The Sun exudes a constant stream of hydrogen, called the "solar wind."

Paul D. Spudis

The Once & Future Moon

Smithsonian Air & Space

New data returned from a fleet of orbiting satellites changes our perceptions of the history and processes of the Moon.  Concentrated at both lunar poles, and to date the most striking discovery, is the documentation of the presence of large amounts of water.  Though this water has been confirmed by several differing techniques (from multiple missions), we remain uncertain about its source.  Two principal origins have been proposed: 1) water added by the in-fall of water-bearing meteorites and comets during the impact bombardment of the Moon; and 2) the manufacture of water from hydrogen implanted in the lunar soil by the wind from the Sun.

A recent discovery may shed some new light on the origin of lunar water.  Researchers conducting detailed examination of tiny fragments of glass in soil returned by the Apollo astronauts found the molecule hydroxyl (OH) present in the glass.  Interestingly, the isotopic composition of these OH molecules indicates the bulk of the hydrogen comes from the Sun, not from cometary and asteroidal impacts.

The Moon has no atmosphere and no global magnetic field.  As a result, the solar wind – the stream of atoms and molecules constantly emitted by the Sun – directly impinges upon the lunar surface.  Most of this solar wind consists of hydrogen, either in the form of neutral atoms or positively charged ions (i.e., protons).   After it encounters the Moon, this spray of hydrogen has a complex fate, with at least some of it being implanted into the lunar dust.  In a process called adsorption, many of the hydrogen atoms stick to the surfaces of the dust grains.  The amount of adsorbed hydrogen varies by position and chemical composition around the Moon, but it can be present in quantities ranging from less than 10 to over 100 parts per million (ppm).

Impact glass is a major component of lunar regolith – up to 60% by weight of the soil at some landing sites.  The constant bombardment of the lunar surface by microscopic meteorites crushes and grinds up the surface rock, continually mixing the outer layer of the Moon.  When a micrometeorite strikes a rock, it forms a micro-crater (wholly melting the surface beneath this pit) and creates a clear, chemically homogeneous glass particle.  However, when a micrometeorite strikes lunar soil instead of rock, its energy is converted mostly into heat.  This flash heating creates a mixture of melt and mineral debris called agglutinate glass.

The new work details results of analyses of agglutinates returned from several lunar landing sites.  Their study measured both the amounts of hydroxyl present and its isotopic composition.  A normal atom of hydrogen is a single proton and an electron.  But in a rare form of hydrogen, called deuterium, the nucleus contains both a proton and a neutron.  The ratio of this form of “heavy hydrogen” to “normal” hydrogen is unique for different materials throughout the Solar System.  By tracking the D/H ratio in the sample, one can assign a source origin to the measured hydrogen.

When the lunar agglutinate glasses were studied, it was found that their D/H ratios indicated that most of the hydrogen in the hydroxyl molecules came from the Sun and not from cometary or meteoritic sources.  However, the source of the hydrogen is not completely solar, as the D/H ratios suggest some mixing with a subordinate component of either lunar or cometary origin.  The authors of this study suggest that the hydroxyl found on the Moon was created when a small impact flash heated the soil, releasing the adsorbed hydrogen and chemically reducing the metallic oxides in the soil into native metal (found as extremely tiny grains on the surfaces of the agglutinates) and hydroxyl molecules.  Multiplied by billions, such a process could account for the generation of water on the lunar surface.  Subsequent migration of these molecules toward cooler-than-average areas of the Moon (i.e., the higher latitudes, up to and including the poles) may have created the polar ice deposits found by numerous techniques.  In the view of the authors of this study, lunar water comes mostly (but not entirely) from the Sun.  This constant process, occurring on the sunlit hemisphere of the Moon, could create an enormous reservoir of hydroxyl molecules (in motion due to their thermal instability), slowly but constantly moving toward the poles.

If such a process occurs on the Moon, one might expect the accumulation of water in every location where water is stable (i.e., within every permanently dark and cold region near both poles).  But it appears that ice at the poles is not uniformly distributed, occurring in high concentration in some areas while absent in others.  This pattern suggests that the source of polar water might be controlled by a non-equillibrium process, such as episodic bombardment by asteroids and comets.  In fact, both solar wind-produced and cometary water may be present at the poles, but until the ice there is actually analyzed for its D/H content, we cannot be certain of its origin.  Such a measurement does not require the return of a polar ice sample to the Earth.  It could be made remotely in situ on the Moon with a properly instrumented robotic spacecraft.

