Mercury Planetary Orbiter Research Articles

Development of the High-resolution FUV Detector for the BepiColombo Mission

The PHEBUS (Probing of Hermean Exosphere By Ultraviolet Spectroscopy) instrument on Mercury Planetary Orbiter in the BepiColombo mission is a dual FUV-EUV spectrometer (EUV: 55-155 nm, FUV: 145-315 nm). We are now developing the compact detector system sensitive to FUV airglow emissions of the Mercury. The FUV detector is required to have high spatial resolution (512×512 pixels) so that the wavelength resolution of the PHEBUS instrument should be 2 nm at the FUV range. The FUV detector consists of a Cs2Te photocathode, microchannel plates (MCPs), and a resistive anode encoder. In a position-sensitive system with a resistive anode encoder, the spatial resolution is determined by the signal-to-noise ratios at the anode terminals. Therefore, a high and stable electron gain of MCPs allows the position determination of each photoelectron event with high spatial resolution. We studied a method for achieving a high and stable electron gain. We fabricated a test model of the FUV detector incorporating a clamped pair of MCPs (V-stack) followed by a gap and a clamped triplet of MCPs (Z-stack) in cascade. We investigated the effect of the negative inter-stack potential on the PHD and the spatial resolution by means of calculation and experiments. As a result, the negative inter-stack potential made the electron gain more stable and the spatial resolution higher by ∼14%. In this paper we report the specific performance of the test model of the FUV detector.

Solar Intensity X-ray and particle Spectrometer (SIXS)

The Solar Intensity X-ray and particle Spectrometer (SIXS) on the BepiColombo Mercury Planetary Orbiter (MPO) will investigate the direct solar X-rays, and energetic protons and electrons which pass the Spacecraft on their way to the surface of Mercury. These measurements are vitally important for understanding quantitatively the processes that make Mercury's surface glow in X-rays, since all X-rays from Mercury are due to interactions of the surface with incoming highly energetic photons and space particles. The X-ray emission of Mercury's surface will be analysed to understand its structure and composition. SIXS data will also be utilised for studies of the solar X-ray corona, flares, solar energetic particles, and the magnetosphere of Mercury, and for providing information on solar eruptions to other BepiColombo instruments. SIXS consists of two detector subsystems. The X-ray detector system includes three identical GaAs PIN detectors which measure the solar spectrum at 1–20 keV energy range, and their combined field-of-view covers ∼1/4 of the whole sky. The particle detector system consists of an assembly including a cubic central CsI(Tl) scintillator detector with five of its six surfaces covered by a thin Si detector, which together perform low-resolution particle spectroscopy with a rough angular resolution over a field-of-view covering ∼1/4 of the whole sky. The energy range of detected particle spectra is 0.1–3 MeV for electrons and 1–30 MeV for protons. A major task for the SIXS instrument is the measurement of solar X-rays on the dayside of Mercury's surface to enable modeling of X-ray fluorescence and scattering on the planet's surface. Since highly energetic particles are expected to also induce a significant amount of X-ray emission via particle-induced X-ray emission (PIXE) and bremsstrahlung when they are absorbed by the solid surface of the planet Mercury, SIXS performs measurements of fluxes and spectra of protons and electrons. SIXS performs particle measurement at all orbital phases of the MPO as the particle radiation can occur also on the night side of Mercury. The energy ranges, resolutions, and timings of X-ray and particle measurements by SIXS have been adjusted to match with the requirements for interpretation of data from Mercury's surface, to be performed by utilising the data of the Mercury Imaging X-ray Spectrometer (MIXS), which will measure X-ray emission from the surface.

The Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) for the BepiColombo mission

Scheduled for launch on board the BepiColombo Mercury Planetary Orbiter (MPO) in 2014, the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is an innovative instrument for studying the surface composition and mineralogy of planet Mercury. MERTIS combines an uncooled grating push broom IR-spectrometer (TIS) with a radiometer (TIR), which will operate in the wavelength region of 7–14 and 7–40 μm, respectively. The spatial resolution of the MERTIS observations will be about 500 m globally and better than 500 m for approximately 5–10% of the surface. The thermal infrared range offers unique diagnostic capabilities to study the surface composition of Mercury. In particular, feldspars can easily be detected and characterized, because they show several diagnostic spectral signatures in the 7–14 μm range: the Christiansen feature, reststrahlen bands, and the transparency feature. In addition, MERTIS will allow the identification and mapping of elemental sulfur, pyroxenes, olivines, and other complex minerals. The scientific objectives of MERTIS include: (1) characterization of Mercury's surface composition, (2) identification of rock-forming minerals, (3) mapping of the surface mineralogy, and (4) study of surface temperature variations and the thermal inertia. In preparation for the MERTIS data interpretation, we are performing spectral measurements of appropriate analogue materials in the Planetary Emissivity Laboratory (PEL) and are building a spectral library (Berlin Emissivity Database (BED)) of these materials for a variety of grain sizes.

The fluxgate magnetometer of the BepiColombo Mercury Planetary Orbiter

The magnetometer (MAG) on the Mercury Planetary Orbiter (MPO) of the joint European–Japanese BepiColombo mission to planet Mercury is a low-noise, tri-axial, dual-sensor, digital fluxgate instrument with its sensors mounted on a 2.8-m-long boom. The primary MPO/MAG science objectives are to determine the spatial and temporal structure of the magnetic field in the Hermean system, in particular the structure and origin of the intrinsic magnetic field of Mercury. MPO/MAG has a dynamic measurement range of ± 2000 nT with a resolution of 2 pT during operation along the near-polar orbit of the MPO spacecraft around Mercury. MPO/MAG is designed to provide measurements with rates between 0.5 and 128 vectors/s. In cooperation with its sister magnetometer instrument, MMO/MGF on board the BepiColombo Mercury Magnetospheric Orbiter (MMO), MPO/MAG will be able to distinguish between temporal and spatial magnetic field variations in the magnetically closely coupled Hermean system.

Magnetic field investigation of Mercury's magnetosphere and the inner heliosphere by MMO/MGF

The Mercury magnetospheric orbiter (MMO) of the Japanese–European BepiColombo mission carries a dual-sensor magnetometer, MMO/MGF. The sensors are of the classical fluxgate type mounted on a boom. For redundancy, each sensor carries its own electronics and is connected to a different data processing unit. MMO/MGF can sample the magnetic field at a rate of up to 128 Hz. The resulting comparatively high time resolution of the magnetic field measurements, i.e., down to 8 ms, will be necessary when studying the dynamics of and processes within the Hermean magnetosphere, since the Mariner 10 observations have shown that their typical time scales are much shorter than in the Earth's magnetosphere, by about a factor of 30. The high time resolution will also be very useful for studying the evolution of the still young solar wind plasma as well as interplanetary shocks at 0.3–0.46 AU. Of course, MMO/MGF is also well-prepared to assist the sister magnetometer aboard the Mercury planetary orbiter, MPO/MAG, in measuring Mercury's intrinsic magnetic field, in particular by helping to distinguish between temporal fluctuations and spatial variations.

