Mercury Helium Research Articles

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

AbstractOn 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.

Numerical study of helium solubility and helium bubble stability in mercury

Dispersing small helium bubbles in the liquid mercury target of the high-power spallation neutron sources was proposed to add compressibility to the target made of liquid mercury. The pressure rise from proton beam deposition is reduced due to added compressibility, which in turn mitigates cavitation damage to the target boundary. A gas volume fraction of ∼0.5% with a nominal bubble diameter of ∼30μm is desired for optimal pressure pulse relaxation at the beam power of >1MW. Initial gas injection experiments performed in the Oak Ridge National Laboratory encountered difficulty in obtaining the required volume fraction in mercury. Gas dissolution and diffusion in mercury were candidate mechanisms for this behavior. To clarify this, the solubility of helium in mercury is evaluated in this study and compared to the available experimental data. The results indicate that helium has very small solubility in mercury and that the solubility increases with system temperature. Based on the predicted solubility values, bubble size evolution due to mass diffusion is simulated numerically. Mass diffusion induced bubble size evolution does not significantly affect bubble behavior for conditions expected in high power spallation targets.

Solubility of helium in mercury for bubbling technology of the spallation neutron mercury target

The pitting damage of mercury target container that originates in the pressure wave excited by the proton beam incidence becomes a large problem to reach the high-power neutron source in JSNS and SNS. The lifetime of mercury container is decreased remarkably by the pitting damage. As one of solutions, the pressure wave is mitigated by injecting the helium micro bubbles in mercury. In order to inject the helium micro bubbles into mercury, it is important to understand the characteristic of micro bubbles in mercury. The solubility of mercury–helium system is a key factor to decide bubbling conditions, because the disappearance behavior, i.e. the lifetime of micro bubbles, depends on the solubility. In addition, the bubble generation method is affected by it. Moreover, the experimental data related to the solubility of helium in mercury hardly exist. In this work, the solubility was obtained experimentally by measuring precisely the pressure drop of the gas that is facing to mercury surface. The pressure drop was attributed to the helium dissolution into mercury. Based on the measured solubility, the lifetime of micro bubbles and the method of the bubble generation is estimated using the solubility data.

Mercury-rare gas van der Waals complexes: From the lightest HgHe to the heaviest HgXe

Mercury helium and mercury xenon van der Waals complexes have been observed in a supersonic free jet expansion by laser-induced fluorescence in the vicinity of the Hg( 3P 1 1S 0) transition. The helium complex very weakly bonded in the ground state ( D 0 = 8 ± 1 cm −1) exhibited a simple rotational structure. The strongly bonded xenon complex has shown a clear example of isotopic broadening in its vibronic spectra.

Mercury's helium exosphere after Mariner 10's third encounter

From a comparison of the Mariner 10 third‐encounter UV spectrometer data with intensities generated from a newly constructed model exosphere, we have derived a new value of 4.5 × 10−4 for the fraction of the solar wind He++ flux to be intercepted and captured by Mercury's magnetosphere if the observed He atmosphere is maintained by the solar wind. If an internal source for He prevails, the corresponding upper bound for the global outgassing rate is estimated to be 4.5 × 1022 s−1. These values differ from those given earlier because of the present use of a surface temperature distribution satisfying the heat equation over Mercury's entire surface which employs Mariner 10 determined mean surface thermal characteristics. We also use the mean standoff distance of Mercury's magnetopause averaged over Mercury's orbit. We find the agreement between the observed and calculated intensities to be good and believe that the minor discrepancies that exist on the nightside of the terminator are explicable in terms of differences between actual and computed surface temperatures and the resulting scale height structure changes. We attribute these temperature differences to the inhomogeneity of the physical properties of the surface of Mercury.