For centuries humans have dreamed of traveling to the Moon. We achieved that dream in 1969, but found our sister world to be a dry airless rock. Most of the early stories of a journey to the Moon painted a very different picture, giving the Moon a breathable atmosphere, and perhaps even exotic life. We now know the Moon is barren of life, but there was a time when the Moon had an atmosphere. 

Our Moon doesn’t have an atmosphere because it is too small and doesn’t have a strong magnetic field. Any atmosphere it might have had would be stripped away by the solar wind that barrages the small world. In contrast, our planet has more mass to hold its atmosphere close, and a strong magnetic field to protect it. But that doesn’t mean the Moon couldn’t have had an atmosphere for a short time, and new evidence shows that it did.

About 3.5 billion years ago, the broad dark patches we see on the lunar surface first formed. Known as maria, they were created by large lava flows that later cooled to become basalt plains. During the Apollo missions of the 1960s and 1970s, astronauts brought back samples of these maria, and we found they contained traces of gas, such as carbon monoxide. This gas erupted from the Moon’s interior at the same time as the maria formed.

Recently a team calculated just how much gas would have been released from this process, and found it was more than originally suspected. So much gas was released that it would have formed a thin atmosphere around the Moon. The atmosphere only lasted about 70 million years, which is brief for geologic scales, but it could have deposited ice and other molecules in the cold sunless regions of craters.

And that could be important for future astronauts. In order to build a permanent presence on the Moon, humans will need water and soil to sustain us. If water and other compounds can be found on the Moon, we won’t have to bring it from Earth.

So our Moon is barren and airless today, but we might be able to live there thanks to the brief period when the Moon had a sky.

Paper: Debra H.Needham and David A.Kring. Lunar volcanism produced a transient atmosphere around the ancient Moon. Earth and Planetary Science Letters, Volume 478 (2017)

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Impact strikes are a rare but dangerous threat to spacecraft. The highest threat is for spacecraft in near-Earth orbit, where decades of satellite debris have accumulated. Given the tremendous speeds of orbiting spacecraft, even a fleck of paint can pose a threat. Beyond Earth orbit the threat is less, but it isn’t zero, as demonstrated by a rare impact with the Lunar Reconnaissance Orbiter (LRO) currently orbiting the Moon. 

The impact was small, and the spacecraft survives. The effect was so subtle that it would likely not have been noticed if the orbiter wasn’t taking images at the time. You can see the effect of the impact in the image above. It was taken in 2014 by one of the LRO’s Narrow Angle Cameras. These cameras take high resolution black and white images of the lunar surface. To take these images, the camera scans the surface line by line, Taking a narrow line image of the surface one after the other to create a complete picture. As you can see in the image, the sharp resolution near the top shifts suddenly to a wiggly image. This means the camera was jostled suddenly as it gathered lines of images. The LRO has two Narrow Angle Cameras, as well as a wide angle one, and only one Narrow Angle Camera showed this wiggly effect. This means the satellite itself was not jostled in a significant way, but only one camera.

This can be explained by a meteoroid impact with the camera. Based upon computer recreations of the image, the meteoroid was only about 0.8 millimeters in diameter, or about the size of the ball at the tip of a ball point pen. But the meteoroid was moving at more than 15,000 miles per hour relative to the spacecraft, which is about ten times faster than a speeding bullet. This gave it enough energy to jostle the camera in a noticeable way.

It did not, however, have enough energy to seriously damage the spacecraft. The impact occurred in 2014, and since that time the Lunar Reconnaissance Orbiter has worked perfectly well. It continues to gather data on the lunar surface, and will help us determine future landing sites for crewed missions to the Moon.

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Of all the moons in the solar system, ours is unique. It has surface composition similar to Earth’s, pointing to a common origin, and it’s unusually large to be orbiting such a small planet. Just how such a large moon came to orbit Earth remains a bit of a mystery. With a similar composition to Earth, it couldn’t have been captured by Earth’s gravity, and the Earth and Moon didn’t likely form at the same time from the primordial gas and dust of the solar system. So the dominant theory is that of a single large impact. Early in its history, Earth was struck by a Mars-sized object, sometimes called Theia. A combination of material from Earth and Theia coalesced to form the Moon. 

