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Internal structure, the magnetic field and magneto-sphere
Models predict that the core of Mercury is very large (42% in volume) (see Fig. 2.5) and consists of mostly metallic (70%) and silicate (30%) materials, making it one of the densest planets in the solar system (5.43 g cm−3), only topped by Earth (5.51 g cm−3).
While the Moon completely lacks a magnetic field, Mercury possesses a very large liquid iron core sustained by tidal eﬀects and generating through a dynamo eﬀect a weak but stable dipolar magnetic field, probably fueled by the cooling of the core and the contraction of the entire planet after the ”late heavy bombardment” period, between 4.1 and 3.8 billion years ago (Breuer et al., 2007; Glassmeier et al., 2007).
The intensity of the magnetic field reaches around 300 nT at the equator, which is about 1.1% the corresponding value of Earth’s. Following the recent measurements of the MESSENGER probe, the magnetic field poles were found to be tilted between 5 and 12◦ from the spin axis of the planet (Anderson et al., 2008).
The stable magnetic field is strong enough to make an obstacle to the solar wind creating a magnetospheric cavity probed by Mariner 10 and more recently by MESSENGER (Fig. 2.5). The magnetosphere traps solar wind ions and electrons that participate to the space weathering of the surface.
During its first flyby on 14 January 2008 followed by a second flyby on 06 October 2008, NASA MESSENGER showed that Mercurys magnetic field can be extremely dynamic (Slavin et al., 2008, Fujimoto et al., 2007) with the discovery of recurrent flux transfer events being a signature of magnetic reconnection between the solar wind frozen-in field and the planet’s own magnetic field (Slavin et al., 2009; Slavin et al., 2009).
MESSENGER’s third flyby on 29 September 2009 showed that the solar wind plasma can then be allowed to enter the magnetosphere in bursts lasting 2 to 3 minutes and at a uniquely high rate owing to the proximity of Mercury to the Sun (Slavin et al., 2010). Following this new picture, two sources are thought to contribute to the existence of a magnetospheric plasma: the solar wind (through reconnection or direct entry along open field lines), but also pick-up ions created by UV photoionisation and electron impact ionisation of the neutral exosphere and subsequently driven along magnetic field lines (Slavin et al., 2008).
Accelerated by intense electric fields in the magnetotail, the magnetospheric plasma can in turn impact the surface of the planet and release volatiles (Orsini et al., 2007). In doing so, it contributes as an additional source to the formation of the neutral exosphere. Charged particle acceleration mechanisms occurring at Mercury are now under close investigation by MESSENGER (Zelenyi et al., 2007). The study of the magnetosphere of Mercury, its energy content and dynamics is only starting.
Morphologically speaking, Mercury shows an astonishing similarity to the Moon (see Figure 2.2) with its mare-like plains, montes, planitiae, rupes and valles, and heavy cratering (Figure 2.6), proving that it has been partly geologically inactive for billions of years.
Strom et al. (2008) concluded from new observations of the cratering record during the first flyby of MESSENGER that the plains formed no earlier than 3.8 billion years ago while lower densities of craters on certain basins (like Raditladi) may be younger than 1 billion year. The curious feature of high ridges in Mercury’s surface is believed to be due to the cooling of the core and mantle and their subsequent shrinking, when the surface had already solidified (Solomon et al., 2008).
The discovery of volcanic evidence at Mercury was one of the main highlights of the first MESSENGER flyby (Head et al., 2008) and is thought to explain the formation of plains on Mercury. By comparison with the Moon, high resolution images (150 m) showed deformation vents of pyroclastic origin around the Caloris basin (the youngest known large basin) which were significantly diﬀerent from the lunar impact basins known properties (albedo, geomorphological structure) (Murchie et al., 2008).
New images and data from the second and third flybys suggested that the role of volcanism was predominant in explaining these formations, while volcanic activity might have spanned previously unsuspected long periods, from the planet’s formation to well into the second half of its history (Prockter et al., 2010).
Mercury, with an equatorial radius of 2440 km, is the smallest planet in the solar system but is also too small to retain a thick atmosphere. However, Mariner 10, ground-based instruments and now MESSENGER have shown that a tenuous surface-bound exosphere exists, where barely any collisions between constituents occur. It is mainly composed of (see Hunten et al., 1988; Killen et al., 2007, Table 2.1):
• Light species such as hydrogen (H and H2)
• Noble gases such as helium
• Alkali species such as sodium Na, potassium K, calcium Ca and magnesium Mg.
