Dataset Information

Photoionization Loss of Mercury's Sodium Exosphere: Seasonal Observations by MESSENGER and the THEMIS Telescope.

ABSTRACT: We present the first investigation and quantification of the photoionization loss process to Mercury's sodium exosphere from spacecraft and ground-based observations. We analyze plasma and neutral sodium measurements from NASA's MESSENGER spacecraft and the THEMIS telescope. We find that the sodium ion (Na⁺) content and therefore the significance of photoionization varies with Mercury's orbit around the Sun (i.e., true anomaly angle: TAA). Na⁺ production is affected by the neutral sodium solar-radiation acceleration loss process. More Na⁺ was measured on the inbound leg of Mercury's orbit at 180°-360° TAA because less neutral sodium is lost downtail from radiation acceleration. Calculations using results from observations show that the photoionization loss process removes ∼10²⁴ atoms/s from the sodium exosphere (maxima of 4 × 10²⁴ atoms/s), showing that modeling efforts underestimate this loss process. This is an important result as it shows that photoionization is a significant loss process and larger than loss from radiation acceleration.

SUBMITTER: Jasinski JM

PROVIDER: S-EPMC8243941 | biostudies-literature | 2021 Apr

REPOSITORIES: biostudies-literature

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Photoionization Loss of Mercury's Sodium Exosphere: Seasonal Observations by MESSENGER and the THEMIS Telescope.

Jasinski Jamie M JM Cassidy Timothy A TA Raines Jim M JM Milillo Anna A Regoli Leonardo H LH Dewey Ryan R Slavin James A JA Mangano Valeria V Murphy Neil N

Geophysical research letters 20210401 8

We present the first investigation and quantification of the photoionization loss process to Mercury's sodium exosphere from spacecraft and ground-based observations. We analyze plasma and neutral sodium measurements from NASA's MESSENGER spacecraft and the THEMIS telescope. We find that the sodium ion (Na<sup>+</sup>) content and therefore the significance of photoionization varies with Mercury's orbit around the Sun (i.e., true anomaly angle: TAA). Na<sup>+</sup> production is affected by the ...[more]

PMID: 34219841

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Project description:Understanding how spacecraft alter planetary environments can offer important insights into key physical processes, as well as being critical to planning mission operations and observations. In this context, it is important to recognize that almost any powered lunar landing will be an active volatile release experiment, due to the release of exhaust gases during descent. This presents both an opportunity to study the interaction of volatiles with the lunar surface, and a need to predict how non-indigenous gases are dispersed, and how long they persist in the lunar environment. This work investigates these questions through numerical simulations of the transport of water vapor during a nominal lunar landing and for two lunar days afterwards. Simulation results indicate that the water vapor component of spacecraft exhaust is globally redistributed, with a significant amount reaching permanently shadowed regions (cold traps) near the closest pole, where temperatures are sufficiently low that volatiles may remain stable over geological timescales. Exospheric evolution and surface deposition patterns are highly sensitive to desorption activation energy, providing a means to constrain this critical parameter through landed or orbital measurements during future missions. Contamination of cold traps by exhaust gases is likely to scale with exhaust mass and proximity of the landing site to the poles. Exhaust propagation is perhaps the most widespread and long-lived impact of spacecraft operations on a nominally airless solar system body, and should be a key consideration in mission planning and in interpreting measurements made by landed lunar missions, particularly at near-polar regions. There has been increasing interest lately in learning more about the origin and distribution of water on the Moon. However, whenever a spacecraft descends to land on the lunar surface, it releases water vapor and other gases into the lunar environment, complicating the situation. In this work, we use computer simulations to understand what happens to the water released by a spacecraft during a typical landing. The simulated landing creates a temporary, very thin atmosphere all around the Moon. The behavior of this atmosphere depends on how strongly water sticks to the lunar surface, such that comparing simulations to measurements of water in the lunar environment during and after future lunar landings could help us figure out the "stickiness" of the lunar surface - something that we don't yet accurately know, but is important to understanding the past, present and future distribution of water on the Moon. Our simulations also show that some spacecraft-delivered water travels to regions near the poles that are cold enough to trap water for very long periods of time. If the spacecraft is heavier, or lands closer to the poles, its influence on the lunar surface and atmosphere may be more significant.

Dataset Information

Photoionization Loss of Mercury's Sodium Exosphere: Seasonal Observations by MESSENGER and the THEMIS Telescope.

Publications

Photoionization Loss of Mercury's Sodium Exosphere: Seasonal Observations by MESSENGER and the THEMIS Telescope.

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