While the impact model has a lot going for it, getting the Moon to form in a relatively close and roughly circular orbit requires just the right kind of collision. The best fit is a collision that was a fast, glancing impact at an odd angle. It’s not impossible, but such a collision between two large bodies would be extremely rare, even in the early solar system. So it’s worth wondering if there is another, more likely impact scenario. A new paper in Nature Geoscience argues that there is.

In this new model, our Moon wasn’t formed by a single impact, but by multiple impacts over time. Each impact would have created a ring of material around Earth, which collapsed into one or a few larger moons over time. Multiple impacts would have created multiple moons over the ages. If this is really what happened, why do we have just one moon instead of several? The key is the long term tidal effects on these moons.

Currently tidal forces between the Earth and Moon gradually slow down the Earth’s rotation, while simultaneously causing the Moon to drift ever farther from the Earth. The same effect would occur with multiple moons, causing them to move slightly away from Earth over time. But the closer a moon is to Earth, the stronger the tidal forces and the faster its orbital distance would increase. So if Earth’s multiple moons formed with roughly similar orbits, the orbits of the inner moons would drift outward until they collided with outer moons, eventually forming the single Moon we see today.

Computer simulations run by the team show that a multiple collision, multiple moon model could have created our single large Moon. The real question is whether that’s actually what happened. That really comes down to which is more likely, a single unusual large impact, or multiple large impacts over time. The jury’s still out on that one. But this new paper does show that there is more than one way to form a large moon around a small, rocky planet like ours.

Paper: Raluca Rufu,et al. A multiple-impact origin for the Moon. Nature Geoscience doi:10.1038/ngeo2866 (2017)

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The dominant model for the origin of our Moon is the impact model. In this model, about 4.5 billion years ago proto-Earth was hit by a Mars-sized body known as Theia. It’s generally been thought that the collision was somewhat off-center, causing the remnants of Theia and some outer layer material from Earth to form the Moon.  But new evidence suggests the collision was more head-on. 

If the Earth-Moon system was caused by an off-center collision with Theia, then we would expect to see close similarities in the chemical compositions the Earth and Moon, but still some differences. Earlier research found some differences in the amount of isotopes, such as the ratio of oxygen-17 to oxygen-16 differing by about 12 parts per million between the Earth and Moon. But new work analyzing oxygen isotopes in lunar rocks and volcanic rocks on Earth found their oxygen isotopes to be indistinguishable. Since oxygen is common in both rock samples, the fact that they are indistinguishable suggests that the material forming the Earth and Moon were mixed together before they formed. This could be achieved by a more head-on collision between proto-Earth and Theia.

If that’s the case, then much of Theia became a part of Earth’s core, and our planet is actually the product of two worlds.  It’s an interesting twist on the impact origin of the Moon.

Paper: Edward D. Young, et al. Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact. Science Vol. 351, Issue 6272, pp. 493-496 (2016)

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Different wavelengths of light interact with matter in different ways. Our atmosphere, for example, is largely transparent to visible wavelengths, but absorbs much of the ultraviolet. Dust surrounding the center of our galaxy absorbs visible light, but is more transparent to radio waves. As a result it is useful to observe objects at a range of wavelengths. Sometimes this light is reflected from natural sources or emitted by the objects themselves, but sometimes we actively shine light on an object to get an image. 

A radio map of Venus. Credit: B. Campbell, Smithsonian, et al., NRAO/AUI/NSF, Arecibo

One way to do this is through radio waves. The radio telescope at Arecibo not only detects radio waves, it also has a large transmitter capable of sending radio signals into space. While it was once used to send a message to potential aliens, the transmitter’s main use it to reflect radio signals off solar system bodies. The timing of these signals can be used to measure the distance to planets more precisely, but with high resolution imaging we can also make detailed maps. Recently maps were made of the Moon (seen above) and Venus by beaming a radio signal from Arecibo and observing its reflection with the 100-meter radio telescope at Green Bank.

Reflected radio images such as these are useful because radio waves are not only transparent to the atmospheres of Earth and Venus, but they are also largely transparent to fine dust. For Venus this means we can get a high resolution image of the surface obscured by thick atmosphere. For the Moon this means we can get a map of any surface features that may be obscured by dust on the Moon’s surface.

