NASA captures, quite literally, gravity's rainbow.
Scientists care about the moon in part because it is the moon -- our moon. But they care about it, also, because its body is in some ways a proxy for our Earth's: Its surface, like ours, bears the scars of a long existence in our little corner of the Milky Way. And new research suggests that that existence might have been much more violent -- and perhaps more hospitable to life -- than we humans initially believed. Yes.
The story begins, officially, around five years ago. In 2007, the Japanese lunar satellite Kaguya released two small probes into orbit around the moon. Those satellites, working in tandem, created the first gravity map of the far side of the moon -- a chart that showed, in color-coded detail, the variations in mass on our nearest planetary neighbor.
The same year, NASA announced plans for a similar mission: the launch of twin spacecraft that would spend several months doing their own moon-mapping work, measuring the lunar gravity field in unprecedented detail. In late 2011, NASA began that mission, sending a pair of spacecraft -- collectively known as GRAIL (Gravity Recovery and Interior Laboratory) -- to orbit the lunar surface. The pair of satellites (named, awesomely, Ebb and Flow) fly in formation around the moon, sending each other -- and Earth -- microwave measurements of our only natural satellite. The twin vehicles, each about the size of a washing machine, work by detecting tiny changes in the distance between them -- variations caused by lunar mountains, craters, and subsurface mass concentrations.
The GRAIL satellites and their operators use a technique pioneered by GRACE, the Gravity Recovery and Climate Experiment, launched in 2002 and run jointly by NASA and the German Aerospace Center. The Grace satellites have been taking detailed measurements of Earth's gravity field -- tracking, in particular, gravity changes related to the movement of mass within Earth (like, say, the melting of ice at its poles and changes in its ocean circulations). GRAIL has been applying the lessons of GRACE to our planetary neighbor -- helped along by the fact that the moon lacks an atmosphere, which allows the satellites to orbit wonderfully close to its surface: between 10 and 30 miles above the lunar crust. (The ESA's GOCE satellite, which does gravitational mapping of Earth, has to stay 10 times farther away from its target to avoid atmospheric drag.)
So. Between March and May of 2012, the Ebb and Flow satellites essentially X-rayed the moon, assessing our nearest planetary neighbor both from and beyond its surface. Scientists took measurements from the moon's crust to its core to reveal both the moon's subsurface structures and -- indirectly -- its thermal history. And partial results of that work are reflected in the amazing graphic shown above, which is, GRAIL says, the highest-resolution map of this kind ever generated for a celestial body. The colors in question represent the variations in the moon's structure, with red indicating more massive areas and blue representing less-massive.
And here, along those lines, is a map depicting the bulk density of the lunar highlands on both the near (left) and far (right) sides of the moon -- generated using both gravity data from the GRAIL mission and topography data from NASA's Lunar Reconnaissance Orbiter. Solid circles correspond to prominent impact basins. White denotes regions that contain mare basalts (thin lines) and that were not analyzed; red corresponds to higher than average densities; and blue corresponds to lower than average densities.
So what do these maps actually mean? Well, first, consider the craters. The color-coding suggests that the moon was pummeled by impacts that were much more violent than previously assumed. The moon's crust is pretty much covered by craters -- and, if those impacts were caused by asteroids and other space debris, it would stand to reason that Earth, along with Mercury, Venus, and Mars -- our nearest neighbors -- once endured similar punishment. As Maria Zuber, GRAIL's principal investigator, put it in a press conference announcing the findings: The discovery "really opens a window to this early stage of just what a violent place the surfaces of all terrestrial planets were early in their history."
The data also suggest that the moon's crust is thinner than previously thought -- just 21 to 27 miles. (Earlier estimates had it ranging from 30 to 40 miles deep.) And underneath that crust, Ebb and Flow detected several large, linear structures -- structures, composed of solidified magma, that can run for up to 300 miles. Those lunar "dikes" are covered by craters, which would suggest that they predate most of the moon's violent impacts. And they could have formed, most interestingly, only if the moon's crust were extending -- only if the moon's interior were heating up and expanding.
This gives more evidence in favor of the Giant Impact hypothesis, the leading theory for the moon's origin -- a theory suggesting that the moon was formed from the fragments of Earth that gathered after a Mars-size body smashed into our planet about 4.5 billion years ago. A warmer-on-the-inside/cooler-on-the-outside moon would be consistent with that kind of coalescent lunar formation. As GRAIL guest scientist Jeff Andrews-Hanna noted in the team's press conference: "This had been predicted theoretically a long time ago, but there was no direct observational evidence to support this period of early lunar expansion until this GRAIL data."
And! Finally! The new information about the moon also suggests some evidence for a theory of how life might exist, or have existed, on other rocky planets. Because the cracks on the moon's surface could provide a pathway for fluids -- which might explain what happened to the ocean that some scientists believe once existed on the surface of, yes, Mars. As Zuber noted, "that ocean could well be underground." As the Martian surface dried out, the sub-surface water could have provided a hospitable environment for any microbes that were living on the planet's surface. Microbes, Zuber said, "could have gone very deep within the crust of Mars."
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