Mercury: Space Environment, Surface, and Interior (2001) 8040.pdf

X-RAY SCIENCE ON ESA’s BEPICOLOMBO MISSION TO MERCURY S. K. Dunkin1,2, M. Grande1, and B.J. Kellett1 1 Space Science and Technology Department, Rutherford Appleton Laboratory, Chilton Didcot, OX11 0QX, UK 2 Department of Geological Sciences, University College London, London WC1E 6BT, UK ([email protected])

Introduction: Mercury has been visited only the lunar maria (dark lava plains) although with very once by a spacecraft, with Mariner 10 completing 3 important differences. They have a much higher visible flybys in 1974/5. It closeness to the Sun makes ground- albedo than the lunar maria and no areas of low albedo based observations very difficult, and hence our blue-colour material representative of the high-titanium knowledge of Mercury is extremely limited. This paper mare basalts found on the Moon. However, the smooth outlines the science drivers of an X-ray instrument (i.e. plains appear to fill in craters, suggesting a flowing CIXS, [1]) to fly on a spacecraft such as ESA’s Bepi- nature (i.e. volcanic deposition). This, along with their Colombo mission. comparative youth relative to the basins they occupy, The Surface of Mercury: Only 45% of the and some irregular depressions which suggest volcanic surface of Mercury was imaged by Mariner 10, at only origin, all imply that volcanism has played an impor- 2 wavelengths, and only ~1% with a resolution of bet- tant part in Mercury’s past. There are suggestions that a ter than 0.5km. Our knowledge of the surface of Mer- highly differentiated magma may have produced more cury is therefore very limited, but what we do know alkaline basalts (i.e. those rich in Na and K, [3]) an X- tells us that any resemblance to the Moon is superficial. ray spectrometer such as CIXS is ideal to detect such Mercury has a systematically higher albedo at visible elements. wavelengths than comparable terrains on the Moon, but Spectra of Mercury have average spectral at UV wavelengths has a very much lower albedo, in- characteristics midway between Na-rich plagioclase dicating that the surface composition is very different. (NaAlSi3O8), and Ca-rich plagioclase (CaAl2Si2O8) Mercury can be split into a number of terrains. implying that Mercury may be more Na-rich than the The cratered areas are analogous to the lunar high- lunar surface (which is mainly of the Ca-rich variety). lands and represent the oldest terrain on Mercury. Im- An X-ray spectrometer with sufficiently high spectral pact craters are a probe into the crust, and hence resolution will be able to resolve the Na line from the studying their features and ejecta may provide insight neighbouring Mg line. In addition, by observing the X- into different compositional layers beneath the surface. ray albedo of the regolith at different phase angles, we The highest albedo areas on Mercury are all associated will be able to probe the fine structure of the soil. This with crater rays (fresh material ejected from impact new technique [4] is being prototyped on DCIXS, an craters) supporting the idea of different compositions X-ray spectrometer due to fly on the SMART-1 mis- and/or maturities being related to impact ejecta. sion to the Moon [5]. There is controversy over the origin of the The Origin and Evolution of Mercury: plains units on Mercury, which could either be of im- There are a number of theories relating to the origin pact origin or of a volcanic nature. The intercrater and evolution of Mercury, relating specifically to its plains are older than smooth plains, since they have a high uncompressed density compared to the other ter- greater crater density, and cover the largest area of all restrial planets. Each theory predicts different compo- the terrains imaged. They show evidence of all the sitions of the present day planet. phases of Mercury’s geological history except the very 1. Selective accretion: This theory predicts that Fe is earliest, and hence are believed to be around 4 billion enriched due to dynamical and mechanical accre- years of age [2]. They therefore represent one of the tion processes in the innermost part of the Solar oldest materials still visible on the surface, and it is of System. In this model, the silicate portion of Mer- great importance to determine their true nature. An X- cury should contain around 3.6-4.5% alumina, 1% ray spectrometer will be able to distinguish between a alkali oxides (Na2O and K2O) and between 0.5 and volcanic and impact origin as it is expected that abun- 6% FeO [6]. MgO is expected to vary between 32 dances of Al and Mg in particular would differ between and 38wt% [7]. the two (Al would be richer in the impact ejecta, Mg 2. Post accretion vaporisation: proposes that the solar richer in the volcanic rocks). Their wide extent over the wind in the early phases of the Sun’s evolution va- whole of the imaged side of Mercury means that only a porised most of the silicate portion of Mercury. In global survey of the planet can obtain a complete pic- this scenario, there should be almost no alkali ox- ture of the nature of these deposits. The smooth plains ides present at all, with very little FeO and a high are much younger and appear to be related to impact enrichment of refractory elements (CaO and basins in the most part. Smooth plains are analogous to Al2O3) to the level of over 30wt%. MgO would Mercury: Space Environment, Surface, and Interior (2001) 8040.pdf

