Dr. Jean-Philippe Combe on the effect of the Solar Wind on the Moon
Listen to this on our Podcast : the Field Guide to Particle Physics
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We all take soil and dirt for granted here on Earth. But sand, soil and dirt is all made by natural processes that are unique to our planet. We have rain, wind, and organisms constantly grinding rocks down into dust. Plants take root and accelerate this process. The lava fields around Kilauea on the Island of Hawaii is an excellent place to see this process in action.
Because it’s common for us to see, it probably surprised nobody that the moon would also have soil. Moon dust. A place for those Apollo astronauts to leave foot prints.
But the moon has no weather. It has no running water or organisms to grind down moon rocks into moon dust. So just what is all that moon dust doing there? Where did it come from?
Today, we are taking our humble field guide TO THE MOON. Literally. We’ll be discussing how those subatomic particles coming from our sun - and cosmic rays - blast the moon’s surface.
To share the story of how all these subatomic particles change the landscape of the moon, we’ve brought in an expert. Dr. Jean-Philippe Combe.
About Dr. Combe
Jean-Philippe is planetary scientist who studies the surfaces of planets and other bodies in our solar system. After studying physics and geophysics at Grenoble and Toulouse, he got his doctorate at the University of Nantes in 2005, and has specialized in reflectance imaging spectroscopy of planetary surfaces.
In short, he’s a natural person to ask about how particle physics impacts the surface of the moon.
From 2005 he was a full time research scientist at Methow Valley’s own Planetary Science center, the Bear Fight Institute, and as of March 2021 has been affiliated with the Planetary Science Institute in Tucson, Arizona.
In Jean-Philippe’s own words:
“I was born in France, about 10 km from the impact location of a 92kg meteorite that fell in the small town of Juvinas on June 15th, 1821 at 3pm. No victim[s] were reported; only a 1m hole in a potato field. The Juvinas meteorite has since been classified as a eucrite, which likely originated from the asteroid Vesta.”
Amusingly, Vesta plays an important role in Jean-Philippe’s current research. For more details, please check out Dr. Combe’s professional website.
Interaction of the solar wind with the surface of the Moon
by Jean-Philippe Combe, PhD, Winthrop, Washington State
Senior scientist at the Planetary Science Institute, Tucson, Arizona
April 1, 2022
The coming narration comes largely from one scientific article entitled
by Thomas. B. McCord, Lawrence. A. Taylor, Jean‐Philippe Combe, Georgiana Kramer, Carle M. Pieters, Jessica M. Sunshine, and Roger N. Clark
published in 2011 in the JOURNAL OF GEOPHYS ICAL RESEARCH PLANETS, VOL. 116.
The Solar Wind
The Solar Wind was postulated in the mid-19th century, as a flow of particles and energy (photons) traveling away from the Sun into the Solar System. The observations that led scientists to this theory were
aurorae in the upper Earth’s atmosphere
terrestrial magnetic storms
their correlations with solar flares
and comet’s tails always pointing away from the Sun .
The solar wind was first observed directly by the Soviet satellite Luna 1 in 1959 and verified by measurements from Luna 2, Luna 3 and Venera 1, and then it was observed by a U.S. spacecraft, Mariner 2, in 1962.
The solar wind is composed mostly of protons and electrons, with about 4% helium and smaller amounts of heavier element ions. The flux at the Earth is about four hundred thousand particles per square centimeter and per second, with average energy of around half of a kilo-electronvolt per atomic mass unit, and a flux energy half width between 300 and 1500 keV as measured in 2009 by the Indian lunar orbiter Chandrayaan‐1. For this energy range, protons (H+) have a penetration depth in the surface grains of 5 to 10 nm. The solar wind plasma is almost completely absorbed by the Moon’s illuminated surface. However, up to 20% of the impinging solar wind protons are reflected from the lunar surface back to space as neutral hydrogen atoms.
The solar wind impacts the lunar surface, and the resulting i nteraction depends largely on the nature of the lunar soil exposed to space at the molecular level. Most known minerals of the solar system are made of molecules that contain large amounts of oxygen, mostly in oxides. The lunar surface has two major types of terrains that can be distinguished with the naked eye: the bright ones are called the highlands, and the dark ones are called by the latin word mare, which means seas, although they are not made of water, but are instead made of volcanic rocks, with various types of minerals rich in iron oxides and magnesium oxides. The lunar soil surface consists in a layer of crushed rock, minerals, and glass called “regolith”.
On the Moon, regolith formation results from combination of all the physical and chemical factors that occur on airless bodies, and that is called space weathering . This is unlike the processes that occur on Earth, and that are largely driven by tectonic activity and erosion due to the water cycle and atmospheric circulation. On the Moon, the agents of space weathering include a wide range of types and sizes of impactors, such as meteorites, micrometeorites (<1 mm), solar wind particles, solar‐ ultraviolet photons and galactic cosmic rays.
