Since the Apollo program wrapped up in the early 1970s, people all around the world have dreamed of the day when we might return to the Moon, and stay there. And in recent years, however, that actual proposals for a lunar settlement have begun to take shape. As a result, a great deal of attention and research has been focused on whether or not the Moon has indigenous sources of water.
Thanks to missions like Chandrayaan-1 and the Lunar Reconnaissance Orbiter (LRO), scientists know that there are vast amounts of surface ice on the Moon. However, according to a new study, researchers from Brown University have found evidence of widespread water within volcanic deposits on the lunar surface. These findings could indicate that there are also vast sources of water within the Moon’s interior.
For their study – titled “Remote Detection of Widespread Indigenous Water in Lunar Pyroclastic Deposits” – Brown researchers Ralph E. Milliken and Shuai Li combined satellite data with new thermal profiles to search for signs of water away from the polar regions. In so doing, they addressed a long-standing theory about the likelihood of water in the Moon’s interior, as well as the predominant theory of how the Moon formed.
As noted, scientists have known for years that there are large amounts of frozen water in the Moon’s polar regions. At the same time, however, scientists have held that the Moon’s interior must have depleted of water and other volatile compounds billions of years ago. This was based on the widely-accepted hypothesis that the Moon formed after a Mars-sized object (named Theia) collided with Earth and threw up a considerable amount of debris.
Essentially, scientists believed that it was unlikely that any hydrogen – necessary to form water – could have survived the heat of this impact. However, as of a decade ago, new scientific findings began to emerge that cast doubt on this. The first was a 2008 study, where a team of researches (led by Alberto Saal of Brown University) detected trace amounts of water in samples of volcanic glass that were bought back by the Apollo 15 and Apollo 17 missions.
This was followed by a 2011 study (also from Brown University) that indicated how crystalline structures within those beads contained as much water as some basalt mineral deposits here on Earth. These findings were particularly significant, in that they suggested that parts of the Moon’s mantle could contain as much water as Earth’s. The question though was whether these findings represented the norm, or an anomaly.
“The key question is whether those Apollo samples represent the bulk conditions of the lunar interior or instead represent unusual or perhaps anomalous water-rich regions within an otherwise ‘dry’ mantle. By looking at the orbital data, we can examine the large pyroclastic deposits on the Moon that were never sampled by the Apollo or Luna missions. The fact that nearly all of them exhibit signatures of water suggests that the Apollo samples are not anomalous, so it may be that the bulk interior of the Moon is wet.”
To resolve this, Milliken and Li consulted orbital data to examine lunar volcanic deposits for signs of water. Basically, orbiters use spectrometers to bounce light off the surfaces of planets and astronomical bodies to see which wavelengths of light are absorbed and which are reflected. This data is therefore able to determine what compounds and minerals are present based on the absorption lines detected.
Using this technique to look for signs of water in lunar volcanic deposits (aka. pyroclastic deposits), however, was a rather difficult task. During the day, the lunar surface heats up, especially in the latitudes where volcanic deposits are located. As Milliken explained, spectronomers will therefore pick up thermal energy in addition to chemical signatures which this can throw off the readings:
“That thermally emitted radiation happens at the same wavelengths that we need to use to look for water. So in order to say with any confidence that water is present, we first need to account for and remove the thermally emitted component.”
To correct for this, Milliken and Li constructed a detailed temperature profile of the areas of the Moon they were examining. They then examined surface data collected by the Moon Mineralogy Mapper, the spectrographic imager that was part of India’s Chandrayaan-1 mission. They then compared this thermally-corrected surface data to the measurements conducted on the samples returned from the Apollo missions.
What they found was that areas of the Moon’s surface that had been previously mapped showed evidence of water in nearly all the large pyroclastic deposits. This included the deposits that were near the Apollo 15 and 17 landing sites where the lunar samples were obtained. From this, they determined that these samples were not anomalous in nature, and that water is distributed across the lunar surface.
What’s more, these findings could indicate that the Moon’s mantle is water-rich as well. Beyond being good news for future lunar missions, and the construction of a lunar settlement, these results could lead to a rethinking of how the Moon formed. This research was part of Shuai Li’s – a recent graduate of the University of Brown and the lead author on the study – Ph.D thesis. As he said of the study’s findings:
“The growing evidence for water inside the Moon suggest that water did somehow survive, or that it was brought in shortly after the impact by asteroids or comets before the Moon had completely solidified. The exact origin of water in the lunar interior is still a big question.
What’s more, Li indicated that lunar water that is located in volcanic deposits could be a boon for future lunar missions. “Other studies have suggested the presence of water ice in shadowed regions at the lunar poles, but the pyroclastic deposits are at locations that may be easier to access,” he said. “Anything that helps save future lunar explorers from having to bring lots of water from home is a big step forward, and our results suggest a new alternative.”
Between NASA, the ESA, Roscosmos, the ISRO and the China National Space Administration (CNSA), there are no shortage of plans to explore the Moon in the future, not to mention establishing a permanent base there. Knowing there’s abundant surface water (and maybe more in the interior as well) is therefore very good news. This water could be used to create hydrazine fuel, which would significantly reduce the costs of individual missions to the Moon.
It also makes the idea of a stopover base on the Moon, where ships traveling deeper into space could refuel and resupply – a move which would shave billions off of deep-space missions. An abundant source of local water could also ensure a ready supply of drinking and irrigation water for future lunar outposts. This would also reduce costs by ensuring that not all supplies would need to be shipped from Earth.
On top of all that, the ability to conduct experiments into how plants grow in reduced gravity would yield valuable information that could be used for long-term missions to Mars and other Solar bodies. It could therefore be said, without a trace of exaggeration, that water on the Moon is the key to future space missions.
Ever since we began sending crewed missions to the Moon, people have been dreaming of the day when we might one day colonize it. Just imagine, a settlement on the lunar surface, where everyone constantly feels only about 15% as heavy as they do here on Earth. And in their spare time, the colonists get to do all kinds of cool research trek across the surface in lunar rovers. Gotta admit, it sounds fun!
More recently, the idea of prospecting and mining on the Moon has been proposed. This is due in part to renewed space exploration, but also the rise of private aerospace companies and the NewSpace industry. With missions to the Moon schedules for the coming years and decades, it seems logical to thinking about how we might set up mining and other industries there as well?
Several proposals have been made to establish mining operations on the Moon; initially by space agencies like NASA, but more recently by private interests. Many of the earliest proposals took place during the 1950s, in response to the Space Race, which saw a lunar colony as a logical outcome of lunar exploration.
For instance, in 1954 Arthur C. Clarke proposed a lunar base where inflatable modules were covered in lunar dust for insulation and communications were provided by a inflatable radio mast. And in 1959, John S. Rinehart – the director of the Mining Research Laboratory at the Colorado School of Mines – proposed a tubular base that would “float” across the surface.