It is important to emphasize that although the quantities of water generated by this process are potentially very large, the hydroxyl in agglutinate glass should not be considered an economic resource.  These molecules occur globally but at very low levels of concentration (tens of ppm).  Even if this water is the primary and ultimate source reservoir of lunar water, the migration of the molecules and their subsequent collection by the cold traps near the poles serve as a concentrating mechanism, where ice accumulates in large quantities, confined within small areas — the classic definition of an ore body.

What a change has occured in the mindset the lunar science community in the past few years!  From a bone-dry lump of rock in space to a complex, still mysterious body with a dynamic hydrological cycle.  It’s clear that many more discoveries about our Moon and its resources have yet to be revealed.  The more we learn about the Moon, the greater the range of processes we must account for and the more subtle and complex its history becomes.

Originally published at his Smithsonian Air & Space blog The Once and Future Moon, Dr. Spudis is a senior staff scientist at the Lunar and Planetary Institute. The opinions expressed are those of the author and are better informed than average.

Read the NASA/GSFC News Release, HERE.

Dream Team - NASA/GSFC/NLSI

In 2007 NASA launched a fleet of five spacecraft into Earth's magnetosphere to study the physics of geomagnetic storms. Collectively, they were called THEMIS, short for "Time History of Events and Macroscale Interactions during Substorms." P1 and P2 were the outermost members of the quintet.

Working together, the probes quickly discovered a cornucopia of previously unknown phenomena such as colliding auroras, magnetic spacequakes, and plasma bullets shooting up and down Earth’s magnetic tail. This has allowed researchers to solve several longstanding mysteries of the Northern Lights.

The mission was going splendidly, except for one thing: Occasionally, P1 and P2 would pass through the shadow of Earth. The solar powered spacecraft were designed to go without sunlight for as much as three hours at a time, so a small amount of shadowing was no problem. But as the mission wore on, their orbits evolved and by 2009 the pair was spending as much as 8 hours a day in the dark.

"The two spacecraft were running out of power and freezing to death," says Angelopoulos. "We had to do something to save them."

The team brainstormed a solution. Because the mission had gone so well, the spacecraft still had an ample supply of fuel--enough to go to the Moon. "We could do some great science from lunar orbit," he says. NASA approved the trip and in late 2009, P1 and P2 headed away from the shadows of Earth.

With a new destination, the mission needed a new name. The team selected ARTEMIS, the Greek goddess of the Moon. It also stands for "Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun."

The first big events of the ARTEMIS mission are underway now. On August 25, 2010, ARTEMIS-P1 reached the L2 Lagrange point on the far side of the Moon. Following close behind, ARTEMIS-P2 entered the opposite L1 Lagrange point on Oct. 22nd. Lagrange points are places where the gravity of Earth and Moon balance, creating a sort of gravitational parking spot for spacecraft.

Because they lie just outside Earth's magnetosphere, Lagrange points are excellent places to study the solar wind. Sensors onboard the ARTEMIS probes will have in situ access to solar wind streams and storm clouds as they approach our planet—a possible boon to space weather forecasters. Moreover, working from opposite Lagrange points, the two spacecraft will be able to measure solar wind turbulence on scales never sampled by previous missions.

"ARTEMIS is going to give us a fundamental new understanding of the solar wind," predicts David Sibeck, ARTEMIS project scientist at the Goddard Space Flight Center. "And that's just for starters."

ARTEMIS will also explore the Moon's plasma wake—a turbulent cavity carved out of the solar wind by the Moon itself, akin to the wake just behind a speedboat. Sibeck says "this is a giant natural laboratory filled with a whole zoo of plasma waves waiting to be discovered and studied."

Another target of the ARTEMIS mission is Earth's magnetotail. Like a wind sock at a breezy airport, Earth's magnetic field is elongated by the action of the solar wind, forming a tail that stretches to the orbit of the Moon and beyond. Once a month around the time of the full Moon, the ARTEMIS probes will follow the Moon through the magnetotail for in situ observations.