What’s amazing about these images is how high their resolution is. While they look like regular telescope images, they are actually radio images. The shadows you see are radio shadows from the transmitted beam, similar to the way a rough surface can cast shadows from the beam of a flashlight. The resolution of these images are a good demonstration of just how sophisticated radio astronomy can be.

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Modern astronomical instruments rely upon digital sensors. Images are gathered through CCDs, which can be manipulated to create wonderfully brilliant images. But it wasn’t that long ago that images still required photographic film. Then, the only way we could gather images from space was to physically transport the film into space and somehow get it back. Like family trips of the day you took photographs hoping they would turn out, and wait to develop them when you got home. This was true of our trips to the Moon and the images astronauts brought home.

A vacation picture from Apollo 17. Credit: NASA

Like many photographs of old, they tended to be stored away and not often looked at. To bring an old photo to the modern age you have to scan a digital copy. Most of the digital copies are of small resolution, since that’s all you need for most purposes. But recently The Apollo Project has posted high resolution scans of images from the Moon missions. By and large they are raw scans, and haven’t been color corrected or modified.

What’s striking about them is how they have the feel of old vacation photographs. The kind you might find in your parent’s shoebox, or you might remember from your own trips. In a very real sense that’s what they are. The cameras were more expensive than your usual Polaroids, and the trip was out of this world, but they were still the record of a human vacation. So along with images of great views there are pictures of friends, selfie photos of your own shadow, and little things here and there that seemed interesting at the time.

If you have a chance its worth looking through the Apollo Project images. Sure, they’re a great archive of one of humanity’s greatest journeys, but they also convey just how deeply human our first vacations into space really were.

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You might have heard that tonight’s full moon is a blue moon, since it is the second full moon in the same month. While this is the most common definition for “blue moon,” it is not the only definition, nor even the oldest.

The earliest references to a blue moon dates to the 1500s, and referred to an impossibility or absurdity, much like we would use the phrase “when pigs fly.” It is possible, however for the moon to truly appear blue. The most widespread incidence of modern history occurred after the eruption of Krakatoa in 1883, which sent so much ash into the atmosphere it produced brilliantly red sunsets and visibly blue moons all across the globe for nearly two years. As a result, the phrase “once in a blue moon” came to mean a rare occurrence.

Astronomically, the phrase has three possible meanings. The first is the second full moon of a given month, of which tonight’s full moon is an example. The second is a truly blue moon. Whether tonight’s moon falls into that category, you’ll just have to go outside tonight and find out. The third definition is the third moon of four in a given season marked by the solstices and equinoxes. Our calendar year consists of 12 months, and so each season is about 3 months long. But a lunar cycle (the time it takes the moon to go from full moon to full moon) is about 29.5 days, so it is possible to have 13 full moons in a given year, which means one season will have four full moons, the third being a “blue moon.”

Whichever definition you prefer, if your evening is clear you can look up at our moon in its full phase. It is a huge moon for a planet our size, and there is no other planet in our solar system where you could have such a view.

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One of the striking features of Earth (besides the living things upon it) is that it possesses a relatively large moon. While our Moon is not the largest in the solar system, it is large for such a small planet. The mass of the Moon is more than 1% that of Earth, compared to Jupiter’s largest moon Ganymede, which is just 0.008% of Jupiter’s mass. The origin of our large moon has been a matter of some debate.

The most popular model is the collision model. In this model, the young proto-Earth collided with a Mars-sized planetoid sometimes named Theia. Part of the mass of Theia was captured to become Earth, and the remains formed a debris ring around Earth, much of which coalesced to form the Moon. This model has a lot going for it. For example, the Moon has a density about 60% that of Earth, which is exactly what is predicted by the collision model, where the lighter outer layers of Theia and proto-Earth are scattered to the debris disk, while the heavy core of proto-Earth remains.

The main downside of this model is the fact that the Earth and Moon have a very similar chemical composition. The only way to explain that fact with the collision model is to assume that Theia and Earth had a very similar composition, which would seem unlikely. Now new research published in Nature has found that similarities between colliding bodies aren’t as unlikely as we thought.

