International Workshop on Instrumentation for Planetary Missions (2012) 1067.pdf

Mercury Dust Monitor for the BepiColombo MMO. M. Kobayashi1, H. Shibata2, K. Nogami3, M. Fujii4, T. Miyachi1, H. Ohashi5, S. Sasaki6, T. Iwai7, M. Hattori7, H. Kimura8, T. Hirai9, S. Takechi10, H. Yano11, S. Hasega- wa11, R. Srama12 and E. Grün12, 1Chiba Inst. Tech., 2Kyoto Univ., 3Dokkyo Medical Univ., 4FAM Science, 5Tokyo Univ. Marine Sci. Tech., 6NAOJ, 7Univ. Tokyo, 8CPS, 9Sokendai,10Osaka City Univ., 11ISAS/JAXA, 12Stuttgart Univ. 13MPI-K.

Abstract: Mercury Dust Monitor (MDM) has been selected as a payload onboard the Mercury Mag- netosphere Orbiter (MMO) of the ESA-JAXA joint Mercury exploration mission Bepi-Colombo [1] for aiming to investigate dust environment around Mercu- ry. The spacecraft will be launched in 2015 and, after arriving at Mercury in 2020, the MDM will observe dust particles in orbit around Mercury during 1 year as nominal operation. In this paper, we summarize ob- servational significance of Mercury ambient dust and report an overview of our instrument onboard the Bepi-Colombo MMO. In-situ Observation in the Inner Solar System: Previous mission: Helios is the only mission that conducted in-situ observation of dust particles near Mercury’s orbit [2]. Micrometeoroid Analyzers onboard the Helios spacecraft measured dust fluxes in the inner solar system between 0.31 and 1.0 AU from the sun in 1970s. The Helios Micrometeoroid Ana- lyzer detected 235 dust particles with masses between 10-14 and 10-10 g using two sensors, the ecliptic sensor and the south sensor. The total detection area is 120 cm2 and the total observation period was about 140 days near Mercury’s orbit (0.31 – 0.47 AU heliocentric distance) [2]. The number of detected particles is statistically of insignificance, but Helios in-situ meas- Figure 1. Picture of the flight models of MDM-S urements revealed low fluxes of dust particles near and MDM-E as the top and bottom ones, respec- Mercury’s orbit. tively. MDM of the Bepi-Colombo MMO. In contrast oroid Analyzers. Figure 1 shows a picture of the to the Helios Micrometeoroid Analyzer, the Bepi- flight models of MDM-S and MDM-E. The Bepi- Colombo MDM will observe dust particles only Colombo MDM uses four plate sensors of piezoelectric around Mercury between 0.31 and 0.47 AU heliocen- lead zirconate titanate (PZT) because it has (1) a sim- tric distances. The Bepi-Colombo MMO is designed ple configuration, (2) a large sensitive area compared to be in an elliptic orbit around Mercury with the with the mass of the system (220 g for sensor part, 381 perihermion of 400 km and the aphermion of 12000 g for electronics), (3) high-temperature tolerance up to km (as of writing this paper). Thus the MDM can +230 °C, and (4) no bias voltage needed. The Bepi- detect dust particles of predominantly interplanetary Colombo MMO is smaller than Helios by about 100 kg, origin near the aphermion, while dust cloud particle thus the resource for its science payloads especially for which is ejecta from the surface of Mercury by micro- occupied volume is very severe; however, the MDM meteoroid bombardment are major detectable dust par- configuration met such tight requirement for the sci- ticles near perihermion [3]. The in-situ observation ence instruments. The MDM has a sensitive area of of dust clouds will contribute the study of the Na at- 64 cm2 with open aperture and the nominal observation mosphere of Mercury as mentioned below. is for 1 year. Thus, the MDM is predicted to observe Bepi-Colombo MMO is a spin-stabilized spacecraft almost similar number of interplanetary dust particles as well as the Helios and the MDM will be installed on to the Helios although it depends on the lower limit of the side panel to face to in-plane direction of ecliptic as detectable mass range. similar to the ecliptic sensor of the Helios Micromete- Internat ional Workshop on Instrumentation for Planetary Missions (2012) 1067.pdf