"We are particularly hoping to catch some magnetic reconnection events," says Sibeck. "These are explosions in Earth's magnetotail that mimic solar flares--albeit on a much smaller scale." ARTEMIS might even see giant 'plasmoids' accelerated by the explosions hitting the Moon during magnetic storms.

These far-out explorations may have down-to-Earth applications. Plasma waves and reconnection events pop up on Earth, e.g., in experimental fusion chambers. Fundamental discoveries by ARTEMIS could help advance research in the area of clean renewable energy.

After six months at the Lagrange points, ARTEMIS will move in closer to the Moon—at first only 100 km from the surface and eventually even less than that. From point-blank range, the spacecraft will look to see what the solar wind does to a rocky world when there's no magnetic field to protect it.

"Earth is protected from solar wind by the planetary magnetic field," explains Angelopolous. "The Moon, on the other hand, is utterly exposed. It has no global magnetism."

Studying how the solar wind electrifies, alters and erodes the Moon's surface could reveal valuable information for future explorers and give planetary scientists a hint of what's happening on other unmagnetized worlds around the solar system.

Orbiting the Moon is notoriously tricky, however, because of irregularities in the lunar gravitational field. Enormous concentrations of mass (mascons) hiding just below the surface tug on spacecraft in unexpected ways, causing them over time to veer out of orbit. ARTEMIS will mitigate this problem using highly elongated orbits ranging from tens of km to 18,000 km.

"We'll only be near the lunar surface for a brief time each orbit (accumulating a sizable dataset over the years)," explains Angelopoulos. "Most of the time we'll linger 18,000 km away where we can continue our studies of the solar wind at a safe distance."

The Dead Spacecraft Walking may have a long life ahead, after all.

Related Posts:
NASA update: ILN Anchor Nodes
and Robotic Lunar Lander Project
August 17, 2010

THEMIS becomes ARTEMIS
Aviation Week
July 30, 2010

Robotic Lunar Landers
for Science and Exploration
41st Lunar and Planetary Science Conference, #2616
March 4, 2010

ARTEMIS, A Two Spacecraft, Planetary
and Heliospheric Lunar Mission
41st Lunar and Planetary Science Conference, #1425
March 4, 2010

Update on the new lunar phase
of THEMIS mission
UC Berkeley Daily Tech
October 30, 2009

ARTEMIS to Lagrange points
to lunar orbit
April 26, 2009

Map of moderated neutrons over the whole Moon. Lunar Prospector data. [Elemental content from 0 to 500 keV neutrons: Lunar Prospector results, Genetay et al. / Planetary and Space Science 51 (2003) 271 – 280].

A lot has happened, suddenly it seems, since the $60 million Lunar Prospector mission first mapped fast and slow neutrons reflecting off the Moon in 1998. Five years after the small, optically blind spacecraft was deorbited into Shoemaker crater, in permanent darkness near the lunar South Pole in a forlorn last-minute attempt to accomplish what LCROSS eventually would do a decade later.

By late 2009, scientists around the world had put one and one and one together, beginning with the Russian-built neutron detector aboard Lunar Prospector in 1998, data originally thought erroneous that was detected during a sling-shot maneuver accelerating Cassini on its way to Saturn with more pieces of the puzzle collected by India's lunar orbiter Chandrayaan-1. Among other things, sniffing the Moon has shown us the Moon is wet in more ways than one, wettest in its Permanently darkened Cold Spots and at the equator. It's becoming more and more clear that the dusty, radioactive lunar exosphere is a very dynamic place.

The STEREO solar satellites only very recently confirmed the presence of neutral hydrogen in the solar wind, so the driving force behind 99 percent of the Moon's most dynamic processes and its volatiles is none other than our modest yellow dwarf Home Star. The heavier elements patiently pile up perhaps mostly from bombardment by far more energetic cosmic rays.

Put more simply, Larry O'Hanlon of Discovery News has called our remote sensing a sniffing of the Moon's solar-driven perfume, in this case calling attention to yet another player in this drama and an experiment early on in the mission of Japan's lunar orbiter Kaguya, monitoring radio signals to and from it's two sub-satellites as they rose into line-of-sight up over the horizon in 2007 and 2008.

The moon's whiff of an atmosphere has been sniffed by a Japanese spacecraft under very special conditions and confirmed as coming largely from sunlight brutally hammering the lunar surface.