Since that time, NASA, the US Army and Air Force, and other space agencies have issued proposals for the creation of a lunar settlement. In all cases, these plans contained allowances for resource utilization to make the base as self-sufficient as possible. However, these plans predated the Apollo program, and were largely abandoned after its conclusion. It has only been in the past few decades that detailed proposals have once again been made.
For instance, during the Bush Administration (2001-2009), NASA entrtained the possibility of creating a “lunar outpost”. Consistent with their Vision for Space Exploration (2004), the plan called for the construction of a base on the Moon between 2019 and 2024. One of the key aspects of this plan was the use of ISRU techniques to produce oxygen from the surrounding regolith.
These plans were cancelled by the Obama administration and replaced with a plan for a Mars Direct mission (known as NASA’s “Journey to Mars“). However, during a workshop in 2014, representatives from NASA met with Harvard geneticist George Church, Peter Diamandis from the X Prize Foundation and other experts to discuss low-cost options for returning to the Moon.
The workshop papers, which were published in a special issue of New Space, describe how a settlement could be built on the Moon by 2022 for just $10 billion USD. According to their papers, a low-cost base would be possible thanks to the development of the space launch business, the emergence of the NewSpace industry, 3D printing, autonomous robots, and other recently-developed technologies.
In 2010, NASA established the Robotic Mining Competition, an annual incentive-based competition where university students design and build robots to navigate a simulated Martian environment. One of the most-important aspects of the competition is creating robots that can rely on ISRU to turn local resources into usable materials. The applications produced are also likely to be of use during future lunar missions.
And the NewSpace industry has also been producing some interesting proposals of late. In 2010, a group of Silicon Valley entrepreneurs came together for create Moon Express, a private company that plans to offer commercial lunar robotic transportation and data services, as well as the a long-term goal of mining the Moon. In December of 2015, they became the first company competing for the Lunar X Prize to build and test a robotic lander – the MX-1.
In 2010, Arkyd Astronautics (renamed Planetary Resources in 2012) was launched for the purpose of developing and deploying technologies for asteroid mining. In 2013, Deep Space Industries was formed with the same purpose in mind. Though these companies are focused predominantly on asteroids, the appeal is much the same as lunar mining – which is expanding humanity’s resource base beyond Earth.
Based on the study of lunar rocks, which were brought back by the Apollo missions, scientists have learned that the lunar surface is rich in minerals. Their overall composition depends on whether the rocks came from lunar maria (large, dark, basaltic plains formed from lunar eruptions) or the lunar highlands.
Rocks obtained from lunar maria showed large traces of metals, with 14.9% alumina (Al²O³), 11.8% calcium oxide (lime), 14.1% iron oxide, 9.2% magnesia (MgO), 3.9% titanium dioxide (TiO²) and 0.6% sodium oxide (Na²O). Those obtained from the lunar highlands are similar in composition, with 24.0% alumina, 15.9% lime, 5.9% iron oxide, 7.5% magnesia, and 0.6% titanium dioxide and sodium oxide.
These same studies have shown that lunar rocks contain large amounts of oxygen, predominantly in the form of oxidized minerals. Experiments have been conducted that have shown how this oxygen could be extracted to provide astronauts with breathable air, and could be used to make water and even rocket fuel.
The Moon also has concentrations of Rare Earth Metals (REM), which are attractive for two reasons. On the one hand, REMs are becoming increasingly important to the global economy, since they are used widely in electronic devices. On the other hand, 90% of current reserves of REMs are controlled by China; so having a steady access to an outside source is viewed by some as a national security matter.
Similarly, the Moon has significant amounts of water contained within its lunar regolith and in the permanently shadowed areas in its north and southern polar regions.This water would also be valuable as a source of rocket fuel, not to mention drinking water for astronauts.
But since that time, multiple missions have not only found samples of water on the lunar surface, but revealed evidence of where it came from. The first was India’s Chandrayaan-1 mission, which sent an impactor to the lunar surface on Nov. 18th, 2008. During its 25-minute descent, the impact probe’s Chandra’s Altitudinal Composition Explorer (CHACE) found evidence of water in the Moon’s thin atmosphere.
In November 2009, the NASA LCROSS space probe made similar finds around the southern polar region, as an impactor it sent to the surface kicked up material shown to contain crystalline water. In 2012, surveys conducted by the Lunar Reconnaissance Orbiter (LRO) revealed that ice makes up to 22% of the material on the floor of the Shakleton crater (located in the southern polar region).
But perhaps the most valuable commodity on the surface of the Moon might be helium-3. Helium-3 is an atom emitted by the Sun in huge amounts, and is a byproduct of the fusion reactions that take place inside. Although there is little demand for helium-3 today, physicists think they’ll serve as the ideal fuel for fusion reactors.
The Sun’s solar wind carries the helium-3 away from the Sun and out into space – eventually out of the Solar System entirely. But the helium-3 particles can crash into objects that get in their way, like the Moon. Scientists haven’t been able to find any sources of helium-3 here on Earth, but it seems to be on the Moon in huge quantities.
From a commercial and scientific point of view, there are several reasons why Moon mining would be beneficial to humanity. For starters, it would be absolutely essential to any plans to build a settlement on the Moon, as in-situ resource utilization (ISRU) would be far more cost effective than transporting materials from Earth.
Also, it is predicted that the proposed space exploration efforts for the 21st century will require large amounts of materiel. That which is mined on the Moon would be launched into space at a fraction of the cost of what is mined here on Earth, due to the Moon’s much lower gravity and escape velocity.
In addition, the Moon has an abundance of raw materials that humanity relies on. Much like Earth, it is composed of silicate rocks and metals that are differentiated between a geochemically distinct layers. These consist of is iron-rich inner core, and iron-rich fluid outer core, a partially molten boundary layer, and a solid mantle and crust.
In addition, it has been recognized for some time that a lunar base – which would include resource operations – would be a boon for missions farther into the Solar System. For missions heading to Mars in the coming decades, the outer Solar System, or even Venus and Mercury, the ability to be resupplied from an lunar outpost would cut the cost of individual missions drastically.
Naturally, the prospect of setting up mining interests on the Moon also presents some serious challenges. For instance, any base on the Moon would need to be protected from surface temperatures, which range from very low to high – 100 K (-173.15 °C;-279.67 °F) to 390 K (116.85 °C; 242.33 °F) – at the equator and average 150 K (-123.15 °C;-189.67 °F) in the polar regions.
Radiation exposure is also an issue. Due to the extremely thin atmosphere and lack of a magnetic field, the lunar surface experiences half as much radiation as an object in interplanetary space. This means that astronauts and/or lunar workers would at a high risk of exposure to cosmic rays, protons from solar wind, and the radiation caused by solar flares.
Then there’s the Moon dust, which is an extremely abrasive glassy substance that has been formed by billions of years of micrometeorite impacts on the surface. Due to the absence of weathering and erosion, Moon dust is unrounded and can play havoc with machinery, and poses a health hazard. Worst of all, its sticks to everything it touches, and was a major nuisance for the Apollo crews!