PHEBUS: A double ultraviolet spectrometer to observe Mercury's exosphere

Probing of Hermean exosphere by ultraviolet spectroscopy (PHEBUS) is a double spectrometer for the Extreme Ultraviolet range (55–155 nm) and the Far Ultraviolet range (145–315 nm) devoted to the characterization of Mercury's exosphere composition and dynamics, and surface–exosphere connections. This French-led instrument is implemented in a cooperative scheme involving Japan (detectors), Russia (scanner) and Italy (ground calibration). PHEBUS will address the following main scientific objectives relative to Mercury's exosphere: determination of the composition and the vertical structure of the exosphere; characterization of the exospheric dynamics: day to night circulation, transport between active and inactive regions; study of surface release processes; identification and characterization of the sources of exospheric constituents; detection and characterization of ionized species and their relation with the neutral atmosphere; space and time monitoring of exosphere/magnetosphere exchange and transport processes; study and quantification of escape, global scale source/sink balance and geochemical cycles synergistically with other experiments of BepiColombo (Mercury Sodium Atmospheric Spectral Imager (MSASI), Mercury Plasma Particle Experiment (MPPE) on Mercury Magnetospheric Orbiter (MMO); Mercury imaging X-ray spectrometer (MIXS), Search for exosphere refilling and emitted neutral abundance (SERENA) on Mercury Planetary Orbiter (MPO)). Two gratings and two detectors are used according to a specific, compact design. The spectrum detection is based on the photon counting method and is realized using micro-channel plate (MCP) detectors with Resistive Anode Encoder (RAE). Typical photocathodes are CsI or KBr for the extreme ultra-violet (EUV) range, CsTe for the far ultra-violet (FUV) range. Extra visible lines are monitored using a photo-multiplier (PM) that is also used in photon counting mode. In order to prevent sensitivity losses which are critical in UV ranges, a minimum of reflections is achieved inside the instrument using only an off-axis parabola and a set of holographic gratings. A one degree-of-freedom scanning system allows to probe, at the highest possible signal-to-noise ratio, selected regions and altitude ranges of interest. Different modes of observation will be used sequentially (vertical scans, along-orbit scans, grazing observations at twilight, etc.). During the mission, the instrument will be regularly calibrated on well chosen stars, in such a way to quantitatively estimate the overall degradation of the sensitivity of the instrument.

The BepiColombo mission to planet Mercury and the fundamental rôle of ISA accelerometer for the Radio Science Experiments

The ESA BepiColombo mission, scheduled for launch in 2013 with a two–satellite configuration, will represent an important step in the exploration of the Solar System and will contribute to fundamental physics. Among the main scientific objectives of this mission are the Radio Science Experiments: gravimetry and rotation of Mercury and tests of Einstein's theory of general relativity, to be achieved by the MPO (Mercury Planetary Orbiter) satellite. The ISA (Italian Spring Accelerometer) instrument will give a fundamental contribution to these experiments by providing a continuous measurement of the non–gravitational accelerations acting on the MPO. After a description of the mission, the role of the ISA accelerometer will be discussed, together with the current activity for its development.

Development of the EUV detector for the BepiColombo mission

An ultraviolet spectrometer, PHEBUS (Probing of Hermean Exosphere by Ultraviolet Spectroscopy) that is loaded onto the Mercury Planetary Orbiter in the BepiColombo mission is under development. The instrument, basically consisting of two spectrophotometers (EUV: 50–150 nm, FUV: 145–330 nm) and one scanning mirror, aims at measuring emission lines from molecules, atoms and ions present in the tenuous atmosphere of Mercury. The detectors employ microchannel plates as 2-D photon-counting devices. In order to enhance the quantum detection efficiencies, the surface of the top microchannel plates of EUV detector is covered with photocathode. This method enables us to identify weak atmospheric signatures such as neon (73.5 nm) and argon (104.8 nm), which could not be detected with conventional detector systems. This paper presents measurements of the performance characteristics of potassium bromide and esium iodide photocathodes, which have been evaluated for use in the EUV channel.

The BepiColombo Laser Altimeter (BELA): Concept and baseline design

The BepiColombo Laser Altimeter (BELA) has been selected for flight on board the European Space Agency's BepiColombo Mercury Planetary Orbiter (MPO). The experiment is intended to be Europe's first planetary laser altimeter system. Although the proposed system has similarities to the Mercury Laser Altimeter (MLA) currently flying on board NASA's MESSENGER mission to Mercury, the specific orbit and construction of the MPO force the use of novel concepts for BELA. Furthermore, the base-lined range-finding approach is novel. In this paper, we describe the BELA system and show preliminary results from some prototype testing.