Using the very first direct measurements of the moon's "exosphere" as the moon passed through the streaming tail of Earth's protective magnetic field, researchers were able to watch the short-lived and ever-changing exosphere in the absence of the hot, magnetized solar wind.

What they found confirmed that it's really just powerful ultraviolet light knocking beat-up atoms, or ions, off the lunar surface and manufacturing the bulk of the weak lunar perfume.

This discovery is important for several reasons, explains NASA lunar scientist Menelaos Sarantos. One is that it could help interpret what kinds of minerals are on the moon's surface.

"What comes out [as exosphere] more or less tells you the mineralogy of the surface," Sarantos said.

The ions and how they change over time also provide direct evidence of how much of a beating the lunar surface is taking, which is invaluable information for anyone hoping to house humans on the moon in the future.

"If you want to build a lunar base or put humans on the surface for any time," Sarantos said, "you want a well-defined radiation environment."

Measurements of hydrogen flux recorded in lunar orbit February 6, 2009, by the Sub Kev Atom reflecting Analyser (SARA) on-board India's Chandrayaan-1. According to ESA, SARA was the first lunar experiment dedicated to the direct study of plasma-surface interactions in space. [ISRO/ESA/Swedish Institute Of Space Physics (IRF)]

The Moon absorbs electrically charged particles emitted by the Sun. These particles interact with oxygen present in some dust grains on the lunar surface, producing water. This conclusion, based on discoveries made using the ESA-ISRO Sub Kev Atom reflecting Analyser (SARA) on-board India's lunar orbiter Chandrayaan-1 "confirms," ESA announced, is a way that the water very recently confirmed as both transitory and materially-fixed in the uppermost layer of the lunar surface is likely to have been created.

The detection also presages an new method of imaging the Moon and other airless bodies in the solar system, ESA wrote.

The lunar surface is blanketed with a loose collection of very fine, irregularly shaped silicate dust called lunar regolith. Incoming particles from the Sun, along with some cosmic radiation, are believed trapped in spaces between and within these grains, becoming chemically bound. with oxygen.

Protons, by far the largest constituent of incoming radiation, are thought to interact with oxygen to produce the hydroxyl and water molecules that have been detected on the lunar surface.

A signature for such molecules was recently confirmed the team of investigators for the NASA-build Moon Mineralogy Mapper (M3), on-board Chandrayaan-1.

The SARA results confirm hydrogen nuclei are emitted from the Sun and are also being absorbed at the lunar surface.

In this process protons join with electrons to become atoms of hydrogen. The results "also highlights a mystery" said ESA. "Not every proton is absorbed." One of every five rebound back into space."

“We didn’t expect to see this at all,” said Stas Barabash of the Swedish Institute of Space Physics (IRF) and European Principal Investigator for SARA. Barabash and his colleagues do not know the cause of the predictable ratio of proton scatter but the discovery introduces a new way, they claim, to gather images.

Reflected hydrogen departs off the lunar surface at speeds of ~ 200 km/s, without attenuation by the Moon’s gravity. Hydrogen is also generally electrically neutral and not refracted by the crustal magnetic fields found in various places on the lunar surface, or in Space around the Moon. These atoms fly in virtually straight lines, according to Barabash, like photons.

In principle, each atom can be traced back to its origin and an image of the surface could be made. Those areas emitting the most hydrogen should appear brightest.

Some areas on the Moon are characterized by very strong magnetic fields, however, some locally strong enough to create an actual bow shock in the Solar Wind. Barabash and his team are currently making images to map these lunar magnetic anomalies.

Lunar features with strong local magnetic fields (and also apparently slowed rates of optical maturity, like Reiner Gamma, Descartes Formation, Airy and Gerasimovich craters, and Mare Marginis and Ingenni, etc.) appear as magnetic "bubbles" that scatter incoming protons into surrounding regions, making magnetic anomalies appear dark in a "hydrogen image."

The imaging technique would result in something opposite of what is seen in optical images of such areas, where they are characterized by an unusually bright albedo.

The incoming protons are part of the solar wind, a constant stream of particles given off by the Sun. They collide with feature in the Solar System and are usually stopped by planetary atmospheres. On airless bodies, like the asteroids or Mercury, the Solar Wind strikes the ground unimpeded. The SARA team expects such objects also reflect a regular ratio of incoming solar protons back into space as hydrogen.

