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)

The post The Moon Once Had An Atmosphere appeared first on One Universe at a Time.

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)

The post Many Moons appeared first on One Universe at a Time.

Mars has two small moons, Deimos and Phobos. Deimos is smaller and more distant from Mars, while Phobos is quite close to the red planet. Its orbital radius only 2.8 Mars radii, compared to our Moon at 60 Earth radii. Phobos is so close to Mars that it orbits the planet three times a day. It’s also so close that the small moon is doomed.

We’ve known for a while that Phobos’ time was limited. Tidal forces between Phobos and Mars cause the moon to move ever closer to the planet. Measurements of its orbit since the 1950s have found its orbit is decaying at a rate of about 1.8 centimeters per year. This and the fact that early observations of the moon had a rubble-pile look to them led some astronomers to speculate that Phobos could be artificial. More recent observations show that it is natural in origin, and (along with Deimos) was likely captured from the asteroid belt.

The inward spiral of Phobos means that it will only be around for about 30 million years. By then it will either be broken up by the tidal forces of Mars, or it will remain solid and impact Mars. Recent observations of the moon point to fragmentation. In fact it may have already begun. Notable on its surface are long grooves. If Phobos has a rubble like interior with a thick outer layer of dust, then these grooves are what you’d expect from tidal forces. If that’s the case, Phobos will gradually break apart, and may even form a ring system around Mars.

There has been talk about sending a mission to Phobos to land on the moon and study its interior. If that happens we may find out just how much time Phobos has left.

The post Phobos Is Running Out Of Time appeared first on One Universe at a Time.

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.

The post Once In A Blue Moon appeared first on One Universe at a Time.

Iapetus is a moon of Saturn known for two distinctive features. One is that it has a two-tone coloration, where roughly half of the moon is a dark, reddish-brown color while the other half is white and almost as bright as Jupiter’s moon Europa. It’s not entirely clear what gives Iapetus is yin yang coloring, but the most popular view is that it is cause by sublimation of the moon’s warmer side. Ice evaporates away leaving the dark remnant material. We know, for example, that the dark layer is no more than a foot thick, and has a bright layer underneath it.

Iapetus’ yin yang coloring. Credit: NASA/JPL-Caltech

Another strange feature is the moon’s large equatorial ridge. It’s about 1,300 km long, and 13 km high. We know that the ridge is old because it is heavily cratered. Again, we aren’t entirely sure how such a ridge could have formed, but generally fall into two camps.  One is that it was produced by some type of internal mechanism such as a convective overturn in its youth, the other is that is was caused an external mechanism such as the accumulation of debris from an ancient ring system. A recent paper in Icarus gives support to the accumulation model.

In this work the team made a detailed model of the ridge system based upon observations from the Cassini probe. They then measured the shapes of the mountain peaks in the ridge, and found that they were within the angle of repose. That is, the angle at which accumulated matter tends to form a peak. Any steeper and the material will tend to collapse to a shallower peak. A geologic upheaval would likely produce a wide range of peak angles, so this suggests the ridge was produced by accumulation. Accumulation from a collapsed ring system would explain why the ridge lies along the equator.

Paper: Erika J. Lopez Garcia, et al. Topographic constraints on the origin of the equatorial ridge on Iapetus. Volume 237, Pages 419–421 (2014)

The post Yin Yang Moon appeared first on One Universe at a Time.

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)

The post Shifting Moon appeared first on One Universe at a Time.

If you’ve ever experienced a thunderstorm, you’re well familiar with the ability of Earth to build a static charge on its surface. When that static build-up reconnects with a similar build-up in the sky, the resulting current is seen as lightning. We’ve long known that a similar static buildup can occur on other solar system bodies. We’ve observed lightning storms on Jupiter, Saturn and Venus, for example. Of course these planets all have thick atmospheres, so what about bodies without atmospheres?

One example we know of is the Moon. Data from the Lunar Prospector mission found that the portions of the Moon’s surface could build electrostatic potentials as high as 4,50o volts. They are generated either when the Moon passes through Earth’s magnetotail, or when a solar storm bombards the Moon with charged particles. With no atmosphere the Moon can’t discharge these as lightning, so it generally leaves the surface gradually.  Sometimes static charge can build within the dust of the lunar surface. The charged dust particles repel each other, and this can create levitated dust clouds. Such an effect was seen during the Apollo missions.

It has generally been thought that charge could build on the surface of other airless bodies, but there hasn’t been any direct evidence of it. Now a new paper confirms the effect for Saturn’s moon Hyperion. The authors looked at data from the Cassini mission, specifically a detector known as the Cassini Plasma Spectrometer (CAPS). This device looks at the energy of charged particles striking Cassini. During a close approach of Hyperion, CAPS detected a strong current of electrons. It was a discharge of about 200 volts over a distance of 2,000 kilometers.

Hyperion doesn’t interact strongly with Saturn’s magnetosphere, so it’s thought that the moon’s charge build-up is due to ultraviolet light striking its surface, which can knock electrons away from the surface via the photoelectric effect. This supports the idea that other outer planet moons can experience similar charges on their surface.

Just as we can get a charge out of seeing our spacecraft make a close approach of a moon, it seems the spacecraft itself can also get a charge.