ISA accelerometer onboard the Mercury Planetary Orbiter: error budget

We have estimated a preliminary error budget for the Italian Spring Accelerometer (ISA) that will be allocated onboard the Mercury Planetary Orbiter (MPO) of the European Space Agency (ESA) space mission to Mercury named BepiColombo. The role of the accelerometer is to remove from the list of unknowns the non-gravitational accelerations that perturb the gravitational trajectory followed by the MPO in the strong radiation environment that characterises the orbit of Mercury around the Sun. Such a role is of fundamental importance in the context of the very ambitious goals of the Radio Science Experiments (RSE) of the BepiColombo mission. We have subdivided the errors on the accelerometer measurements into two main families: (i) the pseudo-sinusoidal errors and (ii) the random errors. The former are characterised by a periodic behaviour with the frequency of the satellite mean anomaly and its higher order harmonic components, i.e., they are deterministic errors. The latter are characterised by an unknown frequency distribution and we assumed for them a noise-like spectrum, i.e., they are stochastic errors. Among the pseudo-sinusoidal errors, the main contribution is due to the effects of the gravity gradients and the inertial forces, while among the random-like errors the main disturbing effect is due to the MPO centre-of-mass displacements produced by the onboard High Gain Antenna (HGA) movements and by the fuel consumption and sloshing. Very subtle to be considered are also the random errors produced by the MPO attitude corrections necessary to guarantee the nadir pointing of the spacecraft. We have therefore formulated the ISA error budget and the requirements for the satellite in order to guarantee an orbit reconstruction for the MPO spacecraft with an along-track accuracy of about 1 m over the orbital period of the satellite around Mercury in such a way to satisfy the RSE requirements.

Future gravitational physics tests from ranging to the BepiColombo Mercury planetary orbiter

Milani et al. recently have published careful and fundamental studies of the accuracy with which both gravitational physics information and the solar quadrupole moment can be obtained from Earth-Mercury distance data. To complement these results, a quite different analysis method is used in the present paper. We calculate the first-order corrections to the Keplerian motion of a single planet around the Sun due to the parameterized post-Newtonian theory parameters {beta}, {gamma}, {alpha}{sub 1}, {alpha}{sub 2}, and {xi}, as well as corrections due to the solar quadrupole moment J{sub 2} and a possible secular change in GM{sub {center_dot}}. The Nordtvedt parameter {eta} that is used in tests of the strong equivalence principle also is included in this analysis. The expected accuracies are given for 1 yr, 2 yr, and 8 yr mission durations, assuming that the planet-planet and asteroid-planet perturbations are accurately known. The ''modified worst-case'' error analysis method that we use is quite different from the usual covariance analysis method based on assumed uncorrelated random errors, plus a bias that is fixed or that changes in a prescribed way. We believe this is appropriate because systematic measurement errors are likely to be the main limitation on the accuracy of themore » results. Our final estimated uncertainties are one-third of the errors that would result if a 4.5-cm rms systematic error had the most damaging possible variation with time. We discuss the resulting uncertainties for several different subsets of orbital and relativity parameters.« less

The Non-Gravitational Perturbations impact on the BepiColombo Radio Science Experiment and the key rôle of the ISA accelerometer: direct solar radiation and albedo effects

The paper is focused on the estimate of the impact of the non-gravitational perturbations on the orbit of the Mercury Planetary Orbiter (MPO), one of the two spacecrafts that will be placed in orbit around the innermost planet of the solar system by the BepiColombo space mission. The key role of the Italian Spring Accelerometer (ISA), that has been selected by the European Space Agency (ESA) to fly on-board the MPO, is outlined. In the first part of the paper, through a numerical simulation and analysis we have estimated, over a time span of several years, the long-period behaviours of the disturbing accelerations produced by the incoming direct solar radiation pressure, and the indirect effects produced by Mercury’s albedo. The variations in the orbital parameters of the spacecraft, together with their spectral contents, have been estimated over the analysed period. The direct solar radiation pressure represents the strongest non-gravitational perturbation on the MPO in the very complex radiation environment of Mercury. The order-of-magnitude of this acceleration is quite high, about 10−6 m/s2, because of the proximity to the Sun and the large area-to-mass ratio of the spacecraft. In the second part of the paper, we concentrated upon the short-period effects of direct solar radiation pressure and Mercury’s albedo. In particular, the disturbing accelerations have been compared with the ISA measurement error and the advantages of an on-board accelerometer are highlighted with respect to the best modelling of the non-gravitational perturbations in the strong radiation environment of Mercury. The readings from ISA, with an intrinsic noise level of about \(10^{-9}\,m/s^{2}/\sqrt{Hz}\) in the frequency band of 3·10−5–10−1 Hz, guarantees a very significant reduction of the non-gravitational accelerations impact on the space mission accuracy, especially of the dominant direct solar radiation pressure.