"This knowledge provides timely advice for the scientists and engineers who are readying ESA’s BepiColombo mission to Mercury," according to ESA. "That spacecraft will be carrying two similar instruments to SARA and may find that the inner-most planet is reflecting more hydrogen than the Moon, because the Solar Wind is more concentrated closer to the Sun."

Until recently confirmed by instruments on-board IBEX and other vehicles, the Solar Wind was thought to be composed primarily of helium, the direct product of the Sun's fusion of hydrogen atoms at very high temperatures. This discovery was confirmed by the Southwest Research Institute last June.

SARA was one of three instruments ESA contributed to Chandrayaan-1, that shut down prematurely last August. The instrument was built jointly by scientific groups from Sweden, India, Japan, and Switzerland: Swedish Institute of Space Physics, Kiruna, Sweden; Vikram Sarabhai Space Centre, Trivandrum, India; University of Bern, Switzerland; and Institute of Space and Astronautical Science, Sagamihara, Japan.

(This article reflects findings presented in ‘Extremely high reflection of solar wind protons as neutral hydrogen atoms from regolith in space’, by M. Wieser, S. Barabash, Y. Futaana, M. Holmström, A. Bhardwaj, R. Sridharan, M.B. Dhanya, P. Wurz, A. Schaufelberger and K. Asamura, in press, Planetary and Space Science, 2009.)

"...after impacting the lunar surface, 0.1 to 1 percent of solar wind ions lose some energy and are reflected and scattered; solar wind ions normally contain helium nuclei and protons (hydrogen nuclei), but the solar wind ions that are reflected and scattered from the lunar surface contain no helium nuclei and are almost entirely made up of protons; and the reflected solar wind ions from the lunar surface are accelerated. Regarding lunar magnetic fields, many subtle magnetic nomalies have been confirmed near the South Pole on the farside. KAGUYA (in) a lower orbit observed the magnetic fields in more detail. Magnetic anomalies on the Moon are thought to have begun three to four billion years ago. By investigating plasma around the Moon and its magnetic fields, KAGUYA aims to explain the evolution of the Moon's magnetic environment."

At the moment the Sun's magnetic field, "the interplanetary magnetic field," is bottoming out with a long solar minimum, a low point in solar activity between the slow dying of Cycle 23 and the unexpectedly slow start to Cycle 24.

The Sun's relative quiet presents an opportunity to further determine any natural floor, what the "absolute" bottom background might be, to the interplanetary magnetic field, without the background noise of its eleven-year swing between often dramatic and chaotic peak activity.

This may not seem as immediately important to the earthbound as the health of Earth's magnetic field (though Earth's magnetic field's shielding energy is, in some measure, determined by our orbital vector perpendicular through the Sun's particle streams and magnetic field lines) but for machines and humans traveling outside Earth's magnetic field an improved understanding if the interplanetary magnetic field is essential.

Between its peak strength at solar max and its weakest at solar minimum , the in-fall of sometimes very heavy and energetic cosmic rays varies by half. The stronger the interplanetary magnetic field the greater the protection against interstellar cosmic rays, and it is literally a toss up if travel beyond Low Earth Orbit is "safer" for humans and their equipment at solar max or solar minimum.

At solar max the interplanetary magnetic field is strongest, providing a statistically important shield against Galactic Cosmic Rays, but it also increases likelihood travelers will encounter a dangerous "Solar Particle Event."

Solar Flares and attendant Coronal Mass Ejections can, of course, occur at anytime,though they are more likely at solar max. Even during the present protracted solar minimum, flares have been observed, along with coronal holes allowing the hot breath of solar wind, consisting mostly of protons, to gust away from the Sun's photosphere.

It is within today's design and materials technologies and spacecraft design to greatly shield life and equipment from most solar wind. Though rarer, a heavy cosmic ray consisting of a stripped nucleon of primordal metal and traveling near the speed of light might be shattered by its encounter with an aluminum-titanium hull of sufficient thickness but a resulting shower of secondary particles actually would increase the likelihood that an astronaut would receive a wider cone of ionizing radiation.

Generating a magnetic field has been suggested as a way to change the direction of infalling cosmic rays. But the effective size and strength of such a field's strength would need to be more than hundreds of kilometers in radius, to name just one known issue with this solution.