• Heavier species such as oxygen O, with expected traces of molecular oxygen O2, nitrogen N2 and carbon dioxide CO2
The relative abundances of these species are badly constrained and need further investigation from current and upcoming space missions (Killen et al., 2007). It is also believed that Mercury has traces of water ice H2O located in the eternally dark polar regions. Species such as C, CO, CO2, N2 and Li have been suggested (Sprague et al., 1996; Hunten et al., 1988) but were never detected, while hypothetical OH or S species were suggested to account for bright deposits at the poles.
According to models and observations (see for instance Leblanc and Johnson, 2003), the neutral species originate from the surface of Mercury and are released in the atmosphere by a number of processes that will be detailed in Chapter 2. They can be classified into three categories:
• Photon- and low energy electron-induced mechanisms (photon-stimulated desorp-tion, electron-stimulated desorption) due to electronic excitation of a surface atom
• Sputtering eﬀects due to impact (solar wind, magnetospheric ions, meteorites)
• Thermal vaporization of atoms (hydrogen and alkali atoms mostly) due to the high surface temperature of Mercury as a dayside and a nightside component (see Milillo et al., 2005) The column density of observed constituents has an upper limit of 1012 cm−2 (Killen and Ip, 1999).
During the diﬀerent flybys of Mariner 10 in 1974 (Shemansky and Broadfoot, 1977) and more recently by MESSENGER (discovery of Mg by McClintock et al., 2009) as well as ground-based observations (discovery of Na and K by Potter and Morgan, 1985, 1986, and of Ca by Bida et al., 2000), the exospheric emissions arising from the excitation (resonant or not) of neutral species were observed and can serve to get an estimate of exospheric neutral densities.
Vervack et al. (2010) performed observations with the spectrometer on board MES-SENGER of Na, Ca and Mg atoms with evidence of Ca+ ions extending far back in the magnetotail. The neutral species exhibited very diﬀerent distributions (for instance, sodium Na displayed a two-temperature distribution, while the weak Mg emission appeared to be nearly uniformly distributed) which indicated that multiple processes might drive the formation of these exospheric species.
History and science: Ancient
Surprisingly old records of Mercury have been discovered. The Mul.Apin tablets, dating back to 500 BC account for observations describing the risings and settings of stars believed to have been made by Babylonian astronomers around 1000 BC. The tablets, containing the earliest and most comprehensive surviving star catalogue from Mesopotamia, refer to Mercury as Ninurta, a planet who ”rises or sets in the east or in the west within a month” (Hobson, 2009).
The Greeks knew around 450 BC that there were five planets, called Astra Planeta −the wandering stars−, and named them Phainon (Saturn), Phaethon (Jupiter), Pyroeis (Mars), Eosphoros (Venus), and Stilbon (Mercury). Believing Mercury to be two planets they named it Stilbon/Apollo in the morning and Hermeaon/Hermes in the evening. However around the 4th century BC they came to realise that it was two aspects of the same planet, naming it as the messenger of the gods, Hermes. The Romans later named the planet ’Mercury’, in reference to the Greek God Hermes in their mythology.
Cicero, the famous Roman rhetorician of the first century BC, mentioned Mercury in his De Natura Deorum:
Below this [the orbits of Saturn, Jupiter and Mars] in turn is the Stella of Mercurius, called by the Greeks Stilbon (the gleaming), which completes the circuit of the zodiac in about the period of a year, and is never distant from the sun more than the space of a single sign, though it sometimes precedes the sun and sometimes follows it.
In Southern America, the Mayas charted the motion of the visible planets including Mercury. Records of their detailed observations are found in the Dresden Codex, a Mayan book from 1000 AD, that is deemed to be a copy of an earlier text at least 300 years older. The astonishingly accurate text describes Mercury’s visibility, location in the sky and a description of conjunctions with other planets at specific dates. The Mayans also calculated that Mercury would rise and set in the same place in the sky every 2200 days.
The Mayan had the notion of Mercury as the messenger of the gods of the underworld Xibalba, represented by four owls, two for the waxing/waning morning star aspect and two for the waxing/waning evening star aspect (Tedlock, 1985). Skull owl and Shooting owl, two of these aspects, can be seen in Figure 2.7.
Around 100 AD the Roman-Egyptian astronomer Ptolemy described a complex system of circles to describe the motion of the planets, based on a geocentric world view. In this system, Earth was in the middle instead of the Sun, and then Mercury went in a circular orbit around it, moving in a epicycle, see Figure 2.8. Later, in his large treatise Almagest, Ptolemy correctly suggested that no transits of planets, such as Mercury, across the face of the Sun had been observed either because it was too small to see, or because the transits were too infrequent.
