moon

The Moon

The moon is our closest neighbor in space and the only natural satellite to orbit the earth. It has a lower density than the earth (3.3 g/cm3 vs. 5.5 g/cm3) and is about ¼ the size of the earth. The moon has no atmosphere and only 1/6 the surface gravity of the earth. It orbits the earth in such a way that it always keeps the same face towards the earth. The side facing the earth consists of dark (basaltic) lava filled basins called mare (maria) or seas. The lighter areas are heavily impacted terrain called the highlands. Look at the picture above.

This page has a link to a QuickTime movie (1.8 Mb) that lets you see the entire surface of the moon.

The Apollo astronauts brought back over 800 pounds of moon rocks from both the maria and highlands and found that the maria formed 3.2 - 3.8 billion years ago and the highlands are even older ... some rocks dating up to 4.5 billion years old (the oldest earth rocks are about 4 billion years old ... and very rare).

What the moon tells us

The moon tells us an interesting story about the early history of the solar system that we can't get from the earth (because much of this time period from earth is lost because of plate tectonics, weathering and erosion). The story is that the early period of the solar system was a time of massive bombardments. There must have been a lot of debris floating around just waiting to hit something. This story is also evident when you examine other surfaces in the solar system. Look at the surface of Mercury ... the surface of Mars (southern hemisphere) ... the surface of asteroids like Eros, Ida, or Gaspra ... the surface of some other moons like Saturn's Mimus. All these surfaces show signs of major impacts. The maria (on the moon) represent the last time the moon was geologically active. Lava flowed up from the interior of the moon into the beep basins (most likely formed from major impacts). The lava cooled and solidified. A close examination of the maria show that these surfaces are relatively impact free (there are some exceptions). This means that the rate of impacts suddenly decreased (to almost zero) after 3.8 billion years ago.

This suggests that the moon, earth, and other planets all formed (rather quickly) from this smaller debris that was floating around. Astronomers call this the planetesimal hypothesis. Although we can't look back in our own solar system ... astronomers can look at other young stars, and this is exactly what they see ... evidence of debris orbiting in a disk around these stars (the first found was around a star called Beta Pictoris). We know that there still exists evidence of this earlier state in our solar system ... the asteroid belt. Astronomers believe that the entire solar system 4.6 billion years ago (the common date of most meteorites) was like this area. Under gravity, these smaller objects collected into larger and larger objects (by collisions) until they reached planetary sizes. It would also explain why the rates of impact would suddenly cease. As the planets grow (from the collection of these smaller objects), there would be less and less of these smaller objects floating around as time went on. By 3.8 billion years ago, most were swept up into making planets. The planetesimal hypothesis received another boost when planets were discovered orbiting other stars. At last count, over 100 planets (called extra-solar planets) have been discovered outside our solar system.

(animation)

The bad news is ... not all of these objects are gone. Estimates are that there are several thousand rocky fragments ½ mile in diameter which have earth crossing orbits, and they will hit the earth sooner or (hopefully) later. Eyes were wide open in 1994 when the earth witnessed a collision of a comet Shoemaker-Levy 9 with Jupiter. When chicken little cried "the sky is falling" ... she could be very correct! Most geologist now accept the idea that an impact with the earth 65 million years ago (known as the K/T event) lead to the mass extinction of almost ¾ of all species (including the dinosaurs). Other mass extinction event about 250 million years ago also appears to be from an impact as well as an event that may have lead to a mass extinction during the "age of fishes" 380 million years ago. Finally another smaller impact about 700,000 - 800,000 years ago severely changed the human population on earth. This is based on the age of Australasian tektites. It's not a matter of if it could happen again ... only when!

How did the moon form?

Good question ... and one that has eluded astronomers until the 1980's. All prior models had to account for the fact that the moon has a lower density than the earth, has very little iron and contained less volatile gasses than earth rocks. No adequate theory held up. That is, until the Collision Ejection Theory was proposed. It proposes that the moon was formed when a large (mars sized) impactor hit the young earth. The collision put large amounts of debris in orbit, which formed into the moon. This theory explains the similarities between earth and moon rocks. In addition, the energy from the impact depleted the moon in volatile gasses and the ejected material was mostly mantle rock, so the moon would have a lower density. This hypothesis is consistent with the idea that collisions were plentiful in the early ages of the solar system.

Click here to see a nice on-line lecture about the moon.