Paper: Nordheim, T. A., et al. Detection of a strongly negative surface potential at Saturn’s moon Hyperion. Geophys. Res. Lett., 41, doi:10.1002/2014GL061127 (2014)

The post Short Circuit appeared first on One Universe at a Time.

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)

The post Water from the Sun appeared first on One Universe at a Time.

When we look at the Moon, we see a surface pocked with craters, scattered between seas of basalt from ancient lava flows. Since the Moon is not geologically active, it’s easy to imagine that the formation of lunar seas was triggered by large impacts. That’s actually been the dominant theory for some time. Now new research indicates that for at least one of the great seas, Oceanus Procellarum, that isn’t the case.

The results have been recently published in Nature, and shows that the great sea seems to be the result of geological activity. The team looked at data from the Gravity Recovery and Interior Laboratory (GRAIL), which is a pair of satellites that mapped the gravity of the Moon in great detail. When they analyzed the data, the team found rift zones bordering Oceanus Procellarum. These rift zones (seen on the right of the image above) are fairly straight with sharp angles, which is not the type of thing you see with impact zones.

The Pacific Ring of Fire is a similar structure on Earth

We have observed rift zones on several planets, as well as on Saturn’s moon Enceladus, but finding them on the Moon is rather surprising. The Moon is not massive enough to drive plate tectonic activity on its own, and it isn’t driven by strong tidal effects like some moons of Jupiter and Saturn. So it isn’t clear how such rift zones could have formed on the lunar surface. One idea proposed by the authors is that the Moon’s crust is rather thin, and the under layers of that region were heated by radioactive decay. The Procellarum region is known to have higher concentrations of radioactive elements such as uranium and thorium, and this could have driven rift formation in the past.

Regardless of the cause, it seems clear that the Moon was not simply a Moon battered by ancient impacts. It also had a few geological tricks of its own, and the famous Man in the Moon feature of Oceanus Procellarum is the result of one of them.

Paper: Andrews-Hanna, J. C. et al. Structure and evolution of the lunar Procellarum region as revealed by GRAIL gravity data. Nature 514, 68–71 (2014)

The post Somewhere Across the Sea appeared first on One Universe at a Time.

Janus is a small moon of Saturn. It is somewhat oval in shape and has a diameter of about 180 kilometers. Epimetheus is another moon of Saturn, with a diameter of about 120 kilometers. The two moons are very similar, even down to their orbits. They share the same orbital plane, and at the moment the orbit of Janus is only about 50 kilometers closer to Saturn than that of Epimetheus. In other words the gap between the orbits is less than the size of the moons.

You might think this is a recipe for unpleasantness. After all, since the orbit of Janus is closer to Saturn, Janus moves around in its orbit faster than Epimetheus. So over time Janus will catch up to Epimetheus, and would overtake its sister moon if it weren’t for that fact that it is in the way. Surely it’s only a matter of time before the two moons collide.

Except that isn’t what happens. Instead of an imminent collision, the two moons do a little dance. Janus and Epimetheus are not only of similar orbits, they are of similar mass. Similar in this case means that Janus is only about four times more massive than Epimetheus, rather than hundreds or thousands. So as Janus begins to approach Epimetheus, the gravitational pull of Janus will cause the orbit of Epimetheus to get a bit smaller. As a result, the speed of Epimetheus will increase. Likewise the gravitational pull of Epimetheus will embiggen the orbit of Janus a bit, causing it to slow down. You can see this in the figure.

So instead of colliding, the two moons do a gravitational dance where they effectively exchange orbits. Janus catches up to Epimetheus (to within about 10,000 kilometers), they do their gravitational dance, and then Epimetheus races ahead of Janus. Eventually Epimetheus catches up to Janus and another dance brings them back to where they started. This exchange happens about once every four years.

A magic dance, driven by the force of gravity.

The post Dance Magic Dance appeared first on One Universe at a Time.

Titan is the largest moon of Saturn, and the second largest moon in our solar system. It has a greater diameter than Mercury. It is also the only moon with a thick atmosphere. It has liquid methane rivers and lakes, and has a seasonal climate.

And like our moon, we have landed a probe on its surface. In 2005 the Huygens probe made a one-way journey to the surface of Titan. You can see a video of that landing above.

The post Titan Fall appeared first on One Universe at a Time.

Saturn’s largest moon Titan is in some ways very similar to Earth.  Sure, it is significantly colder, and has a much thicker atmosphere, but it has something no other world besides Earth. Lakes and seas. These are not water lakes, but methane.  With a surface temperature of about 94 K, and a thick atmosphere, Titan is perfect for a methane cycle similar to the water cycle on Earth.  On titan, methane gas exists as a vapor in the atmosphere, which can condense and fall as rain.  This collects as rivers and lakes on the surface of the moon.

With the Cassini mission we’ve been able to map some of the lakes and seas on Titan.  They resemble the shape of lakes we see here on Earth. But one thing they don’t seem to have is waves.  Some of Titan’s lakes are larger than the Great Lakes of Earth, which can experience sizable waves.  But radar measurements of Titan’s lakes have found no evidence of waves. Since Titan has a lower gravity than Earth, waves should actually be more likely. It’s been calculated that a breeze of only 2 – 3 km/hr would be enough to produce measurable waves. We know that such wind levels exist on Titan, so the lack of waves is a bit of a mystery.

The post Wave the Titanic appeared first on One Universe at a Time.