Present status of the BepiColombo/Mercury magnetospheric orbiter

BepiColombo is an interdisciplinary mission to Mercury, conducted as a joint project by ESA and JAXA: a cornerstone mission in the ESA Cosmic Vision Program and one of the major missions in the JAXA Space Science Program. The mission consists of two spacecraft: the Mercury planetary orbiter (MPO), a three-axis stabilized satellite dedicated to the remote sensing of Mercury, and the Mercury magnetospheric orbiter (MMO), a spin-stabilized satellite devoted to the study of the Hermean magnetosphere and its interaction with the solar wind. JAXA is responsible for development of the MMO and its operation. This paper gives an overview of the scientific payload of the MMO, which was formally selected in the fall of 2004 by JAXA.

Design and performance of the payload instrumentation of the BepiColombo Mercury planetary orbiter

High integration and miniaturization of payload suites have been identified by ESA as a possible strategy for future space missions. Driving argument for this effort is the wish for the future exploration of more solar system bodies in a relatively short time period. Standardizing the payload of missions may contribute to the flexibility of the mission, whereas the reduction of mass and volume could significantly reduce the all over costs. This approach was followed throughout the re-assessment of the payload of the Mercury Planetary Orbiter (MPO) for the BepiColombo mission. The exploration of Mercury imposes several challenges on the mission due to Mercury's environment: The orbiter payload instruments have to face a rather high thermal heat load and high radiation dose levels. Mass savings were intended throughout the instrumentation by definition of the instruments design to a higher level, while meeting the science requirements. The instruments design and the iteration towards a higher maturity level was carried out together with the Science Advisory group of BepiColombo.

On the concepts of a highly integrated payload suite for use in future planetary missions: The example of the BepiColombo Mercury planetary orbiter

Future low resource payload concepts will need to be developed from the viewpoint of a standard integrated payload suite where resources are dramatically reduced through high levels of integration and resource sharing. The study of this approach, its gains together with its limitations was the key objective of this work. The highly compact integration of a specific payload suite was carried out during a reassessment of the technical realisation of all instruments required to form part of the BepiColombo planetary orbiter payload (MPO) for the exploration of Mercury. A study of the heritage of other instruments developed for other missions such as Mars Express and ROSETTA was the precursor to enable identification of typical resource drivers and related problems or technology requirements. Innovative technologies aboard SMART-1 or other technology demonstration reference missions were also taken into account for their potential in miniaturisation without sacrificing performance. In the specific example of the BepiColombo Mercury Planetary Orbiter (MPO) the resource reduction by a Highly Integrated Payload Suite (HIPS) was addressed. Here we give a review on the basic concept and a comparison to the classical approach.

