![]() Slightly larger than Earth’s moon, Io has mountains taller than Mount Everest and hundreds of volcanoes some of which spew plumes of sulfur and sulfur dioxide, some hundreds of kilometers high. Jupiter’s Io looks about as alien as a world could look: bright yellow with swaths of white and spots of red, brown, orange, and black. The dark splotches all over the familiar face of the Moon are made of basaltic rock that erupted from volcanoes millions of years ago. But when we look at the Moon, we can still see evidence of its volcanic past. The Moon has since cooled off enough that there are no active volcanoes and no atmosphere at all. This ongoing volcanic activity is thought to have generated an atmosphere about twice as thick as the current Martian atmosphere. (The Moon is tidally locked to the Earth, meaning it always has the same side facing us.) The near side of the Moon developed a thinner crust, through which magma continued to burst until about 50 million years ago. The Earth would have been very hot from this impact as well, and as the Moon cooled down it did so more quickly on the side facing away from the Earth. And even though the formation of the Moon only took a matter of months, its surface stayed molten for tens or even hundreds of millions of years. The heat generated by the impact, the following rapid accretion, and resulting chemical reactions was likely so intense that the entire surface of the Moon became magma - that is, molten rock. Within the span of about a month to a year, some of these chunks came together, attracted to each other’s gravity, and formed the Moon. The leading theory for the Moon’s formation is that another young planet smashed into the Earth, throwing huge chunks of super-heated rock out into space. The Moon was formed in the early days of the Solar System, roughly 4.5 billion years ago. But it’s actually a fascinating place with a dramatic history. Among the scintillating alien worlds in orbit around other planets, Earth’s natural satellite might seem boring.
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