A detailed photo of the moon reveals a heavenly body devoid of life. Its surface, gray and pitted with craters, hints at a violent past but a 'boring' present. Unlike earthquakes and volcanoes that constantly change the Earth's surface, anything exciting that shaped the lunar surface seems to have already happened, giving the appearance of a moon that went geologically quiet billions of years ago.
But when Thomas Watters, Senior Scientist and Chair of the Center for Earth and Planetary Studies of the National Air and Space Museum, looks at this "dead rock" he sees something very different. He sees a moon that is changing, a moon with recent "moonquakes" and a moon undergoing the puzzling phenomenon where it is contracting globally while simultaneously expanding in small regions. His findings are throwing some of what we thought we understood about the moon into question.
"One of the common misconceptions the public has of the moon is that its pretty much geologically dead," said Watters. "It just looks like everything geologically significant on the moon happened billions of years ago. And what's really exciting is that most of that may not be the case at all. [There are] things that could have happened on the moon very recently." There is new evidence that the surface of the moon has been changing within the last billion years, but, adds Watters, it might be as recent as within the last forty years.
Watters, a member of AAAS, is a participating scientist on a number of current NASA missions. However it is his involvement in the Lunar Reconnaissance Orbiter (LRO) Mission, specifically the high tech camera called LROC, which is producing amazingly detailed images showing new features that provide evidence the moon could still be changing today.
See some of these features and other amazing LROC images here.
"We knew from Apollo era imaging that there were these small contractual faults on the moon, what are called lobate scarps," said Watters. Lobate scarps are small ridges that form on the crust of planets and moons when they contract, like the wrinkles that form on raisins. These faults on the moon were small, only generally tens of meters high and tens of kilometers long, a length most humans can jog in a few hours and about the height of a typical house. The fact that we can see these small features means they are very young. The oldest the scarps could be is 800 million to a billion years old. However, Watters believes they are probably much younger than 100 million years. This would make them much younger then even most dinosaur fossils.
"Features of a certain scale will not survive for very long on the surface of the moon," said Watters. "They just get broken down, because the moon is being constantly hit by meteorites and micro-meteorites that are actively working and breaking up the soil and actually move it around." So scientists can estimate the ages of things on the surface by how much they have been degraded by these meteorites. Small features are erased much faster than big features, so features like the small fault scarps can't be very old, he concludes.
The scarps were first found in images taken during the Apollo era. But, these images only captured in high quality about 10 percent of the moon's surface, so scientists questioned if the fault scarps were a global phenomenon or only found in a few places. Recently images from LROC show lobate scarps all around the moon, indicating they are caused by a global phenomenon. Watters says that it indicates the moon's core was cooling, causing it to contract and the scarps to form.
The high-resolution LROC images have also shown another geologic feature: graben. Graben are indentations or small valleys on the surface of planets and moons, like trenches. Graben are formed by the crust stretching apart causing part of the surface to drop, like stretch marks on human skin. The graben found on the moon are much smaller than the young fault scarps, only a hundred or so meters long and tens of meters wide, and some appear to be as shallow as one meter.
"When you look at these graben, they look pristine," said Watters. "They look sharp; they don't have craters superimposed on them. They actually crosscut or deform very small diameter craters. It became [apparent] that they must be pretty young. And they are so shallow we know they couldn't exist on the moon for more than about 50 million years, and that's the upper end."
Watters even thinks the graben could have been forming in our lifetimes. In fact, in the 1970's there were a number of shallow "moonquakes." Some of these moonquakes were large, the equivalent of a 4.2 magnitude quake on Earth, and they took place near some of the lobate scarps and graben.
"The Apollo astronauts put 4 seismographs on the moon," said Watters. "There was an Apollo seismic network at one time that was operating during the 70s. And it recorded thousands of moonquakes... The really fascinating part of this is that 28 of those moonquakes were shallow. They were in the crustal area of the moon." Moonquakes in the crust could be related to the young graben and fault scarps scientists are seeing today, raising the possibility that some of these features were actively developing as recently as 40 years ago.
However, regardless of the age of the graben, the fact that they exist raises new questions. Graben are formed by the crust's expansion. Some of the graben are found near lobate scarps, and could have been formed by the scarps. But some are found far away, meaning something else must have caused the crust to expand.
"So you've got a bit of a contradiction; where you've got a moon that is contracting, by the evidence of these young fault scarps," said Watters. "But in some areas, not very big, the moon is doing the opposite, it's expanding."
These new findings force us to reconsider a lot of our old ideas about the moon. It raises new questions about how the moon was formed, how it evolved, and what is happening deep inside its interior. Scientists, like Watters, are still looking for answers to these questions, but they all indicate that the moon is a far more dynamic and changing place then the "dead rock" many of us see when we look up into the night sky.