What is the best way to explore remote lands? Is it better to collect a litter of rocks and dust to bring back and study, or is it more valuable to fly super-sharp-eyed cameras above the surface to catalogue every visible and chemical variation?
Now, 32 years after Apollo 11 astronauts walked on the Moon and almost a decade after the orbital missions of the Lunar Prospector and Clementine missions, for Lunar and Planetary Institute scientist Paul Spudis, both methods of exploration seem equally necessary.
The Apollo missions rounded up 841.5 pounds (381.7 kilograms) of lunar rock samples while the two robotic missions sent back detailed maps of the Moon's color, topography, chemistry and gravity. But until all three sets of data came together, the Moon's evolution was unclear.
"It's the samples and the global data together that give us the different pieces of the puzzle," Spudis said. "We understand the maps because of the samples. Each data set tells different things, but together they tell the full story."
, one in which the Moon and Earth are equal players in a two body system that emerged after a Mars-sized meteor struck early Earth. The blow sent molten Moon-making materials into the air that eventually coalesced into a single ball. This is known as the Accretion Model. Our aluminum foil
Samples of the lunar highland from the Apollo missions, and orbital observations from Clementine and the Lunar Prospector, show that the Moon is in fact, almost entirely covered with the aluminum-rich rock called anorthosite. Anorthosite forms when molten rock cools slowly and lightweight aluminum-rich minerals float to the top of the magma.
"The only known source of sufficient heat for such an event is very rapid accretion, as expected if the Moon formed as a result of a giant impact between Earth and a massive asteroid," wrote Spudis in an article titled, 'What is the Moon Made of?' in this week's issue of the scientific journal Science.
Heart of iron
Because a molten proto-Earth would have ejected more surface material than core, goes the Accretion Model, the Moon would have to contain substantially less iron than the Earth. For comparison, Spudis likens the Moon to a raisin muffin with only a single iron core "raisin" in the center, while the Earth is proportionately built like an egg whose yolk is it's iron core
The 1,615.6 mile-wide (2,600 kilometer) South Pole-Aitken basin is one place on the Moon that hints at the lunar heart of iron. The giant impact scar is on the far side of the Moon and all three missions verified that its surface is not covered with aluminum-based mineral, but instead displays a lot of ferrous compounds. Scientists have long thought that this happened when the impact ripped off the top layers of the moon then exposed iron from the core.
In fact, the Lunar Prospector did gravitationally verify the Moon could have an iron heart.
"From the Lunar Prospector, it was determined that there is a core a couple of percent of the mass of the moon," Spudis told SPACE.com. "We think it's an iron core, which would again support the theory that the moon is made up of material blasted off the surface of the proto-Earth."
Water, water anywhere?
Although the Lunar Prospector and Clementine missions mostly verified already-known facts about the Moon, there was one very surprising discovery: that frozen water most likely lies sprinkled throughout the soil of craters at both poles.
The Lunar Prospector found hydrogen compositions at the poles indicating water may have accumulated there. The best explanation for the water so far is that it came from meteorites and accumulated where the Sun's energy could not evaporate it. But to Spudis, this isn't interesting in terms of the Moon's history.
"In terms of survivability in space, water is most important. But the discovery of water is more interesting from a space habitation resource viewpoint, than from a scientific viewpoint," he said.
The ice crystals could be "harvested" to fuel spacecraft, quench the thirst of colonists or provide other forms of energy for future human efforts at the Moon. But there are other possible explanations for the hydrogen reading at the Moon, and some lunar scientists await more direct evidence to confirm the presence of frozen water there.
The dual system
Whether or not there is frozen water is at the poles, and whether or not the Moon was formed from a giant impact, Spudis and nearly all lunar scientists and enthusiasts agree: Life on Earth would not be the same without the Moon, and understanding it better explains life here on Earth.
Spudis points out that without the Moon's stabilizing effect on the Earth's rotational axis, life on Earth would experience extreme climate changes and also wouldn't have evolution-catalyzing tides in the oceans.
"Without the Moon we may have evolved into an intelligent race of dolphins," Spudis conjectured, only half jokingly. "And with fins instead of thumbs, we may not have had a Saturn V rocket to ever get to the Moon in the first place."
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