And while the lower gravity is attractive as far as launches are concerned, it is unclear what the long-term health effects of it will be on humans. As repeated research has shown, exposure to zero-gravity over month-long periods causes muscular degeneration and loss of bone density, as well as diminished organ function and a depressed immune system.
And while there has been plenty of speculation about a “loophole” which does not expressly forbid private ownership, there is no legal consensus on this. As such, as lunar prospecting and mining become more of a possibility, a legal framework will have to be worked out that ensures everything is on the up and up.
Though it might be a long way off, it is not unreasonable to think that someday, we could be mining the Moon. And with its rich supplies of metals (which includes REMs) becoming part of our economy, we could be looking at a future characterized by post-scarcity!
Over the course of the past few decades, our ongoing exploration the Solar System has revealed some surprising discoveries. For example, while we have yet to find life beyond our planet, we have discovered that the elements necessary for life (i.e organic molecules, volatile elements, and water) are a lot more plentiful than previously thought. In the 1960’s, it was theorized that water ice could exist on the Moon; and by the next decade, sample return missions and probes were confirming this.
Since that time, a great deal more water has been discovered, which has led to a debate within the scientific community as to where it all came from. Was it the result of in-situ production, or was it delivered to the surface by water-bearing comets, asteroids and meteorites? According to a recent study produced by a team of scientists from the UK, US and France, the majority of the Moon’s water appears to have come from meteorites that delivered water to Earth and the Moon billions of years ago.
For the sake of their study, which appeared recently in Nature Communications, the international research team examined the samples of lunar rock and soil that were returned by the Apollo missions. When these samples were originally examined upon their return to Earth, it was assumed that the trace of amounts of water they contained were the result of contamination from Earth’s atmosphere since the containers in which the Moon rocks were brought home weren’t airtight. The Moon, it was widely believed, was bone dry.
However, that which was discovered on the surface paled in comparison the water that was discovered beneath it. Evidence of water in the interior was first revealed by the ISRO’s Chandrayaan-1 lunar orbiter – which carried the NASA’s Moon Mineralogy Mapper (M3) and delivered it to the surface. Analysis of this and other data has showed that water in the Moon’s interior is up to a million times more abundant than what’s on the surface.
The presence of so much water beneath the surface has begged the question, where did it all come from? Whereas water that exists on the Moon’s surface in lunar regolith appears to be the result of interaction with solar wind, this cannot account for the abundant sources deep underground. A previous study suggested that it came from Earth, as the leading theory for the Moon’s formation is that a large Mars-sized body impacted our nascent planet about 4.5 billion years ago, and the resulting debris formed the Moon. The similarity between water isotopes on both bodies seems to support that theory.
However, according to Dr. David A. Kring, a member of the research team that was led by Jessica Barnes from Open University, this explanation can only account for about a quarter of the water inside the moon. This, apparently, is due to the fact that most of the water would not have survived the processes involved in the formation of the Moon, and keep the same ratio of hydrogen isotopes.
Instead, Kring and his colleagues examined the possibility that water-bearing meteorites delivered water to both (hence the similar isotopes) after the Moon had formed. As Dr. Kring told Universe Today via email:
“The current study utilized analyses of lunar samples that had been collected by the Apollo astronauts, because those samples provide the best measure of the water inside the Moon. We compared those analyses with analyses of meteoritic samples from asteroids and spacecraft analyses of comets.”
By comparing the ratios of hydrogen to deuterium (aka. “heavy hydrogen”) from the Apollo samples and known comets, they determined that a combination of primitive meteorites (carbonaceous chondrite-type) were responsible for the majority of water to be found in the Moon’s interior today. In addition, they concluded that these types of comets played an important role when it comes to the origins of water in the inner Solar System.
For some time, scientists have argued that the abundance of water on Earth may be due in part to impacts from comets, trans-Neptunian objects or water-rich meteoroids. Here too, this was based on the fact that the ratio of the hydrogen isotopes (deuterium and protium) in asteroids like 67P/Churyumov-Gerasimenko revealed a similar percentage of impurities to carbon-rich chondrites that were found in the Earth’s coeans.
But how much of Earth’s water was delivered, how much was produced indigenously, and whether or not the Moon was formed with its water already there, have remained the subject of much scholarly debate. Thank to this latest study, we may now have a better idea of how and when meteorites delivered water to both bodies, thus giving us a better understanding of the origins of water in the inner Solar System.
“Some meteoritic samples of asteroids contain up to 20% water,” said Kring. “That reservoir of material – that is asteroids – are closer to the Earth-Moon system and, logically, have always been a good candidate source for the water in the Earth-Moon system. The current study shows that to be true. That water was apparently delivered 4.5 to 4.3 billion years ago.“
The existence of water on the Moon has always been a source of excitement, particularly to those who hope to see a lunar base established there someday. By knowing the source of that water, we can also come to know more about the history of the Solar System and how it came to be. It will also come in handy when it comes time to search for other sources of water, which will always be a factor when trying to establishing outposts and even colonies throughout the Solar System.
Look up in the night sky. On a clear night, if you’re lucky, you’ll catch a glimpse of the Moon shining in all it’s glory. As Earth‘s only satellite, the Moon has orbited our planet for over three and a half billion years. There has never been a time when human beings haven’t been able to look up at the sky and see the Moon looking back at them.
As a result, it has played a vital role in the mythological and astrological traditions of every human culture. A number of cultures saw it as a deity while others believed that its movements could help them to predict omens. But it is only in modern times that the true nature and origins of the Moon, not to mention the influence it has on planet Earth, have come to be understood.
Size, Mass and Orbit:
With a mean radius of 1737 km and a mass of 7.3477 x 10²² kg, the Moon is 0.273 times the size of Earth and 0.0123 as massive. Its size, relative to Earth, makes it quite large for a satellite – second only to Charon‘s size relative to Pluto. With a mean density of 3.3464 g/cm³, it is 0.606 times as dense as Earth, making it the second densest moon in our Solar System (after Io). Last, it has a surface gravity equivalent to 1.622 m/s2, which is 0.1654 times, or 17%, the Earth standard (g).
The Moon’s orbit has a minor eccentricity of 0.0549, and orbits our planet at a distance of between 356,400-370,400 km at perigee and 404,000-406,700 km at apogee. This gives it an average distance (semi-major axis) of 384,399 km, or 0.00257 AU. The Moon has an orbital period of 27.321582 days (27 d 7 h 43.1 min), and is tidally-locked with our planet, which means the same face is always pointed towards Earth.
Structure and Composition:
Much like Earth, the Moon has a differentiated structure that includes an inner core, an outer core, a mantle, and a crust. It’s core is a solid iron-rich sphere that measures 240 km (150 mi) across, and it surrounded by a outer core that is primarily made of liquid iron and which has a radius of roughly 300 km (190 mi).