Science: In modern times
Early Optical observations
Even though Mercury,is a very bright object in the sky whith magnitude varying from −2.6 to +5.7 depending on its orbital location, it is very hard to observe the planet from Earth because of its close proximity to the Sun and because of its eccentric orbit. Therefore it is only possible to see it, by eye, in morning or evening when the Sun has not yet risen or when the Sun has just gone down the horizon.
The diﬃculty in observing Mercury from the ground has led to it being the planet least studied, with most erroneously attributed features that have later been disproved. The first known observation using a telescope was made by Galileo early in the 17th century but his telescope was not powerful enough to observe any phases.
In 1631 Pierre Gassendi observed the transit of Mercury on the Sun, as predicted two years before by Johannes Kepler (from his newly completed Rudolphine tables): he used a telescope to project an image 20 cm in diameter of the sun upon a white screen (see Gassendi and the Transit of Mercury, Nature, 128, 3236, 787, 1931).
Using a telescope only slightly more powerful than Galileo, Giovanni Battista Zupi (Zupus) discovered in 1639 that Mercury had phases just like the Moon and Venus, a potent evidence that Mercury was orbiting the Sun and giving strength to the argumentation of heliocentric theories, Figure 2.10. This was later observed independently by Johannes Hevelius in 1644 who also observed a transit in 1661 (Hevelius, 1662), see Figure 2.11.
The very rare occultation of Mercury by Venus in 1737 was recorded by John Bevis. The next one is expected to take place in 2133.
By the turn of the 19th century, with the increasing size of reflecting telescopes, several scientists, like Johann Schr¨oter and his assistant Karl Harding in Germany, Giovanni Schiaparelli in Italy, had described some surface features of Mercury, Figure 2.12. Schr¨oter claimed that due to irregularities in brightness, mountain ranges existed on Mercury, among which one towered at 18 km (Schr¨oter, 1800). Both Schr¨oter and Bessel (1813) incorrectly derived a period of 24 hours with a rotation axis at 70◦ to the orbital plane. At the end of the same century, Schiaparelli estimated a new, though incorrect, value of 88 days for the period (Schiaparelli, 1891), which was not corrected until the advent of radar observations in 1965. By his own words:
Among the planets known by the ancients, none is so diﬃcult to observe as Mercury and none presents as many diﬃculties for the study of its orbit as well as its physical nature. Being impossible to observe during the night, and rarely possible during twilight, there is no other solution than studying it in full daylight, in the presence of the Sun always in the vicinity, and through an illuminated atmosphere.
Concerning the mapping of Mercury, the Greek astronomer Eugene Antoniadi published in 1934 a book in French containing both maps and observations, mostly based on Schiaparelli’s works from the 1880s and on Mercury’s supposed Sun-synchronous rotation (Antoniadi, 1934). A very interesting historical account of Mercury’s optical cartography eﬀorts can be found in McEwen (1935), Dollfus (1953), who resolved features down to 300 km, and more recently in the amateur astronomer’s paper of Frassati et al. (2002).
From 1841 to 1859, the French astronomer Urbain Le Verrier, encouraged by his successful prediction of the presence of Neptune using celestial mechanics, observed an anomalous rate of precession of Mercurys orbit around the Sun that could not be completely explained by Newtonian mechanics and perturbations by the known planets (Le Verrier, 1859). Le Verrier analysed available timed observations of transits of Mercury over the Sun’s disk from 1697 to 1848 and showed that the measured precession rate disagreed with Newton’s theory by an amount initially estimated as 38” (arc seconds) per century and later re-calculated at 43”.
As an analogy with the analysis of the motion of Uranus that led to the discovery of Neptune as a perturbing body, Le Verrier proposed that the motion of Mercury was perturbed by a hypothetical planet or group of corpuscules orbiting inside Mercury’s orbit and soon baptized Vulcan. General Relativity developed by Albert Einstein in the 1910s gave an explanation to the observed advance of perihelion in terms of space-time curvature.
In 400 years, Mercury, this little wandering rock which might look insignificant, had helped to install two major new theories at the time: heliocentrism and general relativity.
Modern ground-based observations
Use of radar techniques
The first modern radio observation was in 1962 when Soviet scientists under V. Kotelnikov in Crimea managed to reflect not less than 53 radar signals oﬀ the surface of Mercury. Three years later, using the newly built incoherent scatter radar of Arecibo (Puerto-Rico), Pettengill and Dyce (1965) showed that its rotational period was 59 ± 5 days completely overturning all that had been believed for the past 150 years. As any change of scientific paradigm, the implications were fascinating, indicating that ”either the planet has not been in its present orbit for the full period of geological time or that the tidal forces acting to slow the initial rotation have not been correctly treated previously” (Pettengill and Dyce, 1965).