BepiColombo: Payload and mission updates

BepiColombo is an interdisciplinary mission to explore the planet Mercury through a partnership between ESA and Japan’s Aerospace Exploration Agency (JAXA). From their dedicated orbits two spacecraft, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO), will be studying the planet and its environment. They will be launched together on a single Soyuz-Fregat. The launch is foreseen for May 2012 with arrival in January 2017. Solar electric propulsion will be used for the journey to Mercury. The MMO provided by JAXA focuses on investigating the wave and particle environment of the planet from an eccentric orbit. The MPO, a three-axis-stabilized and nadir-pointing spacecraft in a low-eccentricity polar orbit, is dedicated to the characterization of Mercury. It will provide high accuracy measurements of the planet’s interior structure, a characterization of Mercury’s exosphere and a full coverage of the planet surface at high resolution, whereby surface morphology will be correlated to surface composition. Major effort was put into optimizing the scientific return by defining the payload complement such that individual measurements can be interrelated and complement each other. In November 2004, the BepiColombo scientific payload has been officially approved. The MPO payload comprises 11 instruments/instrument packages; the MMO payload consists of 5 instruments/instrument packages. Together, the scientific payload of both spacecraft will provide the detailed information necessary to understand Mercury and its magnetospheric environment and to find clues to the origin and evolution of a planet close to its parent star. An overview is given of the status of BepiColombo will with special emphasis on the MPO.

The BepiColombo Mission

AbstractBepiColombo is an interdisciplinary mission to the planet Mercury which will provide the detailed information necessary to understand Mercury and its magnetospheric environment. The mission is envisaged to consist of three spacecrafts, the Mercury Planetary Orbiter (MPO), the Mercury Magnetospheric Orbiter (MMO) and the Mercury Surface Element (MSE). The mission went through a re-assessment with the aim of optimizing resources and and advancing the scientific return. Various mission scenarios were investigated and new pay load concepts were adopted. The newly defined mission will be presented focusing on the launch scenario and the MPO reference payload.

BepiColombo: A visit to Mercury

BepiColombo is the planetary mission of the Cosmic Vision Programme of the European Space Agency (ESA), devoted to the thorough exploration of Mercury and its environment. It will be carried out as a joint project between ESA and JAXA (Japanese Aerospace Exploration Agency). BepiColombo consists of two spacecraft, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO). The MMO studies the wave and particle environment of the planet, while the MPO is dedicated to the characterization of Mercury itself. Both spacecraft will be in dedicated orbits that have been optimized for their tasks. They will be launched together on a single Soyuz-Fregat in July 2012 and reach Mercury about 4 years later. Then BepiColombo will start its one year mission, which will provide the detailed information necessary to understand the process of planetary formation and evolution in the hottest part of the proto-planetary nebula as well as the similarities and differences between the magnetospheres of Mercury and the Earth.

An international program for Mercury exploration: synergy of MESSENGER and BepiColombo

Mariner 10 has been the only spacecraft to visit the innermost planet Mercury. Its three flybys, more than 25 years ago, yielded the first view of this little-understood world. With advances in spacecraft technology and a growing realization of how important Mercury is to our understanding of the solar system and its formation, two missions are now in development for more intensive Mercury exploration. The first is the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission, competitively selected under the NASA Discovery Program, that will send a spacecraft to fly by Mercury in 2007 and 2008 and to orbit Mercury for one Earth year beginning in April 2009. The second is the more comprehensive BepiColombo mission, consisting of three elements: the Mercury Planetary Orbiter (MPO), the Mercury Magnetospheric Orbiter (MMO), and the Mercury Surface Element (MSE). Still in final definition stage, BepiColombo is a partnership between the European Space Agency (ESA) and the Japanese Institute of Space and Astronautical Science (ISAS). With one or two launches (depending upon the final mission architecture) BepiColombo will use solar electric propulsion to send two orbiters (MPO from ESA and MMO from ISAS) and a lander (MSE) to Mercury as early as 2011. The BepiColombo orbiters, in orbits complementary to that of MESSENGER, will extend geochemical, spectral, and photometric mapping of the planet. With its factor-of-ten larger downlink, BepiColombo will complete the intensive study of Mercury begun with the exploration by MESSENGER. Synergistic strategies of exploration will enable efficient use of BepiColombo resources in a more detailed study of the planet than can be accomplished by MESSENGER alone. For example, the earlier MESSENGER mission can help identify appropriate landing sites for the MSE, while BepiColombo can provide complementary orbital measurements of surface features from different phase angles and exploit MESSENGER observations to target high-resolution measurements. Possible joint activities include cooperative use of ground stations and extension of the temporal baseline for geophysical measurements by combining data from all spacecraft. Were MESSENGER to be approved for an extended orbital phase, there is also the possibility of simultaneous dual-spacecraft measurements in the magnetosphere.