Around the core is a partially molten boundary layer with a radius of about 500 km (310 mi). This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon’s formation 4.5 billion years ago. Crystallization of this magma ocean would have created a mantle rich in magnesium and iron nearer to the top, with minerals like olivine, clinopyroxene, and orthopyroxene sinking lower.
The mantle is also composed of igneous rock that is rich in magnesium and iron, and geochemical mapping has indicated that the mantle is more iron rich than Earth’s own mantle. The surrounding crust is estimated to be 50 km (31 mi) thick on average, and is also composed of igneous rock.
The Moon is the second densest satellite in the Solar System after Io. However, the inner core of the Moon is small, at around 20% of its total radius. Its composition is not well constrained, but it is probably a metallic iron alloy with a small amount of sulfur and nickel and analyses of the Moon’s time-variable rotation indicate that it is at least partly molten.
The presence of water has also been confirmed on the Moon, the majority of which is located at the poles in permanently-shadowed craters, and possibly also in reservoirs located beneath the lunar surface. The widely accepted theory is that most of the water was created through the Moon’s interaction of solar wind – where protons collided with oxygen in the lunar dust to create H²O – while the rest was deposited by cometary impacts.
The geology of the Moon (aka. selenology) is quite different from that of Earth. Since the Moon lacks a significant atmosphere, it does not experience weather – hence there is no wind erosion. Similarly, since it lacks liquid water, there is also no erosion caused by flowing water on its surface. Because of its small size and lower gravity, the Moon cooled more rapidly after forming, and does not experience tectonic plate activity.
Instead, the complex geomorphology of the lunar surface is caused by a combination of processes, particularly impact cratering and volcanoes. Together, these forces have created a lunar landscape that is characterized by impact craters, their ejecta, volcanoes, lava flows, highlands, depressions, wrinkle ridges and grabens.
The most distinctive aspect of the Moon is the contrast between its bright and dark zones. The lighter surfaces are known as the “lunar highlands” while the darker plains are called maria (derived from the Latin mare, for “sea”). The highlands are made of igneous rock that is predominately composed of feldspar, but also contains trace amounts of magnesium, iron, pyroxene, ilmenite, magnetite, and olivine.
Mare regions, in contrast, are formed from basalt (i.e. volcanic) rock. The maria regions often coincide with the “lowlands,” but it is important to note that the lowlands (such as within the South Pole-Aitken basin) are not always covered by maria. The highlands are older than the visible maria, and hence are more heavily cratered.
Other features include rilles, which are long, narrow depressions that resemble channels. These generally fall into one of three categories: sinuous rilles, which follow meandering paths; arcuate rilles, which have a smooth curve; and linear rilles, which follow straight paths. These features are often the result of the formation of localized lava tubes that have since cooled and collapsed, and can be traced back to their source (old volcanic vents or lunar domes).
Lunar domes are another feature that is related to volcanic activity. When relatively viscous, possibly silica-rich lava erupts from local vents, it forms shield volcanoes that are referred to as lunar domes. These wide, rounded, circular features have gentle slopes, typically measure 8-12 km in diameter and rise to an elevation of a few hundred meters at their midpoint.
Wrinkle ridges are features created by compressive tectonic forces within the maria. These features represent buckling of the surface and form long ridges across parts of the maria. Grabens are tectonic features that form under extension stresses and which are structurally composed of two normal faults, with a down-dropped block between them. Most grabens are found within the lunar maria near the edges of large impact basins.
Impact craters are the Moon’s most common feature, and are created when a solid body (an asteroid or comet) collides with the surface at a high velocity. The kinetic energy of the impact creates a compression shock wave that creates a depression, followed by a rarefaction wave that propels most of the ejecta out of the crater, and then a rebounds to form a central peak.
These craters range in size from tiny pits to the immense South Pole–Aitken Basin, which has a diameter of nearly 2,500 km and a depth of 13 km. In general, the lunar history of impact cratering follows a trend of decreasing crater size with time. In particular, the largest impact basins were formed during the early periods, and these were successively overlaid by smaller craters.
There are estimated to be roughly 300,000 craters wider than 1 km (0.6 mi) on the Moon’s near side alone. Some of these are named for scholars, scientists, artists and explorers. The lack of an atmosphere, weather and recent geological processes mean that many of these craters are well-preserved.
Another feature of the lunar surface is the presence of regolith (aka. Moon dust, lunar soil). Created by billions of years of collisions by asteroids and comets, this fine grain of crystallized dust covers much of the lunar surface. The regolith contains rocks, fragments of minerals from the original bedrock, and glassy particles formed during the impacts.
The chemical composition of the regolith varies according to its location. Whereas the regolith in the highlands is rich in aluminum and silica, the regolith in the maria is rich in iron and magnesium and is silica-poor, as are the basaltic rocks from which it is formed.
Geological studies of the Moon are based on a combination of Earth-based telescope observations, measurements from orbiting spacecraft, lunar samples, and geophysical data. A few locations were sampled directly during the Apollo missions in the late 1960s and early 1970s, which returned approximately 380 kilograms (838 lb) of lunar rock and soil to Earth, as well as several missions of the Soviet Luna programme.
Much like Mercury, the Moon has a tenuous atmosphere (known as an exosphere), which results in severe temperature variations. These range from -153°C to 107°C on average, though temperatures as low as -249°C have been recorded. Measurements from NASA’s LADEE have mission determined the exosphere is mostly made up of helium, neon and argon.
The helium and neon are the result of solar wind while the argon comes from the natural, radioactive decay of potassium in the Moon’s interior. There is also evidence of frozen water existing in permanently shadowed craters, and potentially below the soil itself. The water may have been blown in by the solar wind or deposited by comets.
Several theories have been proposed for the formation of the Moon. These include the fission of the Moon from the Earth’s crust through centrifugal force, the Moon being a preformed object that was captured by Earth’s gravity, and the Earth and Moon co-forming together in the primordial accretion disk. The estimated age of the Moon also ranges from it being formed 4.40-4.45 billion years ago to 4.527 ± 0.010 billion years ago,roughly 30–50 million years after the formation of the Solar System.
The prevailing hypothesis today is that the Earth-Moon system formed as a result of an impact between the newly-formed proto-Earth and a Mars-sized object (named Theia) roughly 4.5 billion years ago. This impact would have blasted material from both objects into orbit, where it eventually accreted to form the Moon.
This has become the most accepted hypothesis for several reasons. For one, such impacts were common in the early Solar System, and computer simulations modelling the impact are consistent with the measurements of the Earth-Moon system’s angular momentum, as well as the small size of the lunar core.
In addition, examinations of various meteorites show that other inner Solar System bodies (such as Mars and Vesta) have very different oxygen and tungsten isotopic compositions to Earth. In contrast, examinations of the lunar rocks brought back by the Apollo missions show that Earth and the Moon have nearly identical isotopic compositions.
