Despite popular mythology, nobody has ever seriously thought that the Moon is made of cheese. But exactly what it is made of has been a question we\u2019ve only very recently been able to answer.\u00a0<\/p>\n
And the more we learn<\/a>, the more interesting things get. Not only can we now physically touch pieces of the Moon itself, but seismological and spectroscopic techniques have revealed hints of what lies even further beneath the surface.<\/p>\n As it turns out, it\u2019s not so different from what\u2019s down here.<\/p>\n A great big shiny rock in the sky<\/p>\n There\u2019s a reason the Moon has captured imaginations for millennia, and it\u2019s not what\u2019s going on in its core. The \u201clight<\/a>\u201d of the Moon is of course technically no such thing \u2013 it\u2019s just the reflection of sunlight from the other side of the Earth, and not even that much of it either: \u201cthe Moon is one of the least reflective objects in the Solar System,\u201d pointed out Christopher Baird, associate professor of physics at West Texas A&M University and author of The Top 50 Science Questions with Surprising Answers, back in 2015<\/a>.\u00a0<\/p>\n \u201cThe Moon is actually quite dim, compared to other astronomical bodies,\u201d he explained. \u201cThe Moon only seems bright in the night sky because it is so close to the earth and because the trees, houses, and fields around you are so dark at night.\u201d<\/p>\n So, what makes the Moon so\u2026 well, dim? Mostly, it\u2019s just the most boring stuff on Earth: no snow, clouds, ice, or atmosphere; lots of rocks and dust.<\/p>\n And oxygen. \u201cIn rocks of the Earth and Moon, oxygen is the most abundant chemical element,\u201d explained Randy Korotev<\/a>, a now-retired lunar geochemist from Washington University in St Louis. There\u2019s not even all that much in it: on Earth, the crust is about 46 percent oxygen; on the Moon, it\u2019s between 41 and 45 percent.<\/p>\n The similarities don\u2019t stop there. Some 98-99 percent of the Moon\u2019s crust and regolith<\/a> \u2013 that is, the \u201csoil\u201d of the Moon, comprising Moon dust, Moon rocks, and so on \u2013 is made up of oxygen, silicon, magnesium, iron, calcium, magnesium, and aluminum, per Korotev \u2013 startlingly similar<\/a> to our own planet\u2019s crust. Of course, it\u2019s not all equally spread: the brightest, most reflective areas of the Moon are richer in elements like calcium and aluminum, while the darker shadows are home to higher levels of iron and titanium.<\/p>\n The reason for this discrepancy in makeup is fascinating, by the way. The darker depressions on the surface of the Moon are called \u201cmaria<\/a>\u201d, named long ago by astronomers who thought they were looking at far-off seas. While of course no such bodies of water exist on the Moon, they were more right than you might think: between 4.2 and 1.2 billion years ago, the Moon was pummeled by cosmic debris that left huge impact craters across its surface. Those filled with lava and molten lunar material, which eventually cooled to leave the dark mottling we see today.\u00a0<\/p>\n Below the surface<\/p>\n Compared to Earth\u2019s crust, the Moon\u2019s is super-thick. Even at its thinnest, it\u2019s around 40 kilometers (25 miles) deep \u2013 and considering how small the Moon itself is, that\u2019s a pretty hefty wodge, comprising about 2 percent of the total diameter. On Earth, the crust is only about 0.15 percent of the total diameter, making it about 1\/13 the proportion overall.<\/p>\n The Moon\u2019s crust is also kind of\u2026 wonky. It\u2019s \u201ca scientific puzzle,\u201d notes NASA<\/a>: \u201cThe crust is about 37 miles (60 km) thick on the side farthest from Earth, but only 25 miles (40 km) thick on the near side \u2013 quite a difference. Scientists are still trying to find out why the lunar crust is so uneven.\u201d<\/p>\n But this chunky, lopsided crust is nothing compared to the next layer down: the mantle. It\u2019s roughly 1,338 kilometers thick<\/a> (831 miles), which is a whopping 77 percent of the diameter overall \u2013 Earth\u2019s, for comparison, is about 2,900 kilometers (1,802 miles), or about 45.5 percent \u2013 and it\u2019s mainly solid basalt rich in pyroxene and olivine.\u00a0<\/p>\n With so much of the Moon taken up by the mantle and crust, there\u2019s not much room left for the lunar core<\/a> \u2013 and indeed, only about 20 percent of the Moon\u2019s diameter is taken up by the core. That\u2019s really small, from what we know about other rocky bodies \u2013 most, Earth included, have cores that are up to 2.5 times that proportion of their cross-section.<\/p>\n Like Earth, the Moon has a solid inner core comprised mostly of iron \u2013 plus some nickel and sulfur \u2013 surrounded by a molten outer core. While that\u2019s quite similar to Earth\u2019s core makeup, there\u2019s one crucial difference between the two: here on Earth, the outer core is still active, churning out the magnetic field that protects our planet from the sun\u2019s harmful radiation.<\/p>\n Without that magnetic field, the Moon is left to the mercy of solar radiation. The result: striking \u201clunar swirls\u201d, static sparks, and \u2013 potentially, one day<\/a> \u2013 the ingredients for creating water.<\/p>\n Our twin in space<\/p>\n So, what is the Moon made of? Well, as you may have gathered the overwhelming answer is \u201cpretty much the same stuff the Earth is made of\u201d. And there\u2019s a very good reason for that.<\/p>\n \u201cBefore Earth and the Moon, there were proto-Earth and Theia<\/a>,\u201d explains the Natural History Museum, London<\/a> (NHM). \u201cThe giant-impact model suggests that at some point in Earth’s very early history, these two bodies collided.\u201d<\/p>\n It was both supremely devastating \u2013 and wonderful. Theia<\/a> \u2013 before, a planet roughly the size of Mars \u2013 all but disappeared: much of it melted together with Earth, reforming the latter planet to be more than 50 percent larger than it had previously been. The collision fundamentally changed the chemistry of the Earth, imparting the volatile chemicals necessary for life<\/a> to form \u2013 without this apocalyptic event, in other words, Earth would be just another barren rock in the Solar System.<\/p>\n The rest of Theia? Well, that became the Moon. Two planets, likely very similar to begin with in any case, came together and swapped ingredients, making for an Earth and Moon that are extremely close in composition \u2013 just different enough to rule out one body splitting apart; too close to allow for the Moon to have come from somewhere else entirely.<\/p>\n In other words: there\u2019s more of our history out there than we might realize. \u201cThe mineralogy of Earth and the Moon are so close that it’s possible to observe Moon-like landscapes without jetting off into space,\u201d the NHM notes. \u201cThe Moon hasn’t had volcanoes for billions of years, so its surface is remarkably unchanged.\u201d<\/p>\n Thus, it explains, \u201cby looking at the Moon we can tell a lot about what the Earth was like four billion years ago.\u201d<\/p>\n