Diagnóstico e identificación molecular de los aislamientos de

The discovery that Mercury is relatively enriched in volatiles has meant that the formation of the planet has had to be reconsidered (PEPLOWSKI ET AL.,2011). The

partial vaporisation and giant impact formation mechanisms introduced in Section 2.2, which were put forward to explain the presence of a disproportionately large iron core within Mercury, are no longer viable since they would involve a short period of intense heating of the planet, which would be expected to leave the planet depleted in volatile species (PEPLOWSKI ET AL.,2011).

Fig. 2.13 Distribution and orientation of lobate scarps on Mercury mapped by WATTERS ET AL.(2015). (a) Map of lobate scarps >50 km in length superimposed on a map

of smooth plains (light yellow) by DENEVI ET AL.(2013). (b) Double-headed arrows indicate local fault orientation and are scaled to reflect total local fault length. Equatorial lobate scarps are generally north–south, whereas higher latitude scarps are more often east–west. (c) Histogram of total fault length binned by longitude. There is an apparent marked decrease in lobate scarp length between 0–60°E and 150–240°E, probably due to smooth plains from Borealis Planitia and the Caloris smooth plains. From WATTERS ET AL.(2015).

One new suggestion is that Mercury was a hit-and-run impactor whose core was not catastrophically disrupted by the collision and instead migrated away from the collision site (ASPHAUG AND REUFER,2014). If Mercury formed farther out in the

solar system from its present day orbit then it might have accreted a volatile-rich silicate shell larger than it currently possesses. Many numerical simulations of planetary formation predict hit-and-run collisions between proto-planets and proto-planetary migration (ASPHAUG AND REUFER,2014). It is possible that a larger,

naturally volatile-rich Mercury, which formed farther out in the solar system, migrated inward and was involved in a hit-and-run collision. By having Mercury move away from the site of the giant impact, it could not reaccrete much of its silicates stripped by the collision, explaining how the planet has a large core but only a thin mantle (ASPHAUG AND REUFER,2014). However, it would seem that this

model would cause heating of Mercury that would drive off the planet’s volatiles. Whether more volatiles could be retained during multiple, low-energy hit-and-run collisions is unclear.

Currently, the most favoured formation model for Mercury is the iron-rich accretion model of WEIDENSCHILLING (1978). Recently, it has been suggested that

the solar magnetic field in the protoplanetary disk aided rapid accretion of iron- rich aggregates at Mercury’s present-day orbit (KRUSS AND WURM,2018). After the

iron-rich proto-Mercury reached a sufficient size, it would be able to accrete and retain chondritic material that would endow it with the volatile-rich crust observed today (PEPLOWSKI ET AL.,2011).

2 . 5 . 5 Summary

In summation, MESSENGER has shown that Mercury is a unique planet in the solar system. Its bulk composition (volatile- and metal-rich) means that the planet had a formation and early development unlike any of the other terrestrial planets. Furthermore, the cooling of the relatively large core means that Mercury’s lithosphere has been in compression for most of the planet’s history, which has dominated the planet’s tectonics and influenced the expression of its volcanism. The volatile content of Mercury is demonstrated by hollows probably active today and explosive volcanism that has persisted through Mercury’s geological history, and it remains to be seen what further unexpected landforms and processes these volatiles could have contributed to. A graphical summary of the geological history of Mercury based on MESSENGER data is shown in Fig. 2.14.

Fig. 2.14 Summary of the geological history of Mercury. On the right is an absolute

timescale. The time systems of Mercury are shown to the left of the absolute time scale. The basal ages of the Kuiperian and the Mansurian are taken from (BANKS ET AL., 2017). Approximate basal ages for the Calorian and Tolstojan are taken from (SPUDIS AND GUEST, 1988). The base of the Pre-Tolstojan is undefined. To the left of the Hermean time systems are graphical representations of the prevalence and duration of various geological processes during Mercury’s history. References containing the absolute ages indicated in this figure are labelled appropriately. Impact cratering is thought to have been very intense on Mercury before the end of the Late Heavy Bombardment of the inner solar system ~3.8 Ga. As an airless body, impact cratering remained an important geological process throughout Mercury’s history, albeit at a reduced rate. Lobate scarps are thought to have increased in activity following an initial stage of planetary expansion (for which there is no observable geological evidence) during core formation and reached peak activity ~3.5 Ga. After this, lobate scarp activity declined due to the decrease in the rate of secular cooling, but new, young lobate scarps continued to be formed until the relatively recent past. Effusive volcanism, which generated the widespread intercrater plains, was most active before the onset of global contraction. The last large-volume effusive eruptions formed the smooth plains, and these eruptions ceased at around the time lobate scarp activity reached its peak. Explosive volcanism was presumably active during the same interval as effusive volcanism, however there is no evidence that it resurfaced substantial areas of Mercury’s surface, so it must be assumed that this process has always been much less dominant than effusive volcanism when both eruptive styles were active together. However, the superposition of putative explosive volcanic deposits on smooth plains suggests that explosive volcanism outlasted effusive volcanism. Evidence for Kuiperian explosive volcanic activity has been reported, but most volcanic activity appears to have ceased before this. All hollows appear geologically recent, but the materials for their formation appear to be endogenic, which suggests that hollow formation could have occurred throughout Mercury’s history.