Observational Significance of the MDM: The MDM observations will give a direct evidence for the goal of the MDM is the observation of Mercury ambi- Na release process. ent dust particles and the environment of dust particles Dust particles of Mercurial origin. Surface ma- in the inner solar system. In particularly, the Mercury terials might escape from the Mercury, although they ambient dust particles are related to the space weather- have not yet been directly observed. Small bodies ing of the surface materials, the origin of Mercury's such as satellites and asteroids have weak gravitational atmosphere, and the dust particles of Mercurial origin. fields that enable surface materials to easily escape as Space weathering. Dust particles play an important impact ejecta of meteoroid bombardment. The dust role in space weathering of airless bodies such as the detector subsystem (DDS) onboard Galileo detected Moon and Mercury. Bombardment of high-velocity dust particles ejected from the Jovian satellites, Gany- dust particles on Mercury’s surface contributes to the mede, Europa, and Callisto. Theoretical estimate of production of its regolith layer. Space weathering the total mass of impact ejecta is difficult because of affects its optical properties. As the surface under- unknown ejection mechanisms. However, observa- goes space weathering, the overall albedo is reduced, tion by a dust detector with a large detection area may reflectance decreases with wavelength, and the depth determine the total mass of outflow materials. of its diagnostic absorption bands is reduced. The Instrumentation: The MDM system is com- space weathering affects remote-sensing observations posed of a piezoelectric ceramic sensor unit (MDM-S) of reflectance spectrum by the visible-near infrared attached to the side panel and the electronics unit spectrometer complex SIMBIO-SYS onboard Bepi- (MDM-E) installed inside the MMO. Table 1 shows Colombo MPO [4]. The surface materials of old geo- the property of the MDM instrument. The MDM-S logical regions on the airless bodies have undergone consists of four square plates of lead zirconate titanate the space weathering for long duration, while the in- (PZT) piezoelectric ceramic sensors (40 mm × 40 mm side regions of the newer craters and the area of impact × 2 mm) coated with a white paint layer of 60 μm (ab- cratering depots have undergone less space weathering. sorptivity to sunlight α=0.40 and infrared emissivity Thus, even if mineral compositions are the same, re- ε=0.86) to reflect solar light. The white paint is made flectance spectra can have difference. Some craters of a polyimide and has a conductivity to avoid charge can be found to have less space weathering effect. up of the sensors by the sun light exposure. The elec- The changes in spectra depend on time duration for trodes for the signal readout on the PZT sensors are which the surface materials are exposed to the inflow made of silver and have a thickness of 5 μm. dust. Therefore, the dating of the Mercury surface The PZT sensors generate charge signals depend- can be constrained by determining the inflow dust flux into Mercury. Table 1. Properties of the MDM instrument The origin of Na atmosphere. It has been Parameter Value/description known from ground-based spectroscopic observations Sensor Piezoelectric ceramics that Mercury has a thin atmosphere composed of Na Material Lead zirconate titanate and K, but its release process from the surface materi- (PZT) als is still unknown. Neither spattering by solar wind Supplier Honda electronics Co., Ltd. particle or energetic particle with < 1 MeV range can Dimension 4 cm × 4 cm × 2 mm for explains the production of Mercury atmosphere. Ka- individual, 4 plates used meda et al. suggested that vaporization effect by the Detection area Total 64 cm2 impact of interplanetary dust into the Mercury surface Operational –160 to 200 oC might form the Na atmosphere of Mercury being re- temperature leased from the surface [5]. If the observation of Na Frame of the sensor 125 mm × 125 mm × 7 mm, atmosphere is compared to in-situ observation of inter- CFRP planetary dust, it may provide us with crucial infor- Feild of view mation on the release process [6]. The dust cloud Azimuth 360 o density could be associated with the Na density of Elevation ± 90 o Mercury’s atmosphere, if the dust cloud results from Angular resolution < 180 o the bombardment of interplanetary dust. The Mercu- Location On the side panel of the ry Sodium Atmosphere Spectral Imager (MSASI) on spacecraft the MMO is a high-dispersion visible spectrometer Mass working in the spectral region near the Na D2 emission MDM-S (sensor) 220 g (589 nm) and will provide new information of the Na MDM-E (electronics) 381 g atmosphere at the same time as the MDM. The Power comsumption 4.0 W (nominal)