This is the most compelling evidence suggesting that the Earth and the Moon have a common origin.
Relationship to Earth:
The Moon makes a complete orbit around Earth with respect to the fixed stars about once every 27.3 days (its sidereal period). However, because Earth is moving in its orbit around the Sun at the same time, it takes slightly longer for the Moon to show the same phase to Earth, which is about 29.5 days (its synodic period). The presence of the Moon in orbit influences conditions here on Earth in a number of ways.
The most immediate and obvious are the ways its gravity pulls on Earth – aka. it’s tidal effects. The result of this is an elevated sea level, which are commonly referred to as ocean tides. Because Earth spins about 27 times faster than the Moon moves around it, the bulges are dragged along with Earth’s surface faster than the Moon moves, rotating around Earth once a day as it spins on its axis.
The ocean tides are magnified by other effects, such as frictional coupling of water to Earth’s rotation through the ocean floors, the inertia of water’s movement, ocean basins that get shallower near land, and oscillations between different ocean basins. The gravitational attraction of the Sun on Earth’s oceans is almost half that of the Moon, and their gravitational interplay is responsible for spring and neap tides.
Gravitational coupling between the Moon and the bulge nearest the Moon acts as a torque on Earth’s rotation, draining angular momentum and rotational kinetic energy from Earth’s spin. In turn, angular momentum is added to the Moon’s orbit, accelerating it, which lifts the Moon into a higher orbit with a longer period.
As a result of this, the distance between Earth and Moon is increasing, and Earth’s spin is slowing down. Measurements from lunar ranging experiments with laser reflectors (which were left behind during the Apollo missions) have found that the Moon’s distance to Earth increases by 38 mm (1.5 in) per year.
This speeding and slowing of Earth and the Moon’s rotation will eventually result in a mutual tidal locking between the Earth and Moon, similar to what Pluto and Charon experience. However, such a scenario is likely to take billions of years, and the Sun is expected to have become a red giant and engulf Earth long before that.
The lunar surface also experiences tides of around 10 cm (4 in) amplitude over 27 days, with two components: a fixed one due to Earth (because they are in synchronous rotation) and a varying component from the Sun. The cumulative stress caused by these tidal forces produces moonquakes. Despite being less common and weaker than earthquakes, moonquakes can last longer (one hour) since there is no water to damp out the vibrations.
Another way the Moon effects life on Earth is through occultation (i.e. eclipses). These only happen when the Sun, the Moon, and Earth are in a straight line, and take one of two forms – a lunar eclipse and a solar eclipse. A lunar eclipse occurs when a full Moon passes behind Earth’s shadow (umbra) relative to the Sun, which causes it to darken and take on a reddish appearance (aka. a “Blood Moon” or “Sanguine Moon”.)
A solar eclipse occurs during a new Moon, when the Moon is between the Sun and Earth. Since they are the same apparent size in the sky, the moon can either partially block the Sun (annular eclipse) or fully block it (total eclipse). In the case of a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye.
Because the Moon’s orbit around Earth is inclined by about 5° to the orbit of Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes.The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the “Saros Cycle“, which is a period of approximately 18 years.
History of Observation:
Human beings have been observing the Moon since prehistoric times, and understanding the Moon’s cycles was one of the earliest developments in astronomy. The earliest examples of this comes from the 5th century BCE, when Babylonian astronomers had recorded the 18-year Satros cycle of lunar eclipses, and Indian astronomers had described the Moon’s monthly elongation.
The ancient Greek philosopher Anaxagoras (ca. 510 – 428 BCE) reasoned that the Sun and Moon were both giant spherical rocks, and the latter reflected the light of the former. In Aristotle’s “On the Heavens“, which he wrote in 350 BCE, the Moon was said to mark the boundary between the spheres of the mutable elements (earth, water, air and fire), and the heavenly stars – an influential philosophy that would dominate for centuries.
In the 2nd century BCE, Seleucus of Seleucia correctly theorized that tides were due to the attraction of the Moon, and that their height depends on the Moon’s position relative to the Sun. In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance. These figures were greatly improved by Ptolemy (90–168 BCE), who’s values of a mean distance of 59 times Earth’s radius and a diameter of 0.292 Earth diameters were close to the correct values (60 and 0.273 respectively).
By the 4th century BCE, the Chinese astronomer Shi Shen gave instructions for predicting solar and lunar eclipses. By the time of the Han Dynasty (206 BCE – 220 CE), astronomers recognized that moonlight was reflected from the Sun, and Jin Fang (78–37 BC) postulated that the Moon was spherical in shape.
In 499 CE, the Indian astronomer Aryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause of the shining of the Moon. The astronomer and physicist Alhazen (965–1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon in all directions.
Shen Kuo (1031–1095) of the Song dynasty created an allegory to explain the waxing and waning phases of the Moon. According to Shen, it was comparable to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent.
During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognized as a sphere, though many believed that it was “perfectly smooth”. In keeping with medieval astronomy, which combined Aristotle’s theories of the universe with Christian dogma, this view would later be challenged as part of the Scientific Revolution (during the 16th and 17th century) where the Moon and other planets would come to be seen as being similar to Earth.
Using a telescope of his own design, Galileo Galilei drew one of the first telescopic drawings of the Moon in 1609, which he included in his book Sidereus Nuncius (“Starry Messenger). From his observations, he noted that the Moon was not smooth, but had mountains and craters. These observations, coupled with observations of moons orbiting Jupiter, helped him to advance the heliocentric model of the universe.
Telescopic mapping of the Moon followed, which led to the lunar features being mapped in detail and named. The names assigned by Italian astronomers Giovannia Battista Riccioli and Francesco Maria Grimaldi are still in use today. The lunar map and book on lunar features created by German astronomers Wilhelm Beer and Johann Heinrich Mädler between 1834 and 1837 were the first accurate trigonometric study of lunar features, and included the heights of more than a thousand mountains.
Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s, when English astronomer Richard Proctor proposed that they were formed by collisions. This view gained support throughout the remainder of the 19th century; and by the early 20th century, led to the development of lunar stratigraphy – part of the growing field of astrogeology.
With the beginning of the Space Age in the mid-20th century, the ability to physically explore the Moon became possible for the first time. And with the onset of the Cold War, both the Soviet and American space programs became locked in an ongoing effort to reach the Moon first. This initially consisted of sending probes on flybys and landers to the surface, and culminated with astronauts making manned missions.
Exploration of the Moon began in earnest with the Soviet Luna program. Beginning in earnest in 1958, the programmed suffered the loss of three unmanned probes. But by 1959, the Soviets managed to successfully dispatch fifteen robotic spacecraft to the Moon and accomplished many firsts in space exploration. This included the first human-made objects to escape Earth’s gravity (Luna 1), the first human-made object to impact the lunar surface (Luna 2), and the first photographs of the far side of the Moon (Luna 3).