The team ran computer simulations of young solar systems, and looked at how chemically similar large bodies were with their last major impactors. What they found was that 20% – 40% of the time they were similar enough to account for the Earth-Moon compositions. So it’s not so unusual that the Earth and Moon are chemically similar.

There’s only one other body in the solar system with such a relatively large moon as ours, and that’s Pluto’s moon Charon. Charon’s mass is 12% that of Pluto. Charon is also thought to be the result of a collision, so when New Horizons makes its flyby of the planet in July, we find even more clues about planet-shattering collisions.

Paper: Alessandra Mastrobuono-Battisti, et al. A primordial origin for the compositional similarity between the Earth and the Moon. Nature 520, 212–215 (2015)

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Has the orientation of the Moon shifted in the past? According to the distribution of ice on the lunar poles, that seems to be the case. The results were recently presented at the Lunar and Planetary Science Conference, and they tell an interesting story about the history of the Moon. 

The results are based on data from the Lunar Prospector mission, which (among other things) measure neutron emissions from the lunar surface. The neutrons are emitted by radioactive decay, and their energy is a measure of the mass of the elements they decay from. When a neutron is emitted, the atom recoils, which affects the energy of the emitted neutron. The neutrons with the lowest energy are an indicator of hydrogen, which in turn is a measure of the amount of water ice. So from this data the team was able to make a map of the distribution of ice on the lunar poles.

We generally think of the Moon as dry and airless, and that’s basically true, but there is trace amounts of ice near the poles. This is because the poles don’t get much direct sunlight, so it doesn’t evaporate as much off the surface.  When the team created the map, they found that the ice on both poles was lopsided. More specifically, the lopsided region of the north pole was antipodal to the lopsided region on the south pole. Antipodal means you could draw a line from one lopsided region to the other, right through the center of the Moon. One obvious explanation for this is that the offset regions used to be the poles of the Moon. Since they are about 5.5 degrees off from the current poles, that would mean the axis of the Moon shifted about 5.5 degrees sometime in its past.

If that’s the case, there would need to be some mechanism for the shift, which is too big to be due to some kind of impact event. What the authors propose is that it could have been caused by a hot region beneath the lunar surface, which ejected lava to become the Oceanus Procellarum, that large dark region on the Earth-facing side of the Moon. This region formed early in the Moon’s history, so if such a shift really did occur, it also means lunar ice must be as old as the Moon.

That goes against other evidence which points toward water forming on the Moon much later due to interaction with the solar wind. On the other hand, we’ve also found recently that Earth’s water is much older than we’d thought, so it’s possible that the Moon’s water is old as well. This is early research on the idea, so it’s a bit too early to know for sure. But it’s an interesting idea, to say the least.

Paper: M.A. Siegler, et al. Hidden in the Neutrons: Physical Evidence for Lunar True Polar Wander. 46th Lunar and Planetary Science Conference (2015)

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Tomorrow most of the western United States and Canada will experience a partial solar eclipse. For the northern US and Canada the Moon will block up to 90% of the Sun, while for other regions it will block much less.  You can find out more about this eclipse, as well as when to check it out here. I’ll add the standard note of caution that you shouldn’t look at the Sun directly during an eclipse. You should either use eclipse glasses specially made for safe viewing, or make a simple pinhole camera.

This particular eclipse won’t be seen as a total eclipse anywhere. To see a solar eclipse on Earth, the Moon must pass in front of the Sun from our vantage point. If the orbit of the Moon were exactly in line with the orbit of the Sun, then we might expect a solar eclipse once a month. But the Moon’s orbit is tilted about five degrees relative to the orbit of Earth, and this means that the Moon is often slightly above or below the Sun from our perspective when it passes between the Sun and Earth. So most months there is no solar eclipse.

Credit: Armagh Observatory

Not all solar eclipses are alike. Since the Sun is larger than the Moon, there are regions where only part of the Sun is blocked by the Moon, and the resulting shadow is called the penumbra. For a much smaller region where the Sun is completely blocked by the Moon, the shadow is called the umbra. The type of eclipse you observe depends in part on whether you are viewing it from the umbra or penumbra. In this case the Earth will pass within the umbra of the Moon, so it is only a partial eclipse.

