2015.06.17 @DLR, Berlin MESSENGER-BepiColombo Joint Science Meeting Mercury Sodium Atmosphere Spectral Imager (MSASI) Onboard BepiColombo/MMO Go Murakami1, Shingo Kameda2, Ichiro Yoshikawa3, and Oleg Korablev4 1ISAS/JAXA, 2Rikkyo University, 3The University of Tokyo, 4IKI

Scientific objectives Source mechanism of Mercury’s sodium exosphere Photon-stimulated desorption Charged-particle sputtering Micro-meteoroid Impact/vaporisation Still open issue [Yoshioka et al., 2008] MESSENGER/MASCS observation [Ronald et al., 2010] Spectral resolution: 0.6 nm -> Impossible to separate solar reflections from the disc and emissions from the exosphere Key observation: -Imaging the global distribution of the sodium exosphere -Long-term and continuous monitoring [Cassidy et al., 2015]

Mercury Sodium Atmosphere Spectral Imager（MSASI） Fabry-Perot interferometer Detector unit: I. I. + coupling lens + CMOS Band-pass filter Light entrance Scanning mirror: 1-axis scan of FOV MSASI flight model (FM)

Mercury Sodium Atmosphere Spectral Imager（MSASI） MMO spacecraft (FM) Electronics (MSASI-E) Sensor (MSASI-S) Hood (MSASI-H) Field of view

Mercury Sodium Atmosphere Spectral Imager（MSASI） Solar reflection from Mercury’s surface 0.6 nm 0.005 nm Specifications Na D2 MMO/MSASI MESSENGER/MASCS Spectral range 589.158 ± 0.028 nm (Na D2 line) 115-600 nm Spectral resolution 0.009 nm 0.6 nm Spatial resolution 0.18° x 0.18°: 30 km (1/80 Rm) (nominal) 3 km (1/800 Rm) (near Aphelion) 25-km (height) 600-800 km (latitude/longitude) Field of view 1.8° x 1.8° Scan: 30° (lat) x 45°-180° (lon) 1° x 0.04° Exposure time 2 msec 1-2 sec Sensitivity 10 kR (S/N = 5) 100 R Mass 3.48 kg 3.1 kg Power 15.2 W (nominal) 6.7 W -Na D2 only -Resolution > 60,000 -> Exosphere on dayside disc can be seen -High spatial resolution -Wide FOV (by scan)

MSASI observation: 2-D scanning “Fast imaging” Spin Spectral image (1 shot) Spin scan (45°) Mirror scan (30°) ・Spin scan: 0.18 deg (= 1 pix)/2 msec Nominal: 256 shots/spin (max: 1024 shots/spin) 　 →FOV: 45 deg (max: 180 deg) ・Mirror scan: 1.44 deg/step, 21 steps →FOV: ~30 deg

Observation plan: nominal Nominal observation geometry FOV: 30° (latitudinal) x 45°(longitudinal) 8 iamges / orbit (9.3 h) (~30% will be downlinked) -> At least one image every orbit will be available in almost all season MSASI can monitor the temporal variation of exosphere every 9.3h

Observation plan: perihelion #Thermal limitations exist in this season Nominal FOV: 30° (latitudinal) x 45°(longitudinal) 8 iamges / orbit (9.3 h) (~30% will be downlinked) MSASI can observe the dawn and dusk sides at the same time

Observation plan: aphelion #No observation during the eclipse Nominal Wide FOV FOV: 30° (latitudinal) x 45°(longitudinal): 2 images + Wide FOV: 30° (latitudinal) x 90°(longitudinal): 3 images The highest spatial resolution (3 km) of dayside will be achieved

Observation plan: tail observation Nominal + wide Wide FOV FOV: 30° (latitudinal) x 45°(longitudinal): 1 image + Wide FOV: 30° (latitudinal) x 90°(longitudinal): 2 images The longest sodium tail can be observed in this season

Conjunction observations MPO/PHEBUS (UV obs.) -Other exospheric compositions -Relations of temporal and spatial variations MPO/MIXS and MGNS -Relations with surface compositions MPO/SIMBIO-SYS and MERTIS -Relations with surface structures MMO/MDM -Dust monitoring MMO/MPPE and MPO/SERENA -Ionized particles MMO/MPPE and MGF -Solar wind monitoring

Summary MSASI: 2D + spectral imaging of Na D2 line Long-term and continuous monitoring of Mercury’s sodium atmosphere (every 9.3 h) Spectral resolution: 0.009 nm Spin scan + mirror scan -> Global images of Mercury can be obtained

From MESSENGER to BepiColombo Thank you! Nicholas Piccillo - Fotolia From MESSENGER to BepiColombo