Between 1959 and 1979, the program also managed to make the first successful soft landing on the Moon (Luna 9), and the first unmanned vehicle to orbit the Moon (Luna 10) – both in 1966. Rock and soil samples were brought back to Earth by three Luna sample return missions – Luna 16 (1970), Luna 20 (1972), and Luna 24 (1976).
Two pioneering robotic rovers landed on the Moon – Luna 17 (1970) and Luna 21 (1973) – as a part of Soviet Lunokhod program. Running from 1969 to 1977, this program was primarily designed to provide support for the planned Soviet manned moon missions. But with the cancellation of the Soviet manned moon program, they were instead used as remote-controlled robots to photograph and explore the lunar surface.
NASA began launching probes to provide information and support for an eventual Moon landing in the early 60s. This took the form of the Ranger program, which ran from 1961 – 1965 and produced the first close-up pictures of the lunar landscape. It was followed by the Lunar Orbiter program which produced maps of the entire Moon between 1966-67, and the Surveyor program which sent robotic landers to the surface between 1966-68.
In 1969, astronaut Neil Armstrong made history by becoming the first person to walk on the Moon. As the commander of the American mission Apollo 11, he first sett foot on the Moon at 02:56 UTC on 21 July 1969. This represented the culmination of the Apollo program (1969-1972), which sought to send astronauts to the lunar surface to conduct research and be the first human beings to set foot on a celestial body other than Earth.
The Apollo 11 to 17 missions (save forApollo 13, which aborted its planned lunar landing) sent a total of 13 astronauts to the lunar surface and returned 380.05 kilograms (837.87 lb) of lunar rock and soil. Scientific instrument packages were also installed on the lunar surface during all the Apollo landings. Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17 landing sites, some of which are still operational.
After the Moon Race was over, there was a lull in lunar missions. However, by the 1990s, many more countries became involve in space exploration. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its Hiten spacecraft, an orbiter which released the smaller Hagoroma probe.
In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine to lunar orbit to obtain the first near-global topographic map of the Moon and the first global multispectral images of the lunar surface. This was followed in 1998 by the Lunar Prospector mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters.
Since the year 2000, exploration of the moon has intensified, with a growing number of parties becoming involved. The ESA’s SMART-1spacecraft, the second ion-propelled spacecraft ever created, made the first detailed survey of chemical elements on the lunar surface while in orbit from November 15th, 2004, until its lunar impact on September 3rd, 2006.
China has pursued an ambitious program of lunar exploration under their Chang’e program. This began with Chang’e 1, which successfully obtained a full image map of the Moon during its sixteen month orbit (November 5th, 2007 – March 1st, 2009) of the Moon. This was followed in October of 2010 with the Chang’e 2 spacecraft, which mapped the Moon at a higher resolution before performing a flyby of asteroid 4179 Toutatis in December of 2012, then heading off into deep space.
On 14 December 2013, Chang’e 3 improved upon its orbital mission predecessors by landing a lunar lander onto the Moon’s surface, which in turn deployed a lunar rover named Yutu (literally “Jade Rabbit”). In so doing, Chang’e 3 made the first soft lunar landing since Luna 24 in 1976, and the first lunar rover mission since Lunokhod 2 in 1973.
Between October 4th, 2007, and June 10th, 2009, the Japan Aerospace Exploration Agency‘s (JAXA) Kaguya (“Selene”) mission – a lunar orbiter fitted with a high-definition video camera and two small radio-transmitter satellites – obtained lunar geophysics data and took the first high-definition movies from beyond Earth orbit.
The Indian Space Research Organisation (ISRO) first lunar mission, Chandrayaan I, orbited the Moon between November 2008 and August 2009 and created a high resolution chemical, mineralogical and photo-geological map of the lunar surface, as well as confirming the presence of water molecules in lunar soil. A second mission was planned for 2013 in collaboration with Roscosmos, but was cancelled.
NASA has also been busy in the new millennium. In 2009, they co-launched the Lunar Reconnaissance Orbiter (LRO) and the Lunar CRater Observation and Sensing Satellite(LCROSS) impactor. LCROSS completed its mission by making a widely observed impact in the crater Cabeus on October 9th, 2009, while the LRO is currently obtaining precise lunar altimetry and high-resolution imagery.
Upcoming lunar missions include Russia’s Luna-Glob – an unmanned lander with a set of seismometers, and an orbiter based on its failed Martian Fobos-Grunt mission. Privately funded lunar exploration has also been promoted by the Google Lunar X Prize, which was announced on September 13th, 2007, and offers US$20 million to anyone who can land a robotic rover on the Moon and meet other specified criteria.
Under the terms of the Outer Space Treaty, the Moon remains free to all nations to explore for peaceful purposes. As our efforts to explore space continue, plans to create a lunar base and possibly even a permanent settlement may become a reality. Looking to the distant future, it wouldn’t be far fetched at all to imagine native-born humans living on the Moon, perhaps known as Lunarians (though I imagine Lunies will be more popular!)
We have many interesting articles about the Moon here at Universe Today. Below is a list that covers just about everything we know about it today. We hope you find what you are looking for:
Shining like a beacon in Earth’s sky is the Moon. We’ve seen so much of it in our lifetimes that it’s easy to take it for granted; even the human landings on the Moon in the 1960s and 1970s were eventually taken for granted by the public.
Fortunately for science, we haven’t stopped looking at the Moon in the decades after Neil Armstrong took his first step. Here are a few things to consider about Earth’s closest big neighbor.
An image of water-filled debris ejected from Cabeus crater about 20 seconds after the 2009 LCROSS impact. Courtesy of Science/AAAS.
Comets? Asteroids? The Earth? The origins of water now known to exist within the Moon’s soil — thanks to recent observations by various lunar satellites and the impact of the LCROSS mission’s Centaur rocket in 2009 — has been an ongoing puzzle for scientists. Now, new research supports that the source of at least some of the Moon’s water is the Sun, with the answer blowing in the solar wind.
Spectroscopy research conducted on Apollo samples by a team from the University of Tennessee, University of Michigan and Caltech has revealed “significant amounts” of hydroxyl within microscopic glass particles found inside lunar soil, the results of micrometeorite impacts.
According to the research team, the hydroxyl “water” within the lunar glass was likely created by interactions with protons and hydrogen ions from the solar wind.
“We found that the ‘water’ component, the hydroxyl, in the lunar regolith is mostly from solar wind implantation of protons, which locally combined with oxygen to form hydroxyls that moved into the interior of glasses by impact melting,” said Youxue Zhang, Professor of Geological Sciences at the University of Michigan.
Hydroxyl is the pairing of a single oxygen atom to a single hydrogen atom (OH). Each molecule of water contains two hydroxyl groups.
