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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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.

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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)

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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)

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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.

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In 1979 Linda Morabito was processing an image taken by the Voyager 1 spacecraft.  It was an image of Io, one of Jupiter’s moons.  She discovered what appeared to be a plume of material erupting from Io’s surface.  Upon further analysis it was found to be due to a volcanic eruption, as was the first evidence of active volcanism on a body other than Earth.  We now know that Io is the most geologically active body in our solar system.

First image showing a volcanic plume on Io. Credit: NASA

Io is so geologically active because of the gravitational interactions it experiences between Jupiter and other moons.  Because Io’s orbit is in resonance with Europa and Ganymede, its orbit is kept a bit elliptical.  This means that it oscillates between being a bit closer and farther away from Jupiter.  As a result the tidal forces of Jupiter squeeze and stretch Io, kind of like squeezing putty in your hand.  This doesn’t deform Io’s shape that much, but just like squeezing putty makes it warm, squeezing Io keeps its interior warm.

This source of heat is very different the source that drives Earth’s geologic activity, which is radioactive decay and the remnant heat of Earth’s formation.  Because Io is continuously driven by tidal forces it is much more active.  Io erupts about 100 times more lava than Earth, and Io’s lava is much hotter than Earth’s.  No impact craters have been observed on Io, which means volcanic activity likely resurfaces the moon within less than a million years.

Top: Prediction for mantle heating. Bottom: Prediction for asthenosphere heating. Credit: NASA/Christopher Hamilton

About 1oo active volcanos have been observed on Io, which is enough to study the way in which the tidal forces heat the moon.  There are two basic models for Io’s tidal heating.  One is that the heating primarily occurs in the core, while the other is that it occurs closer to the surface, in a layer under the crust known as the asthenosphere.  If the core is the source of heat, then most of the volcanic activity should occur in the polar region, but if the asthenosphere is the source of heat, then Io should be more active in the equatorial regions.  A recent paper in Earth and Planetary Science Letters found that volcanic activity tends to be more equatorial, which indicates that the asthenosphere is the source of Io’s volcanism.

But the authors also discovered that volcanic activity didn’t cluster in the equatorial regions that tidal models predict.  They were shifted along the equator about 30 – 60 degrees east of the predicted model.  Just why that would be the case isn’t clear.  So while the asthenosphere model works in a general sense, there are aspects of Io’s volcanism we still don’t understand.

Paper: Christopher W. Hamilton et al. Spatial distribution of volcanoes on Io: Implications for tidal heating and magma ascent.  Earth and Planetary Science Letters Volume 361, Pages 272–286 (2013)

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