Mercury’s surface and atmosphere mapped by solar wind study

A new study maps how solar wind particles alter Mercury's surface and its thin atmosphere.
Rizwan Choudhury
Colorful view of Mercury produced by using images from the color base map imaging campaign during MESSENGER's primary mission.
Colorful view of Mercury produced by using images from the color base map imaging campaign during MESSENGER's primary mission.

Source: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington 

A new study has revealed how the surface and the very thin atmosphere of Mercury are affected by the charged particles from the Sun, known as the solar wind. The study has mapped the location and energy of the protons and electrons that hit the surface of Mercury, which is the closest planet to the Sun.

The study, published in the Planetary Science Journal, is one of the first to examine the impact of the solar wind on Mercury’s surface in terms of geographic location (longitude) rather than time of day (dawn, noon, dusk). This is important for scientists who study the properties of the surface, said Elizabeth A. Jensen, a senior scientist at the Planetary Science Institute and a co-author of the paper.

How protons and electrons interact with Mercury's surface

Jensen elaborated that their team is focused on studying how protons and electrons interact with Mercury's surface, influencing its regolith and atmosphere. Prior maps have depicted the rate at which these particles hit the surface at specific times, but there's complexity beyond that. While one might expect an even distribution of protons and electrons due to the occurrence of dawn all over the surface, Mercury's unique orbital and rotational characteristics make it more complicated. Specifically, Mercury experiences a 3-to-2 spin-orbit resonance, meaning it completes two orbits around the Sun for every three rotations on its axis. As a result, a day on Mercury is slightly shorter than a year on the planet. Additionally, the planet's elliptical orbit and uneven time spent facing the Sun across its longitudes contribute to varying levels of solar wind material impacting different areas.

The study also took into account the interactions of the solar wind with Mercury’s magnetic field, which is much weaker than Earth’s and varies with distance from the Sun. The researchers used a computer model to simulate how the solar wind particles penetrate through Mercury’s magnetic field and reach the surface. They then integrated the results over a full Mercury day, which is equivalent to two full orbits around the Sun.

The study found that there are significant variations in the location and energy of the solar wind particles that hit Mercury’s surface. The particles have higher energy near the poles and lower energy near the equator. The particles also have different energy levels depending on whether they are protons or electrons. The study also identified regions on Mercury’s surface that receive more or less exposure to the solar wind particles.

The study’s lead author is Federico Lavorenti from France and Italy, who is an expert in modeling the interactions of the solar wind with magnetic fields. Another co-author is Deborah Domingue from PSI, who has studied how planetary surfaces are altered by the solar wind.

Mercury and Earth

Domingue explained that the solar wind has a transformative effect on both the surface and atmosphere of Mercury. This interaction is responsible for creating Mercury's exosphere, its incredibly thin atmosphere, and it also alters the minerals making up the planet's surface. However, the paper in question concentrates not on these phenomena, but rather on quantifying the levels of radiation impacting various areas of Mercury's surface, data that is valuable for researchers investigating these other processes.

The study also compared Mercury with Earth, which has a much thicker atmosphere and a stronger magnetic field that protects it from most of the solar wind particles. Only in some regions near the poles, where the magnetic field leaves openings, can the protons and ions from the solar wind reach the surface. This produces the aurora borealis near the north pole, the aurora australis near the south, as well as the polar rain and other effects.

The study was published in the Planetary Science Journal.

Study abstract:

Mercury is the closest planet to the Sun, possesses a weak intrinsic magnetic field, and has only a very tenuous atmosphere (exosphere). These three conditions result in a direct coupling between the plasma emitted from the Sun (namely, the solar wind) and Mercury's surface. The planet's magnetic field leads to a nontrivial pattern of plasma precipitation onto the surface that is expected to contribute to the alteration of the regolith over geological timescales. The goal of this work is to study the solar wind plasma precipitation onto the surface of Mercury from a geographical perspective, as opposed to the local time-of-day approach of previous precipitation modeling studies. We employ solar wind precipitation maps for protons and electrons from two fully kinetic numerical simulations of Mercury's plasma environment. These maps are then integrated over two full Mercury orbits (176 Earth days). We found that the plasma precipitation pattern at the surface is most strongly affected by the upstream solar wind conditions, particularly the interplanetary magnetic field direction, and less by Mercury's 3:2 spin–orbit resonance. We also found that Mercury's magnetic field is able to shield the surface from roughly 90% of the incoming solar wind flux. At the surface, protons have a broad energy distribution from below 500 eV to more than 1.5 keV, while electrons are mostly found in the range 0.1–10 keV. These results will help to better constrain space weathering and exosphere source processes at Mercury, as well as interpret observations by the ongoing ESA/JAXA BepiColombo mission.

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