I had dinner with my friend’s son Lucas this weekend, and since he is my favorite little grom snowboarder / mini-geologist-in-training, I asked him if he would like to ask me a science question for this week’s column. Luca looked up from Angry Birds just long enough to tell me he was looking up in the sky one night recently, and was wondering where the moon came from.
The first thing that comes to mind when I started pondering the answer to Lucas’s question was, of course, Will Ferrell and his portrayal of legendary Chicago Cubs broadcaster Harry Caray on Saturday Night Live. Those of you old enough to remember who Harry Carray is will get a kick out of this clip, in which “Harry” offers some interesting hypothetical questions to a “scientist” played by Independence Day’s Jeff Goldblum.
Despite not being made of BBQ spare ribs, the moon is an incredibly mysterious celestial object, whose origin is still the source of much speculation. It has a mass of 1/81 the mass of earth, or around 7.3477 × 10^22 kilograms. The moon’s relatively smaller mass gives it a lower gravitational acceleration, about 16.7% that of earth. This means that a 180lb human walking on the moon would only weigh ~30 lbs.
Our tides here on earth owe their ebb and flow to the pull of the gravity from the mass of the moon as it rotates around the earth, on a slightly different plane than the earth’s orbit around the sun. This excellent short video illustration from NASA shows both the effect of the moon on the tides of earth’s oceans, and the reason that we can only see the light side of the moon as it hurtles around us in space as we in turn zoom around the sun. The moon orbits earth synchronously, which means that it rotates at the same rate as the earth as it is spinning around our planet, thus showing us the same side all the time. Only a select few astronauts have actually laid eyes on the dark side of the moon.
Lying about 30 earth-diameters away from us, the moon has roughly the same relative diameter as the sun (which is much, much farther away). This cosmic geometric coincidence allows the moon to almost completely block sunlight as it’s shadow crosses the earth during total solar eclipses. The last one was two months ago on June 15th, and the next one will occur on December 10th of this year.
Check out this amazing sequence of the August 28th 2007 lunar eclipse by renowned Yosemite photographer Michael Frye:
Back to Luca’s original question though – where exactly did the moon come from? Believe it or not, today there are still several different competing hypotheses about the origins of the moon.
The earliest celestial thinkers postulated that the moon and the earth formed at around the same time from the same cloud of floating debris, but formed into two planetary bodies that then started orbiting eachother. This hypothesis is known as the “binary accretion” model. This model would require that the earth and the moon were made of the same material though, and they are not. More on that in a moment.
The” fission” hypothesis, first put forth by George Darwin (son of Charles) holds that the young earth, recently accreted from an enormous mass of coagulated cosmic debris, was spinning much much faster than it is today. At some point, the proto-earth spun off a blob that eventually started orbiting the earth in a different plane, and became the moon. The younger Darwin backed up his hypothesis with the observation that the moon is receding away from the earth about 4cm every year, and thus must have originally been part of the earth until it was flung off from the earth.
The “capture” hypothesis brings forth the idea that the earth’s gravity captured a giant planetary mass that passed by the young earth and pulled it into orbit, much like Darth Vader’s Death Star did to the Millenium Falcon in Star Wars, Episode IV. This theory almost makes sense, until you think about the fact that the moon and Earth have slightly different orbital panes, which throws the theory out the window because if the earth had grabbed onto a round planetary object a very long time ago and forced it into orbit, the orbits of the two would certainly lie in the same plane by now.
Of all the hypotheses about the formation of the moon, the most widely-accepted theory is the “giant impact” hypothesis. This idea holds that an enormous planetary object the size of Mars smashed into the Earth about 100 million years after it had formed and broke off a giant chunk of material that eventually accreted under its own gravity and formed the moon. If the impact was at a slight angle, which would have been more likely than a perpendicular impact in the vastness of space, the two coalesced bodies would develop non-planar orbits.
Another fact in support of this idea of lunar origin is the observations of the mineralogical and physical characteristics of moon rocks. Scientists learned a great deal about these otherworldy rocks from the samples brought back to earth during the Apollo missions of the 1960 and 70s. Moon rocks are much less dense than earth rocks, and are made of minerals that are deficient in water. if the “giant impact” idea were to have occurred, a great deal of the water in the two bodies would have been boiled off in the enormous amounts of energy released during impact. Furthermore, a giant collision would have spewed only the lighter elements and material into space, which would have then become the moon.
No matter where it came from, the moon is a mysterious sight to behold. When I go out photographing at night, I try to time my excursions for nights when there is just a little bit of moon in the sky to light up the foreground of my images.