Between 1960 and 1961, it was also confirmed by analysis of the planetary radio wave emission between 3.45 and 3.75 cm wavelength with the 25.9 m reflector of Michigan University that Mercury’s nightside was warmer than commonly assumed (Howard, 1962), questioning the accepted idea of the 1:1 ratio orbit, as this would have left the planet much colder on the nightside.
Dyce, Pettengill and Shapiro (1967) from the Smithsonian Astrophysical Observatory (SAO) were working on the interpretation of the new puzzling radar data, when Giuseppe ’Bepi’ Colombo (1920 − 1984), an Italian astronomer who was a visiting scientist of SAO, hypothesised that Mercury could be in a 3:2 resonance (Colombo, 1965). As a main collaborator of Colombo (Colombo and Shapiro, 1966), Shapiro recalls:
Colombo realized almost immediately that 58.65 days was exactly two-thirds of 88 days. Mercury probably was locked into a spin such that it went around on its axis one-and-a-half times for every once around the planet. The same face did not always face the Sun. That meant that […] the orbital motion and spin rotation of Mercury were very closely balanced, so that Mercury almost presented the same face to the Sun during this period.
Irwin Shapiro, quoting Giuseppe Colombo, in Butrica (1996).
The 3:2 resonance was later confirmed at the arrival of the American mission Mariner 10 in 1974 and explained why observers had always managed to see only one side of Mercury since it is always showing the same side every second orbit, while all other sides would not be properly viewed due to poor viewing conditions. The formidable intuition of Colombo fostered many studies on the origin and consequences of the rotation of Mercury from Liu and O’Keefe (1965) to Correia and Laskar (2010).
The story of Colombo with Mercury did not stop there: he also suggested in 1970 how to put the Mariner 10 spacecraft into an orbit that would bring it back repeatedly to Mercury, using the gravitational slingshot maneuver provided by Venus to bend its original flight path and decelerate the spacecraft on its way to the inner solar system.
Table of contents :
1.0.1 Layout of thesis
2 Mercury: History and context
2.1 The Planet Mercury
2.1.1 Orbital parameters
2.1.2 Internal structure, the magnetic field and magnetosphere
2.2 History and science: Ancient
2.3 Science: In modern times
2.3.1 Early Optical observations
2.3.2 Modern ground-based observations
2.3.4 Modelling the exosphere of Mercury: a brief history
3 SPICAV: Differentiate ultraviolet signatures
3.1 Stellar occultations and calibrations of the SPICAM and SPICAV instruments
3.2 The UV spectrometers on board the Mars Express and the Venus Express missions
3.2.1 General overview of the instruments
3.2.2 SPICAM and SPICAV datasets
3.3 Star calibration
3.3.1 Theoretical background
3.3.2 Observation of intensity decrease in high wavelengths
4 PHEBUS: Instrumentation for a harsh environment
4.1 Radiometric Modelling and Scientific Performance of PHEBUS
4.2 Theoretical background
4.2.2 Objectives and demands on the instrument
4.2.4 Optical layout
4.2.5 Photometric assessment and spectral resolution
4.3 Theoretical Results
4.3.1 Radiometric modelling of star spectra
4.3.2 In-flight calibrations of stars
5 Modelling Mercury’s hydrogen exosphere
5.2 SECTION I: The physics behind
5.2.1 Definitions and basic exospheric theory
5.2.2 Mechanisms of ejection from the surface: Maxwell-Boltzmann distributions
5.2.3 Temperature mapping
5.2.4 Ballistic motion of particles in the exosphere and external conditions
5.2.5 Sources of hydrogen at Mercury: Thermal processes
5.2.6 Sinks of hydrogen at Mercury: Ionisation
5.2.7 Deriving emission line brightness: radiative transfer and optical thickness (with Jean-Yves Chaufray)
5.3 SECTION II: Monte Carlo model
5.3.1 Coordinate system
5.3.2 Flow of program
5.3.3 Time evolution of the particle
5.3.4 Euler solution to ballistic motion
5.4.1 Convergence criteria
5.4.3 Thermalisation at surface
5.5 Sensitivity study
5.5.1 MB and MBF Velocity distribution functions
5.5.2 Accommodation coefficient
5.5.3 Source regions
5.5.4 Comparison to Mariner 10 data
5.5.5 Prediction of expected PHEBUS signal