Although such glass particles are widespread on the surface of the Moon — the researchers studied samples returned from Apollo 11, Apollo 16 and Apollo 17 missions — the water in hydroxyl form is not something that could be easily used by future lunar explorers. Still, the findings suggest that solar wind-derived hydroxyl may also exist on the surface of other airless worlds, like Mercury, Vesta or Eros… especially within permanently-shadowed craters and depressions.
“These planetary bodies have very different environments, but all have the potential to produce water,” said Yang Liu, University of Tennessee scientist and lead author of the team’s paper.
The discovery of hydroxyl within lunar glasses presents an “unanticipated, abundant reservoir” of water on the Moon, and possibly throughout the entire Solar System.
A team of NASA-funded researchers led by Carnegie Institution’s Erik Hauri has recently announced the discovery of more water on the Moon, in the form of ancient magma that has been locked up in tiny crystals contained within soil samples collected by Apollo 17 astronauts. The amounts found indicate there may be 100 times more water within lunar magma than previously thought… truly a “watershed” discovery!
Orange-colored lunar soil sampled during Apollo 17 EVA missions was tested using a new ion microprobe instrument which measured the water contained within magma trapped inside lunar crystals, called “melt inclusions”. The inclusions are the result of volcanic eruptions on the Moon that occurred over 3.7 billion years ago.
Because these bits of magma are encased in crystals they were not subject to loss of water or “other volatiles” during the explosive eruption process.
“In contrast to most volcanic deposits, the melt inclusions are encased in crystals that prevent the escape of water and other volatiles during eruption. These samples provide the best window we have to the amount of water in the interior of the Moon.”
– James Van Orman of Case Western Reserve University, team member
While it was previously found that water is contained within lunar magma during a 2008 study led by Alberto Saal of Brown University in Providence, Rhode Island, this new announcement is based upon the work of Brown undergraduate student Thomas Weinreich, who located the melt inclusions. By measuring the water content of the inclusions, the team could then infer the amount of water present in the Moon’s interior.
The results also make correlations to the proposed origins of the Moon. Currently-accepted models say the Moon was created following a collision between the newly-formed Earth and a Mars-sized protoplanet 4.5 billion years ago. Material from the Earth’s outer layers was blasted out into space, forming a ring of molten material that encircled the Earth and eventually coalesced, cooled and became the Moon. This would also mean that the Moon should have similarities in composition to material that would have been found in the outer layers of the Earth at that time.
“The bottom line is that in 2008, we said the primitive water content in the lunar magmas should be similar to lavas coming from the Earth’s depleted upper mantle. Now, we have proven that is indeed the case.”
– Alberto Saal, Brown University, RI
The findings also suggest that the Moon’s water may not just be the result of comet or meteor impacts – as was suggested after the discovery of water ice in polar craters by the LCROSS mission in 2009 – but may also have come from within the Moon itself via ancient lunar eruptions.
The success of this study makes a strong case for finding and returning similar samples of ejected volcanic material from other worlds in our solar system.
“We can conceive of no sample type that would be more important to return to Earth than these volcanic glass samples ejected by explosive volcanism, which have been mapped not only on the Moon but throughout the inner solar system.”
– Erik Hauri, lead author, Carnegie’s Department of Terrestrial Magnetism
A new study reveals that the water within the Apollo Moon rocks – and within the Moon itself — likely came from comets bombarding the nascent lunar surface, shortly after it formed following an impact event with a young Earth and Mars-sized protoplanet. The recent findings of abundant water at the lunar poles by the LCROSS impactor and across the Moon’s surface by various spacecraft have turned the long-standing notion of a dry Moon on its head, and the past year and a half, researchers have been trying to determine where this unexpected water came from.
“The water we are looking at is internal,” said Larry Taylor from the University of Tennessee, Knoxville, a member of an international team. “It was put into the moon during its initial formation, where it existed like a melting pot in space, where cometary materials were added in at small yet significant amounts.”
Using secondary ion mass spectrometry, the researchers measured the water signatures within rocks returned from the Apollo 11, 12, 14, and 17 missions that landed on the moon between 1969 and 1972. They found the chemical properties of the lunar water were very similar to signatures seen in three different comets: Hyakutake, Hale-Bopp and Halley.
The team found significant water in the lunar mineral apatite from both mare and highlands rocks, which indicates “a role for water during all phases of the Moon’s magmatic history,” the team wrote in their paper. “Variations of hydrogen isotope ratios in apatite suggest sources for water in lunar rocks could come from the lunar mantle, solar wind protons and comets. We conclude that a significant delivery of cometary water to the Earth–Moon system occurred shortly after the Moon-forming impact.”
Even though comet impacts may also have created the Earth’s oceans, Taylor said the water signatures from the mass spectrometer show that the water on the Earth and Moon are different, as apatite has a ratio of the deuterium and hydrogen that are distinctive from those in normal Earth water.
“The values of deuterium/hydrogen (D/H) that we measure in apatite in the Apollo rock samples is clearly distinguishable from water from the Earth, mitigating against this being some sort of contamination on Earth,” said James Greenwood of Wesleyan University, who led the research team.
Initially after the Apollo program, the Moon was believed to extremely dry. Many of the rocks returned by the astronauts and also the Soviet Luna program contained trace water or minor hydrous minerals, but those signatures were attributed to terrestrial contamination since most of the boxes of the Apollo program used to bring the Moon rocks to Earth leaked. This led the scientists to assume that the trace amounts of water they found came from Earth air that had entered the containers. The assumption remained that, outside of possible ice at the moon’s poles, there was no water on the moon.
Forty years later, a trio of spacecraft found evidence of water across the surface of the Moon: The Chandrayaan-1 spacecraft’s Moon Mineralogy Mapper (M Cubed) found that infrared light was being absorbed near the lunar poles at wavelengths consistent with hydroxyl- and water-bearing materials. A spectrometer on the re-purposed Deep Impact probe showed strong evidence that water is ubiquitous over the surface of the moon, and archival data from a Cassini Moon flyby also agreed with the finding that water appears to be widespread across the lunar surface.
“This discovery forces us to go back to square one on the whole formation of the Earth and moon,” said Taylor. “Before our research, we thought the Earth and moon had the same volatiles after the Giant Impact, just at greatly different quantities. Our work brings to light another component in the formation that we had not anticipated — comets.”
Taylor added that the existence of hydrogen and oxygen – water – on the moon can literally serve as a launch pad for further space exploration.
“This water could allow the moon to be a gas station in the sky,” said Taylor. “Spaceships use up to 85 percent of their fuel getting away from Earth’s gravity. This means the moon can act as a stepping stone to other planets. Missions can fuel up at the moon, with liquid hydrogen and liquid oxygen from the water, as they head into deeper space, to other places such as Mars.”
