Details

Autor(en) / Beteiligte

• Quémerais, Eric
• Koutroumpa, Dimitra
• Lallement, Rosine
• Sandel, Bill R.
• Robidel, Rozenn
• Chaufray, Jean‐Yves
• Reberac, Aurélie
• Leblanc, Francois
• Yoshikawa, Ichiro
• Yoshioka, Kazuo
• Murakami, Go
• Korablev, Oleg
• Belyaev, Denis
• Pelizzo, Maria G.
• Corso, Alain J.

Titel

Observation of Helium in Mercury's Exosphere by PHEBUS on Bepi‐Colombo

Ist Teil von

• Journal of geophysical research. Planets, 2023-06, Vol.128 (6), p.n/a

Ort / Verlag

Washington: Blackwell Publishing Ltd

Erscheinungsjahr

2023

Quelle

Wiley Online Library

Beschreibungen/Notizen

• On 1 October 2021, Bepi‐Colombo performed its first flyby of Mercury. During the maneuver, the short wavelength channel (55–155 nm) of “Probing the Hermean Exosphere by UV Spectroscopy” (PHEBUS) was activated for a total duration of 1 hr. The helium resonance line at 58.4 nm was clearly observed during the whole sequence. At large distance from the planet, the emission was due to helium atoms in the interplanetary medium (interplanetary UV glow). Just after crossing the terminator of the planet and entering the dawn side of the exosphere, PHEBUS observed a clear additional emission due to scattering of solar photons by helium atoms in the exosphere of Mercury. The first detection of the 58.4 nm line in the exosphere of Mercury was reported by Broadfoot et al. (1976, https://doi.org/10.1029/gl003i010p00577) following the Mariner 10 flybys in 1974. The PHEBUS observation of exospheric helium emissions is the first for this element since the UVS measurements. In this paper, we present the results of our analysis of the PHEBUS data at 58.4 nm. Calibration of both instruments are compared with observations of the interplanetary glow, showing that the measurements of both instruments are accurate. However, we find that the exospheric density of helium atoms deduced from the PHEBUS data is 4.5–7.5 times lower than the previous estimate from UVS on Mariner 10. Possible explanations are considered. We show that some of the helium atoms present in the exosphere of Mercury could originate from the local interstellar cloud. Plain Language Summary On 1 October 2021, The Bepi‐Colombo mission performed its first flyby of Mercury. During this maneuver, the “Probing the Hermean Exosphere by UV Spectroscopy” (PHEBUS) ultraviolet spectrometer was activated for 1 hour, starting 30 min before the time of the closest approach to the surface of Mercury. This instrument is able to measure the emission of helium atoms either in the interplanetary medium or in the tenuous atmosphere of Mercury. The presence of helium atoms around Mercury was first discovered by the ultraviolet spectrograph (UVS) of the Mariner 10 mission in 1974. The observations of PHEBUS confirm the detection of helium atoms in the atmosphere of Mercury. However, the amount of helium atoms detected by PHEBUS is 4.5–7.5 times lower than the value reported by UVS after the Mariner 10 flybys. We discuss possible explanations for this difference. Key Points This paper presents helium measurements obtained during the first flyby of Mercury by the Bepi‐Colombo mission Calibrations of UVS/Mariner 10 and of PHEBUS are cross‐checked with observations of the interplanetary glow at 58.4 nm The helium content in the exosphere of Mercury derived by PHEBUS is 4.5–7.5 times lower than the previous estimate from the Mariner 10 mission