. In fact, the dynamic detection of changes in Mercury’s Neptune. Mariner 10 and MESSENGER magnetospheric magnetic field can be magnetosphere and exosphere. observed similar properties to other so large, relative to the internal field, A lack of changes in the global planetary magnetospheres — the bow that the dynamo action in Mercury’s magnetic fi eld would suggest a shock, magnetopause, magnetotail and core may be significantly affected by dynamo that is deeply buried near the plasma sheet, cusp regions, pick-up ions, the interaction with the solar wind21. inner-core boundary15. On the other and Alfvenic and other plasma waves3,4. The possibility of such an interaction hand a multipolar dynamo that operates Particularly intriguing is new evidence of the solar wind with core dynamics near the core–mantle boundary would from MESSENGER of Kelvin–Helmholtz is further facilitated by Mercury’s thin likely result in secular variation of instabilities, which may provide an mantle, which places the planet’s small Mercury’s intrinsic fi eld that would be important entry mechanism for solar magnetosphere in close proximity to its measurable over the time span covered wind into the hermean magnetosphere19, relatively large core. Figure 2 illustrates by the MESSENGER and BepiColombo and a driver for various types of some of the dynamical aspects of this missions. We can hope that at least some magnetospheric waves. remarkably proportioned planet. of Mercury’s secrets will be revealed in the Despite similarities with the diff erent not-to-distant future. planetary magnetospheres, the details The dynamo action in References of physical processes in Mercury’s 1. Anderson, B. J. et al. Science 321, 82–85 (2008). magnetosphere are unique in the solar Mercury’s core may be affected 2. Solomon S. C. et al. Science 321, 59–62 (2008). system, and several features have not been 3. Slavin, J. A. et al. Science 321, 85–89 (2008). found elsewhere to date. One example is by the interaction with the 4. Zurbuchen, T. H. et al. Science 321, 90–92 (2008). 5. McClintock, J. W. et al. Science 321, 92–94 (2008). the structure of the magnetopause, the solar wind on this remarkably 6. Head, J. W. et al. Science 321, 69–72 (2008). thin interface between the solar wind and proportioned planet. 7. Connerney, J. E. P. & Ness, N. F. in Mercury (eds Vilias, F., the magnetosphere. As MESSENGER Chapman, C. R. & Matthews, M. S.) 494–513 exited the hermean magnetosphere (Univ. Ariz. Press, Tucson, 1988). 8. Aharonson, O., Zuber, M. T. & Solomon, S. C. during the fi rst fl yby it detected two Apart from Mercury, all other planets Earth Planet. Sci. Lett. 218, 261–268, (2004). current layers, instead of the usual single in the solar system have gravitationally 9. Stevenson, D. J. Earth Planet. Sci. Lett. magnetopause current sheet3. Th is double bound atmospheres which give rise to 82, 114–120, (1987). 10. Ness, N. F., Behannon, K. W., Lepping, R. P., Whang, Y. C. & layer structure of the magnetopause ionospheres, providing a natural barrier Schatten, K. H. Science 185, 151–160 (1974). may be associated with eff ects of heavy between the space environment of the 11. Margot, J. L., Peale, S. J., Jurgens, R. F., Slade, M. A. & ions gyrating in the magnetic fi eld, planet and its surface. Mercury has no Holin, I. V. Science 316, 710–714 (2007). 12. Olson, P. L. & Christensen, U. R. Earth Planet. Sci. Lett. but a complete theoretical explanation atmosphere and only a tenuous and 250, 4,5 561–571 (2006). is still lacking. Ultra-low frequency constantly recycled exosphere . Th is 13. Heimpel, M. H., Aurnou, J. M., Al-Shamali, F. M. waves observed by both Mariner 10 and leads to the heavy space weathering & Gomez Perez, N. Earth Planet. Sci. Lett. MESSENGER3 have unique properties of Mercury’s surface, which is one of 236, 542–557 (2005). at Mercury because of the small spatial the processes supplying new atoms to 14. Stanley, S., Bloxham, J., Hutchison, W. E. & Zuber, M. T. Earth Planet. Sci. Lett. 234, 2738 (2005). 22 and temporal scales that characterize the exosphere . Th e lack of a highly 15. Christensen, U. R. Nature 444, 1056–1058 (2006). the planet’s magnetosphere. Interaction conducting ionosphere leads to a 16. Chen, B., Li, J. & Hauck II, S. A. Geophys. Res. Lett. of the intense solar wind with Mercury’s yet another mystery of the hermean 35, L07201 (2008). 17. Solomon, S. C. Icarus 28, 509–521 (1976). small magnetosphere causes heavy ions, magnetosphere: how do the electric 18. Hauck II, S. A., Dombard, A. J., Phillips, R. J. & Solomon, S. C. such as sodium, oxygen and potassium, currents close? Th is lack of a conducting Earth Planet. Sci. Lett. 222, 713–728 (2004). to be sputtered from the surface of the layer close to the surface of Mercury may 19. Hasegawa, H. et al. Nature 430, 755–758 (2004). planet. Th e dynamics of this process are produce a truly unique magnetospheric 20. Kabin, K. et al. Icarus 195, 1–15, (2008). 21. Glassmeier, K.-H. et al. Space Sci. Rev. very diff erent from those for planets with current system, which still remains to 132, 511–527 (2007). atmospheres and/or stronger magnetic be discovered. 22. Killen R. et al. Space. Sci. Rev. fi elds (such as Earth), where ions do not 132, 433–509 (2007). interact appreciably with the surface4. FUTURE EXPLORATION The lower Alfven–Mach number Moritz Heimpel* and Konstantin Kabin (the ratio of the solar wind speed Th e fi ndings from the fi rst fl yby of are at the Department of Physics, University of to the Alfven wave speed, which Mercury by MESSENGER1–6 are just Alberta, Edmonton, Alberta, Canada T6G 2J1. controls the relative importance of gas the beginning of an extended period *e-mail: [email protected]

566 nature geoscience | VOL 1 | SEPTEMBER 2008 | www.nature.com/naturegeoscience © 2008 Macmillan Publishers Limited. All rights reserved.