If you happen to catch this eclipse and are waiting for a total one, you won’t have to wait too long. Make plans for August 21, 2017. On that day a total eclipse will cross the US from Oregon to South Carolina, making it within a day’s drive of most of the country.

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The moon is known for its cratered surface. Its lack of atmosphere and proximity to Earth make it easy to observe impact craters from past collisions. Since the Earth and Moon are in the same general region of the solar system, they were likely bombarded at similar rates. While we do find craters on Earth (such as Barringer crater in Arizona), they are typically not as obvious due to wind and water erosion.

Most of the craters we see on the Moon occurred during the late heavy bombardment period, nearly 4 billion years ago. According to a model of the early solar system known as the Nice (pronounced neese) model, Jupiter was roughly at its current distance when it entered a 1:2 resonance with Saturn. The resulting resonance drove Neptune (initially closer than Uranus) to the outer edge of the solar system, pushed Uranus and Saturn outward, and scattered much of the remaining protoplanetary material out to the farthest reaches of the solar system. Some of that material was also strewn through the inner solar system, causing the heavy bombardment. There is some evidence to support this idea, as Mercury appears to have gone through a heavy cratering period at the same time.

Today our solar system is largely cleared of protoplanetary debris, so sizable crater impacts are not as common. This doesn’t mean they never occur. The Chelyabinsk meteor last year comes to mind, though that one didn’t create an impact crater. However an impact on the Moon last year did. You can see the before and after in the image here. It’s not a particularly big change, but it was the largest lunar impact that was observed in real time.

The Lunar Reconnaissance Orbiter has found several impact craters not seen in earlier images from the Apollo missions. So we know the Moon is still being bombarded from time to time. It’s all a part of how the lunar surface changes over time.

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The Moon is a dry, airless rock. At least that is how we imagine it. At basic level, that’s a pretty accurate description. It is drier than any desert on Earth, and its surface would be considered a hard vacuum. But at a more subtle level, that isn’t quite true. The Moon does have the faintest trace of atmosphere, consisting of elements such as argon, helium and hydrogen. The Moon also has traces of water on its surface, mostly locked up within minerals.

That doesn’t mean these minerals are wet by any means. Initial studies of lunar rocks gathered during the Apollo missions found no evidence of water. Only during later, more sophisticated studies was a trace of water discovered. With modern satellites we can detect such traces of water across the lunar surface, such as seen in the image above.

It’s generally been thought that lunar water originated on the Moon in much the same way as it originated on Earth, through water-rich meteorites (chondrites) and comets. But that doesn’t seem to be the case. While some of the Moon’s water clearly did come from impacts, the majority of lunar water is due to a rather surprising source: the Sun.

The discovery was published recently in PNAS, and it looks at isotopes in lunar water. Typical water consists of two parts hydrogen to one part oxygen, hence H₂O. But there are other variations such as D₂O, which is two parts deuterium instead. The ratio of these two varieties of water (known as the D/H ratio) can tell us about the water’s origin. The D/H ratio found in water-rich meteorites is fairly consistent, and it is one of the ways we know meteorites contributed more water to Earth than comets. The D/H ratio found in lunar water doesn’t match that of meteorites. The authors estimate that less than 15% of lunar water could have come from chondrites.

The rest of the water seems to have come from the solar wind. The solar wind consists of protons and electrons that stream away from the Sun. On Earth, these charged particles are caught by our planets magnetic field, causing them to strike the upper atmosphere near the poles, which creates aurora. The Moon lacks a strong magnetic field, so these particles can strike the lunar surface. When protons from the solar wind strike the Moon, they can bond with elements on the surface, such as oxygen. This can lead to the formation of water. Of course, solar-wind produced water also has a distinctive D/H ratio, and the authors were able to show that lunar water was a good match.

So it turns out water can appear on a dry, airless rock. All you need is a bit of solar wind.

Paper: Alice Stephant and François Robert. The negligible chondritic contribution in the lunar soils water.  PNAS, DOI:10.1073/pnas.1408118111 (2014)

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