X-RAY SCIENCE ON BEPICOLOMBO: S. K. Dunkin, M. Grande and B.J. Kellett

also be enriched, with an abundance of 30-40wt% spectra observed at Mercury, on which will be super- [8]. imposed the fluorescent lines which we observe. Lo- 3. Giant impact: a large, planet sized body impacted calised peak emission could be significantly brighter, into Mercury, stripping it of most of its outer sili- since our calculation for Mercury is based on average cate crust and mantle. In this case the FeO content fluxes. Moreover, since all these estimates are based on will be much the same as for the selective accre- a simple scaling of the terrestrial magnetosphere, the tion scenario, but with a depletion in both alkali real situation may be very different. and refractory elements (0.01-0.1% and 0.1-1% The interaction of electrons with the surface will also respectively) (see [3]). produce some line emission. The majority of the en- An important point to note is that these abun- ergy of a 100 keV electron striking the surface goes dances are bulk crust/mantle abundances and therefore into ionisation, and of order 2% ends up as fluores- only a global elemental dataset can fully address the cence. In the case of the Moon there have also been question of which theory most accurately represents the observations of X-rays produced by the high energy present day composition of Mercury. Clearly, many of tail of the solar wind electron striking the surface [17], the important differences are to be seen in elements and energy stored in high charge states of minor ions such as Al, Na, Ca, Fe, and also Mg and K, all of will also be released. It is anticipated that in the case of which can be observed with an X-ray spectrometer that the magnetopause being driven down to the surface of covers the energy range 0.5-10keV and which has suf- Mercury [9], these mechanisms would provide strong ficiently high resolution to resolved such lines emission from the dayside, enabling CIXS to remote (~150ev). sense the global magnetospheric connectivity. Magnetospheric science: Mariner 10 re- Thus, in addition to quantitative surface analysis and vealed the presence of a highly active magnetosphere mapping using the X-ray fluorescence technique, we around Mercury. The field observed is compatible with will also be able to provide valuable insights into the a strong dipole, although higher order terms and magnetospheric precipitation. These comments of anomalies may be more important than for the Earth. course directly imply that very efficient monitoring of Explaining the presence of this field is a highly com- the input solar spectrum is required in order to decon- plex question, which could hinge on establishing the volve the different effects. concentrations of trace elements such as sulphur. While Summary: ESA’s BepiColombo mission and NASA’s the overall picture of the magnetosphere may resemble Mercury Messenger mission will start to unravel many that of the Earth, the end result of space weather effects of the mysteries of Mercury. An X-ray spectrometer on Mercury must be very different. Recently [10] has will provide a vital addition to the instrument comple- presented a model in which the magnetosphere is al- ment of both missions, addressing many of the most most entirely driven, with reconnection on a global important issues reamining about this planet. scale occurring during periods when the interplanetary magnetic field is southward. [11] show images of Na References: [1] Grande M., Dunkin S.K., Kellett emission in the exosphere, which are highly variable, B.J., this volume [2] Strom R.G., Neukum G. (1988), in and may be the footprints of such activity. Mercury, University of Arizona Press [3] Strom R.G., (1997) Mariner 10 found clear signatures of behav- Adv. Space Res., 19, 1471 [4] Muinonen K., et al. (2001) Planetary and Space Science, in press [5] Grande M. et al. iour very reminiscent of terrestrial substorms. Bursts of (2001) Planetary and Space Science, in press [6] Lewis J.S. electrons were observed with energies of several hun- (1988) in Mercury, University of Arizona Press [7] Goettel dred keV [12]. The precipitating auroral electrons K.A., (1988) in Mercury , University of Arizona Press [8] could result [13] in a total energy deposit, during an Cameron et al. (1988) in Mercury , University of Arizona 11 inferred duration of order ten seconds, of between 10 Press [9] Slavin J. A., Holzer R. E., (1979) JGR., 84, 2076 J and 1014 J. Such precipitation would be a copious [10] Luhman, J. G.; et al. JGR, 103, 9113 [11] Potter A, E, source of X-rays [14,15]. At the Earth, the PIXIE X- Killen, R, M and Morgan T, H (1999), Planet. Space Sci., ray imager recently launched on POLAR has produced 47, 1441-1448 [12] Russell C. T.,D. N. Baker and J. A. [eg 16] the first global X-ray images of the aurora. In Slavin, (1988) in Mercury University of Arizona Press [13] general, the bremsstrahlung spectrum has roughly the Baker D. N. et al. (1987), JGR. 92, 4707 [14] Grande, M (1997) Adv. Space Res. 19, 1609-1614 [15] Grande M., same spectrum as the incident electrons. An estimate -4 Dunkin S.K., Kellett B.J., (2001) Planetary and Space Sci- for Mercury is an overall efficiency of 10 [14], im- ence, in press [16] Anderson P. D. et al. (1998), Geophys. plying a peak average emission which is comparable to Res. Lett. 25 , 4105 [17] Schmitt J. H. M. M. et al. (1991) the coherent scatter of reflected solar X-rays. These Nature 349, 583. two components will make up the main continuum in