Internat ional Workshop on Instrumentation for Planetary Missions (2012) 1067.pdf

ing on the magnitude of stress caused by the impacts of incident dust particles. The momenta of the incident dust particles can be derived from the charge signals and the incident direction can be determined from the azimuth of the Bepi-Colombo MMO’s spin axis as of the detection. The Bepi-Colombo MMO spacecraft spins with a period of 4 s. Because the spin axis is per- pendicular to the ecliptic plane throughout the orbiting operations, the normal direction of the MDM is always parallel to the ecliptic plane, resulting in the highest sensitivity to particles moving on the ecliptic plane. The time of an impact event will be recorded by using a clock counter data from the MMO system and the time precision is about 2 ms (1/512 s). The clock Figure 2. Examples of the waveform of the readout counter is reset at the timing when the sun sensor of signal of a PZT sensor when hyper speed dust par- the MMO is oriented to the sun, thus the pointing di- ticles hit the sensors. rection of the MDM can be determined by the clock counter value and the angle between the MDM and the delberg, Germany, in April 2012. This experiment sun sensor positions, then the incoming direction of was implemented for the qualification of flight sensors incident dust particles can be estimated from the clock and the evaluation of functionality of the MDM system. counter value. Figure 2 shows examples of the waveform signal of the The MDM-E uses four charge sensitive amplifiers PZT sensors in the experiment. Four flight PZT sen- (CSA) for the signal readout of the sensors. The sors were selected out of whole sensor sets that we charge signals from the sensors are converted to volt- made including spare sensors and to evaluate the func- age signals by the CSAs and are summed up by the tionality of electronics of the MDM-E in terms of sen- following summing amplifier, and then digitized by an sitivity. This experiment was significant to determine 8-bit flash ADC with a certain sampling rate. The the lower limit of the dynamic range of dust momenta. sampling rate is changeable from 20 MS/s down to 156 In the experiments, the dust particles of iron were ac- kS/s and can be selected by command. celerated with the speeds in the range between 0.5 and The data memory of the MDM is equipped to store 10 km/sec and with the sizes of up to 2 μm. The the digital waveforms for detected dust impact events. waveforms of impact signals from the PZT sensors of Due to the resource limitation, the memory region is the MDM are processed with Fast Fourier Transfor- limited to be 950 sampling points. In the nominal mation to make frequency spectra. The spectra have case, 95 sampling points are used for an impact event a strong resonance peak for real dust impact event and and then 10 events can be stored in the memory. the intensity of the resonance peak around 1.1 MHz Such signal processing as above starts at the moment has linearity with the momentum transfer of an inci- when the PZT signal exceeds a certain threshold that is dent dust particle. For future analysis of observation set by command. The first 10 sampling points of the data, calibration curves will be produced from the rela- individual event are spared for baseline-level signal tion between the momenta of dust particle measured in before the event trigger timing. Stored waveform accelerator and the resonance peak intensity for indi- data eventually will be downlink to ground as wave- vidual sensors. form signals during daily operations. Those wave- Concluding Remarks: The Mercury Dust Moni- form signals are used for identification of signal/noise, tor (MDM) will be onboard the BepiColombo/Mercury and for the measurements of the momenta of incident Magnetosphere Orbiter to investigate the dust envi- dust particles. ronment around Mercury. The information of inter- Ground Experiment for Functionality check and planetary dust and the dust cloud around Mercury will Calibration: To measure the physical parameters of contribute to the clarification of the release process of impacting dust particles, such as velocity, mass and sodium from the Mercury surface to the atmosphere. momentum, from the impact signals of the sensors, we As of writing this paper, we are analyzing the need to obtain empirical formulae for calibration ground experiment data for the calibration of the elec- through ground experiments. For the calibration, we tronics in order to determine the lower limit of the have implemented a dust acceleration test campaign measurable range of dust particle momentum and the using a high voltage Van de Graaff dust accelerators at flight model of the MDM is undergoing the final ad- the Max Planck Institute for Nuclear Physics in Hei- justment and required environment tests.

Internat ional Workshop on Instrumentation for Planetary Missions (2012) 1067.pdf

Acknowledgements: We would like to give heart- felt thanks to Mr. S. Bugiel who operates the Van de Graaff accelerator of the Heidelberg Dust Accelerator Facility in MPI-K and also Mr. T. Omata who operates the Van de Graaff accelerator of HIT in the University of Tokyo for MDM calibration. We also gratefully appreciate the elaborative support from Meisei Electric Co., Ltd. for the development of MDM system. References: [1] Nogami K., et al. (2010) Planet. Space Sci. 58, pp. 108-115. [2] Grün E., Baguhl, M., Svedhem, H., Zook, H. A. (2001) Interplanetary Dust, Springer, pp. 295–346. [3] Müller M., et al. (2002) Planet. Space Sci. 50, pp. 1101-1115. [4] Sgavetti et al., (2007) Planet. Space Sci. 55, pp. 1596-1613 [5] Ka- meda, S. et al. (2009) GRL, 36, Issue 15, L15201. [6] Kameda, S. et al. (2011) 42nd LPSC, LPI Contribution No. 1608, p.1654.