Solar wind particles bombard the exposed surfaces of lunar soil grains, producing an amorphous layer and effecting atoms, ions or molecules to be ejected from a lattice site in the target material; this process is called sputtering . Such ejected particles from the source material can be redeposited as a thin film on a surface. The sputtered particle can be charged, but is most often neutral. The sputtering energy is inherent to the solar wind velocity and particle mass. Protons (H+), which make ∼95% of solar wind particles, have an incident energy range of ∼300–1500 keV and average energy of ∼500 keV under normal solar wind velocities of 300–800 km/s. Heavier ions such as He+ and other heavy species, with greater incident energies, also play a significant role in the sputtering process . Sputtering of cations with the lowest crystalline binding energy occur preferentially, such as for magnesium.
Effects of the Solar Wind
Interaction of the lunar surface with solar wind particles occurs in a context where micrometeorite impacts locally melt the soil to form layers of amorphous glass at the surface of mineral grains, and agglutinates composed of small fragments of minerals in a matrix of glass. As a consequence, the solar wind is able to implant protons (H+) in a lunar soil that is rich in iron oxide FeO, by reducing the FeO component in the soil melt to metallic iron. As a result, in agglutinitic glass of soil grains, nanophase metallic iron particles are ubiquitous.
The texture and major element compositions of these thin, amorphous rinds on the surface of lunar soil grains is a testament to the process of vaporization and subsequent deposition of these silica‐rich patinas, with their myriads of nanophase iron particles.
UV Photons from the Sun
Now let’s talk about photons, and specifically about solar ultraviolet photons. This is a radiation that triggers the emission of electrons from the lunar dayside surface, which makes the surface charged several volts positive and leaves many dangling positive ions. The released photo -electrons move to the unlighted side of the Moon and into the solar wind plasma wake forme d by the Moon absorbing most of the solar wind. Some of these effects were observed by the Electron Reflectometer onboard the Lunar Prospector spacecraft, and they have stimulated the study of the electrical charging of objects on the Moon, such as astronauts. The photoejection of electrons is yet another effect that enhances the chemical activity of the lunar soils.
Cosmic Rays
To continue with flux of particles, we have to mention cosmic and galactic rays, which are high-energy particles, on the order of ∼1 GeV on the Moon, and which can penetrate several meters into the regolith. As they fragment into less energetic products, such as neutrons, they can leave damaging tracks in crystalline grains of minerals. At a high density, these accumulated tracks can cause amorphisization, which is the definition for removal of all crystals in a grain of mineral.
Why is it important to study the interaction of solar wind particles with the lunar surface?
One way of identifying the molecular composition by remote -sensing is reflectance spectroscopy, which measures the proportion of sunlight reflected and scattered by the Moon’s surface, either by a spacecraft in orbit or by telescope.
The processes we just described have major consequences on studying the composition and brightness of the surface of the Moon, which challenges attempts to understand its surface geology, and thus its past activity and early evolution. In particular,
One way of identifying the molecular composition by remote-sensing is reflectance spectroscopy, which measures the proportion of sunlight reflected and scattered by the Moon’s surface, either by a spacecraft in orbit or by telescope.
The processes we just described have major consequences on studying the composition and brightness of the surface of the Moon, which challenges attempts to understand its surface geology, and thus its past activity and early evolution. In particular,
Glass deposits from space weathering add an opaque top layer to mineral grains, which mask the underlaying composition, and makes it difficult to identify remotely by spectroscopGlass deposits from space weathering add an opaque top layer to mineral grains, which mask the underlaying composition, and makes it difficult to identify remotely by spectroscopy.
Glass deposits due to space weathering can be misinterpreted as glass deposits f rom pyroclastic activity; only a geological analysis of the surface texture may distinguish between the two processes.
More importantly, the process of proton implantation has open two major areas of research about the Moon:
The first is the formation of hydroxyl (the O-H) group, and subsequently to H-O-H (H2O), which is so important today to find viable resources for manned missions to the Moon,
The second one is observation of regions of alternate bright and dark areas with wavy patterns, on the mare and that are called respectively “lunar swirls” and “dark lanes” that seem to be associated with remnant crustal magnetic anomalies, which is further evidence of tectonic activity inside of the Moon in the past. As a hypothesis, the bright swirls may be protected from incomingchargedparticlesbythelocalmagneticfield,andthusstillhave largelyundamaged crystals and pristine grain surfaces. Simultaneously, the deviated charged particles are redirected into the dark lanes, which have most likely undergone enhanced glass deposition and crystal damaging over time.
Both phenomena have led to fascinating studies in the past decade, and in early 2022, are still relevant topics in current lunar science.
