Our Moon is unusually large for a small planet like Earth. Did it form from a single impact with a Mars-sized body, or did it form over time from multiple impacts?
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.
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.
Saturn’s moon Iapetus has a strange yin yang coloring, as well as a mysterious equatorial ridge.
Did the axis of the Moon shift in the past? A map of ice on the lunar surface suggests that it did.
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?
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.
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.
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.
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