In Table 2.1 I summarise MESSENGER-derived properties of Mercury as a reference for the reader.

Physical properties

Mass 3.30×1023 _{kg (MAZARICO ET AL.,2014)}

0.055 Earth masses

Mean radius 2439.36 km (PERRY ET AL.,2015)

0.38 Earth radii

Surface gravity 3.70 ms−2 _{(MAZARICO ET AL.,2014;PERRY} ET AL.,2015)

0.38g Axial tilt (relative to the normal of the

orbital plane)

2.06’ (0.034°) (MAZARICO ET AL.,2014)

Rotational period 58.6 Earth days (MAZARICO ET AL.,2014)

Orbital characteristics

Aphelion 0.47 AU

Perihelion 0.31 AU

Orbital period 88 Earth days

Table 2.1 Mercury planetary properties measured by MESSENGER.

2 . 5 . 6 BepiColombo

BepiColombo (BENKHOFF ET AL., 2010), the next spacecraft mission to Mercury,

launched on 20th _{October 2018. It is a joint European Space Agency (ESA) and}

Japan Aerospace Exploration Agency (JAXA) mission comprising two spacecraft with scientific payloads, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO), designed to study Mercury after orbital insertion on 5th _{December 2025. ESA delivered MPO, which will focus on studying the}

surface of the planet (ROTHERY ET AL.,2010).

Before BepiColombo arrives, it is important that as much as possible is known about Mercury based on MESSENGER data. From a planetary geology standpoint, the most valuable tools for generating science targets and objectives for a new spacecraft mission are detailed geological maps. As stated previously, MESSENGER data have been used to create the first global geological map of Mercury (PROCKTER

ET AL.,2016;KINCZYK ET AL.,2018B). This map is being produced at a small map scale

(1:15M), which is useful for a global picture of the planet, but it does not exploit the maximum resolution of MESSENGER data. Larger scale quadrangle geological maps are being produced using MESSENGER data to provide science context and targets for BepiColombo (GALLUZZI ET AL.,2019). These maps are being produced

for publication at the 1:3M-scale, which allows greater detail to be shown on each quadrangle map than can be shown on the global map. MESSENGER-era quadrangle geological maps of three quadrangles have been published so far

(GALLUZZI ET AL.,2016;MANCINELLI ET AL.,2016;GUZZETTA ET AL.,2017) and the maps

of the remaining quadrangles are to be published before the arrival of BepiColombo (GALLUZZI ET AL.,2019).

Fig. 2.15 The BepiColombo spacecraft and the instruments that will study Mercury’s geology. (a) Exploded artist’s conception of the BepiColombo spacecraft stack. MMO:

Mercury Magnetospheric Orbiter. MOSIF: MMO Sunshield and Interface Structure. MPO: Mercury Planetary Orbiter. MTM: Mercury Transfer Module. (b) The instruments of MPO. SYMBIO-SYS: Spectrometers and Imagers for MPO BepiColombo Integrated Observatory System. Encompasses: Stereo Channel (STC) for colour and monochrome imaging of the whole planet at 50 m/pixel resolution or better; the High Spatial Resolution Imaging Channel (HRIC) for 5 m/pixel colour and monochrome imaging, and; Visible and near- Infrared Hyperspectral Imaging channel (VIHI) for imaging in 256 narrow channels (400– 2,000 nm) at up to 100 m/pixel. BELA: BepiColombo Laser Altimeter for measuring the topography of Mercury at all latitudes. MERTIS: Mercury Radiometer and Thermal Infrared Spectrometer for mineralogical, temperature, and thermal inertia mapping. MIXS: Mercury Imaging X-ray Spectrometer for element mapping, including Si, Ti, Al. Fe, Mg, Na, Ca, P, Mn, K, S, Cr, Ni, and O. SERENA: Search for Exosphere Refilling and Emitted Neutral Abundance. Encompasses Emitted Low-Energy Neutral Atoms (ELENA), Start from a Rotating Field Mass Spectrometer (STROFIO), Miniature Ion Precipitation Analyser (MIPA), and Planetary Ion Camera (PICAM). Together these instruments will identify the sources, sinks and compositions of exospheric atoms and ions.