With all the recent news of water on the Moon, a new paper published today in the journal Science may offer a surprise – or it may bring us back to previous assumptions about the Moon. A new analysis of eleven lunar samples from the Apollo missions by Zachary Sharp from the University of New Mexico and his colleagues indicates that when the Moon formed, its interior was essentially dry. While the recent findings of ubiquitous water and hydroxyl on the surface as well as water ice in the lunar poles are not challenged by this new finding, it does dispute — somewhat — two other recent papers that proposed a wetter lunar interior than previously thought. “The recent LCROSS findings were of water on the lunar surface due to cometary impacts, and the ice is from the comets themselves,” Sharp told Universe Today. “We are talking about water that was present in the molten early Moon 4.5 billion years ago.”
The accepted theory of how the Moon formed is that a Mars-sized body slammed into our early Earth, creating a big disk of debris that would ultimately form into the Moon.
Although planetary scientists are still refining models of the Moon’s formation, there is much to suggest a dry Moon. Any water would have been vaporized by the high temperatures generated by the impact and cataclysm that followed, and vapor would have escaped into space. The assumption is that the only way there could be water in the Moon’s interior if is the impactor was especially water-rich, and also if the Moon solidified quickly, which is considered unlikely.
But earlier this year, Francis McCubbin and his team from the Carnegie Institution for Science released their findings of a surprisingly high abundance of water molecules — as high as several thousand parts per million — bound to phosphate minerals within volcanic lunar rocks, which would have formed well beneath the lunar surface and date back several billion years.
Additionally, in 2008, Alberto Saal of Brown University and colleagues found a slightly lower abundance of water in the lunar mantle, but it was significantly higher than the previous estimate of 1 part per billion.
These two findings have been pushing lunar scientists to find possible alternative explanations for the Moon’s formation to account for all the water.
But now, Sharp and his team studied a wide range of lunar basalts and measured the composition of chlorine isotopes. Using gas source mass spectrometry they found a wide range of chlorine isotopes contained in the samples which are 25 times greater than what is found in rocks and minerals from Earth and from meteorites.
Chlorine is very hydrophilic, or attracted to water, and is an extremely sensitive indicator of hydrogen levels. Sharp and his team say that, if lunar rocks had initial hydrogen contents anywhere close to those of terrestrial rocks, then the fractionation of chlorine into so many different isotopes would never have happened on the Moon. Because of this Sharp and his colleagues say their results suggest a very dry interior of the Moon.
Sharp proposes that Saal and McCubbin’s calculations of high hydrogen contents in some lunar samples are not typical, and perhaps those samples are the product of certain igneous processes that resulted in their “extremely volatile enrichment.” They do not, however, represent the high and variable isotopic chlorine values reported in the majority of lunar rocks, Sharp said.
Still, there could be a compromise between the varied findings. “There are uncertainties that one has to take into account when doing this type of study, ” Sharp told Universe Today, “and if we take the low estimates of Saal and McCubbin’s papers, they are not so different from our findings.”
But the discrepancies, however small, show that perhaps we can’t make generalizations about the entire Moon from limited samples.
“We have not yet looked for water in a wide range of lunar samples,” said Jeff Taylor from the University of Hawaii, who was not involved in any of the aforementioned studies. “It is quite possible that the initial differentiation of the Moon and subsequent processes such as mantle overturn concentrated whatever water the Moon had into certain areas. Until we measure more samples, including samples from the farside (represented by many of the lunar meteorites and eventually by sample-return missions), we will not know for sure how much water is in the bulk Moon.”
In combination, all the recent studies of the lunar surface show there is likely a complex chemistry on the Moon that we have yet to understand.
“In other words,” said Taylor, “we need more work!”
“Water cycle on the Moon” is a phrase that many people – including lunar scientists – were never expecting to hear. This surprising new finding of ubiquitous water on the surface of the Moon, revealed and confirmed by three different spacecraft last year, has been one of the main topics of recent discussion and study by lunar researchers. But figuring out the cycle of how water appears and disappears over the lunar day remains elusive. As of now, scientists suspect a few different processes that could be delivering water and hydroxyl (OH) to the lunar surface: meteorites or comets hitting the Moon, outgassing from the Moon’s interior, or the solar wind interacting with the lunar regolith. But so far, none of the details of any of these processes are adding up.
Dana Hurley from The Johns Hopkins University Applied Physics Laboratory is part of team of scientists attempting to model the lunar water cycle, and she discussed the work at the NASA Lunar Science Institute’s third annual Lunar Forum at Ames Research Center, July 20-22, 2010.
“When we do the model, we assume the way that the water is lost is through photodissociation, and so that sets the timescale,” Hurley told Universe Today. “And using that timescale the amount that is coming in through the solar wind or micrometeorites can’t add up to the amount observed if it is in steady state, so something is not jiving.”
Photodissociation involves the breaking up of a substance into simpler components by the radiant energy of sunlight.
It appears the amount of water varies over the course of the lunar day. Two observations a week apart by a spectrometer on the repurposed Deep Impact spacecraft (now called EPOXI) showed the region that was near the Moon’s terminator at dawn had a detectable amount of water and hydroxyl, and a week later when it was near noon, those substances were gone. But the new region at dawn then had H2O and OH.
One theory holds that the water and hydroxyl are, in part, formed from hydrogen ions in the solar wind. By local noon, when the moon is at its warmest, some water and hydroxyl are lost. By evening, the surface cools again, and the water and hydroxyl return.
But, Hurley said, the solar wind in steady state does not reproduce the observed surface density of water and hydroxyl.
Additionally, looking at the other possible sources — the known source rate of micrometeoroids and comets — doesn’t provide the amount of observed H20 and OH either.
“We’d really like to have a lot more observations to understand how it evolves over the course of the day,” Hurley said.
In her talk, Hurley said her team has been trying to look at all possible angles and ideas, including recent larger comet hits on the Moon, or potentially a seasonal event where water deposited at winter poles could be released when it warms up in summer. But so far none of these ideas have been tested or modeled, and as of now do not provide a solution to the daily cycle of water that was observed.
She also noted that since there are obviously some unique processes going on, the interaction between the surface and atmosphere needs more study.
“The surface and atmosphere are coupled,” Hurley said in an interview with Universe Today. “The atmosphere is produced from the surface; there is no atmosphere that lasts for a long time on the Moon and it is constantly being produced and lost. And so it is coming from the surface, either from something that is coming from the lunar regolith grains or something that is interacting with those grains, whether it is solar wind or something that is impacting. So, the surface is the source of the atmosphere and that atmosphere comes back and interacts with the surface again. And you really have to understand that whole system.”
So, what is her best guess as to the source of the water?
Hurley said there has to be some sort of recycling going on within the regolith, and perhaps a complex surface chemistry that allows the H20 and OH to exist for longer periods of time, which would better explain the surface density.
“What I’ve looked at is what could be happening in the atmosphere and how things hop around from the surface up and then back down to the surface,” she said. “The lunar regolith is rather loose, and these small particles and gases can go down within the regolith and be within the top several centimeters and work their way down and back out. So there is an exchange going on in that top layer that is kind of acting as a reservoir. That is my best guess of what is going on.”