The variability of Mercury's exosphere by particle and radiation induced surface release processes

Mercury's close orbit around the Sun, its weak intrinsic magnetic field and the absence of an atmosphere ( P surface<1×10 −8 Pa) results in a strong direct exposure of the surface to energetic ions, electrons and UV radiation. Thermal processes and particle-surface-collisions dominate the surface interaction processes leading to surface chemistry and physics, including the formation of an exosphere ( N⩽10 14 cm −2) in which gravity is the dominant force affecting the trajectories of exospheric atoms. NASA's Mariner 10 spacecraft observed the existence of H, He, and O in Mercury's exosphere. In addition, the volatile components Na, K, and Ca have been observed by ground based instrumentation in the exosphere. We study the efficiency of several particle surface release processes by calculating stopping cross-sections, sputter yields and exospheric source rates. Our study indicates surface sputter yields for Na between values of about 0.27 and 0.35 in an energy range from 500 eV up to 2 keV if Na + ions are the sputter agents, and about 0.037 and 0.082 at an energy range between 500 eV up to 2 keV when H + are the sputter agents and a surface binding energy of about 2 eV to 2.65 eV. The sputter yields for Ca are about 0.032 to 0.06 and for K atoms between 0.054 to 0.1 in the same energy range. We found a sputter yield for O atoms between 0.025 and 0.04 for a particle energy range between 500 eV up to 2 keV protons. By taking the average solar wind proton surface flux at the open magnetic field line area of about 4×10 8 cm −2 s −1 calculated by Massetti et al. (2003, Icarus, in press) the resulting average sputtering flux for O is about 0.8–1.0×10 7 cm −2 s −1 and for Na approximately 1.3–1.6×10 5 cm −2 s −1 depending on the assumed Na binding energies, regolith content, sputtering agents and solar activity. By using lunar regolith values for K we obtain a sputtering flux of about 1.0–1.4×10 4 cm −2 s −1. By taking an average open magnetic field line area of about 2.8×10 16 cm 2 modelled by Massetti et al. (2003, Icarus, in press) we derive an average surface sputter rate for Na of about 4.2×10 21 s −1 and for O of about 2.5×10 23 s −1. The particle sputter rate for K atoms is about 3.0×10 20 s −1 assuming lunar regolith composition for K. The sputter rates depend on the particle content in the regolith and the open magnetic field line area on Mercury's surface. Further, the surface layer could be depleted in alkali. A UV model has been developed to yield the surface UV irradiance at any time and latitude over a Mercury year. Seasonal and diurnal variations are calculated, and Photon Stimulated Desorption (PSD) fluxes along Mercury's orbit are evaluated. A solar UV hotspot is created towards perihelion, with significant average PSD particle release rates and Na fluxes of about 3.0×10 6 cm −2 s −1. The average source rates for Na particles released by PSD are about 1×10 24 s −1. By using the laboratory obtained data of Madey et al. (1998, J. Geophys. Res. 103, 5873–5887) for the calculation of the PSD flux of K atoms we get fluxes in the order of about 10 4 cm −2 s −1 along Mercury's orbit. However, these values may be to high since they are based on idealized smooth surface conditions in the laboratory and do not include the roughness and porosity of Mercury's regolith. Further, the lack of an ionosphere and Mercury's small, temporally and spatially highly variable magnetosphere can result in a large and rapid increase of exospheric particles, especially Na in Mercury's exosphere. Our study suggests that the average total source rates for the exosphere from solar particle and radiation induced surface processes during quiet solar conditions may be of the same order as particles produced by micrometeoroid vaporization. We also discuss the capability of in situ measurements of Mercury's highly variable particle environment by the proposed NPA-SERENA instrument package on board ESA's BepiColombo Mercury Planetary Orbiter (MPO).