The fact remains that reducing the probability of radiation exposure induced death to below 4 percent, over an individual astronaut's lifetime, for a trip to Mars using presently available speeds is still beyond our capability. Though this will change, reducing the likelihood of being dosed by a wide range of cosmic rays by 50 percent, possible within the interplanetary magnetic field at solar max, is not a small factor in mitigating risk in long periods traveling in deep space.

Living on the Moon immediately provides a shield from half the infall of interstellar radiation, and under 15 meters of lunar regolith doses come down to levels on Earth at sea level.

It may be possible, where it would be unthinkable on Earth, to build interplanetary transport vessels shielded by lunar concrete and propelled on trips to Mars and elsewhere using rail guns and unimpeded by atmospheric drag.

Of a more basic concern to engineers and policy makers, it may prove very unwise to travel to Mars without first building an infrastructure on Earth's Moon. This is especially likely if the Sun's present solar minimum persists.

It may be that the downslope from solar max may be driven by CMEs, carrying away kinks of the Sun's internally twisted magnetic field, up and away from the Sun and temporarily reducing the interplanetary magnetic field by as much as ten percent. A CME may be the way the Sun balances out the tensions in its magnetic field that wind up as portions of the Sun rotate at different speeds.

Because the Sun's magnetic field swaps out polarity between its hemispheres from one cycle to the next, the interplanetary magnetic field changes polarity.

On first glance it may seem finding an absolute bottom, a basic and unchanging level to the interplanetary magnetic field, would be impossible. Scientists at the Russian Academy of Sciences, however, have published an examination of the interplanetary magnetic field between 1976 and 2000. They claim there is no point when the interplanetary magnetic field reaches "zero," and this is backed by observations of the Sun over this past year's lengthy solar minimum.

In 2008-2009, the sun has produced brief outcroppings of sunspots at polarities and latitutdes that clearly mark them as a beginning to the next solar maximum, with its next expected peak in activity now expected in 2013. Over the past summer however, months after Cycle 24 officially started, sunspots from Cycle 23 briefly appeared.

In The floor in the interplanetary magnetic field (Yermolaev, et. al. 2009) predicts a floor to the interplanetary magnetic field at 4.65 ± 6.0 nT.

Today's measurement of the interplanetary magnetic field on Spaceweather.com was 4.1 nT, and their report also "agrees well" with observations over the past thirty years.

NASA's Lunar Science Institute (NLSI) has selected a proposal submitter by NASA Goddard to investigate the influence of the Sun on the Moon. The wide-ranging effects of Solar Wind and its surface interaction with the dusty lunar surface was identified as essential research, by the National Academies of Science's Space Studies Board in 2007, before extended human activity on the Moon can begin.

The award, one of seven announced by NASA, devotes $5 million over four years beginning in April. Researchers will build advanced simulations to explore the interaction between the Sun and Moon, emphasizing surface interactions during solar particle events (SPEs) such as intense flares and Coronal Mass Ejections. The study will also investigate spellation along with both primary and secondary high energy Galactic Cosmic Ray impacts and those of micrometeorites.

"Many people think of the moon as dead, but if you look with a different pair of glasses – at the atomic level – it is very active," said Dr. William Farrell of NASA Goddard, Principal Investigator for the proposal, called the Dynamic Response of the Environment at the Moon (DREAM).

"One of our roles will be to provide modeling support to scientists examining data from NASA's lunar science missions, such as the Lunar Reconnaissance Orbiter (LRO). There are always surprises in science, and our computer models can help them understand unexpected results or choose among competing theories," said Farrell.

"The sun is constantly throwing energy and matter into space – radiation and a million-mile-per-hour stream of electrically charged particles called the solar wind. If you put an object in the path of this stuff, such as the moon, that object will get hit and react. This reaction to inflowing solar matter includes surface erosion of gas and dust. There are also other subtle reactions, like the electrostatic charging of the lunar surface and any object on the surface that can be a concern for human explorers. All these effects are enhanced during a solar storm when the sun temporarily spews out a greater amount of energy and matter," said Farrell.

DREAM researchers will study many ways the sun influences the moon, but some interactions will be of special interest to human explorers: solar storms, the electric charging of lunar dust, and the erosion of potential resources at the poles.