Down the Lunar Rabbit-hole

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July 12, 2010:  A whole new world came to life for Alice when she followed the White Rabbit down the hole. There was a grinning cat, a Hookah-smoking caterpillar, a Mad Hatter, and much more. It makes you wonder… what’s waiting down the rabbit-hole on the Moon?

Down the Lunar Rabbit-hole (Marius Hills, 200px)

This pit in the Moon’s Marius Hills is big enough to fit the White House completely inside. Credit: NASA/ LROC/ ASU [larger image]

NASA’s Lunar Reconnaissance Orbiter (LRO) is beaming back images of caverns hundreds of feet deep — beckoning scientists to follow.

"They could be entrances to a geologic wonderland," says Mark Robinson of Arizona State University, principal investigator for the LRO camera. "We believe the giant holes are skylights that formed when the ceilings of underground lava tubes collapsed."

Japan’s Kaguya spacecraft first photographed the enormous caverns last year. Now the powerful Lunar Reconnaissance Orbiter Camera (LROC, the same camera that photographed Apollo landers and astronauts’ tracks in the moondust) is giving us enticing high-resolution images of the caverns’ entrances and their surroundings.

Back in the 1960s, before humans set foot on the Moon, researchers proposed the existence of a network of tunnels, relics of molten lava rivers, beneath the lunar surface. They based their theory on early orbital photographs that revealed hundreds of long, narrow channels called rilles winding across the vast lunar plains, or maria. Scientists believed these rilles to be surface evidence of below-ground tunnels through which lava  flowed billions of years ago.

"It’s exciting that we’ve now confirmed this idea," says Robinson. "The Kaguya and LROC photos prove that these caverns are skylights to lava tubes, so we know such tunnels can exist intact at least in small segments after several billion years."

Down the Lunar Rabbit-hole (Kaguya Pits, 550px)

These Kaguya images show the Marius Hills pit in the context of a meandering system of volcanic rilles. Because the pit is in the middle of a rille, it likely represents a collapse in the roof of a lava tube. Credit: JAXA/SELENE [more]

Lava tubes are formed when the upper layer of lava flowing from a volcano starts to cool while the lava underneath continues to flow in tubular channels. The hardened lava above insulates the molten lava below, allowing it to retain its liquid warmth and continue flowing. Lava tubes are found on Earth and can vary from a simple tube to a complex labyrinth that extends for miles.

If the tunnels leading off the skylights have stood the test of time and are still open, they could someday provide human visitors protection from incoming meteoroids and other perils.

"The tunnels offer a perfect radiation shield and a very benign thermal environment," says Robinson. "Once you get down to 2 meters under the surface of the Moon, the temperature remains fairly constant, probably around -30 to -40 degrees C."

Down the Lunar Rabbit-hole (The Clever Pit, 200px)

This cavern in Mare Ingenii is almost twice the size of the one in the Marius Hills. Credit: NASA/ Goddard/ ASU [more]

That may sound cold, but it would be welcome news to explorers seeking to escape the temperature extremes of the lunar surface. At the Moon’s equator, mid-day temperatures soar to 100 deg C and plunge to a frigid -150 deg C at night.

Paul Spudis of the Lunar and Planetary Institute agrees that lunar lava tubes and chambers hold potential advantages to future explorers but says, "Hold off on booking your next vacation at the Lunar Carlsbad Hilton. Many tunnels may have filled up with their own solidified lava."

However, like Alice’s White Queen, who "believed as many as six impossible things before breakfast," Spudis is keeping an open mind.

"We just can’t tell, with our remote instruments, what the skylights lead to. To find out for sure, we’d need to go to the Moon and do some spelunking. I’ve had my share of surprises in caving. Several years ago I was helping map a lava flow in Hawaii. We had a nice set of vents, sort of like these skylights. It turned out that there was a whole new cave system that was not evident from aerial photos."

As for something similar under the lunar skylights?

"Who knows?" says Spudis. "The Moon continually surprises me."

This could be a white rabbit worth following.


A Molten Flood

Large impacts heat portions of the crust to such high temperatures that rocks melt and flow like lava.

A flood of impact melt swept away from the rim of Necho crater. NAC image M119041553; scene width is 540 m. Credit: NASA/GSFC/Arizona State University
Large impacts are catastrophic events for the local area. Besides the huge craters they leave behind (this one is 30 km or 19 miles across), impacts heat portions of the crust to such high temperatures that rocks melt and flow like lava, as seen in today’s featured image. These melts run downhill, cool and solidify, leaving behind beautiful flow features also highlighted in several past featured images.

Large impacts heat portions of the crust to such high temperatures that rocks melt and flow like lava.

A reduced resolution view of the impact melt outside the eastern rim of Necho crater. Images M119041553L and R; scene width is 5.3 km. Credit: NASA/GSFC/Arizona State University
The scene above is a wider view outside the eastern rim of Necho crater (the detailed view in the first image is in the upper left corner here). Almost everything you see is coated in impact melt, which flowed from the crater, moving boulders along with it and ponding in small topographic lows, which now look like smooth, frozen lakes. What a sight this must have been shortly after the crater formed!

Large impacts heat portions of the crust to such high temperatures that rocks melt and flow like lava.

A WAC monochrome view of the 30-km crater Necho. The impact melt is concentrated outside the northeastern rim, approximate location of the NAC detailed views is indicated by arrow. Credit: NASA/GSFC/Arizona State University
Impact melts play a key role in our understanding of when things happened on the Moon. As rock is melted and then cools and reforms, its internal radiometric clock is reset. By collecting a sample of impact melt scientists can very accurately determine when that crater formed. Since crater rays run out long distances we can determine the relative ages of rays, material that underlies rays, and rays that cross other rays. By sampling a few key craters scientists could easily unravel the absolute chronology of some key events on the Moon over the past billion years – a time not well sampled during the Apollo years. Sample return missions are a high priority for lunar exploration, and absolute age dating is only one of the many reasons why!


The Depths of Mare Ingenii


Mare Ingenii may be best known for its prominent lunar swirls, which are high albedo surface features associated with magnetic anomalies. However, lunar swirls are not the only unique geologic feature found in the farside "sea of cleverness."
pit in Mare IngeniiImpact craters are visible everywhere on the moon, but pits are rare. This pit in Mare Ingenii, the "sea of cleverness," is about 130 meters (427 feet) in diameter! Image width is 550 meters (1,805 feet), illumination is from the upper right, LROC Frame: NAC M128202846LE. Credit: NASA/Goddard/Arizona State University
› Larger image
The high-resolution cameras aboard the Japanese SELENE/Kaguya spacecraft first discovered this irregularly-shaped hole, visible in the above image at LROC’s 0.55 m/pixel resolution. The boulders and debris resting on the floor of the pit are partially illuminated (left side of the pit, above image) and probably originated at the surface, falling through the pit opening during collapse.
Marius Hills regionArrow indicates location of pit. "S" indicates one of the numerous lunar swirls located in this region. Image is a portion of LROC WAC mosaic, 200 meters per pixel resolution; image width is 160 km (100 miles). Credit: NASA/Goddard/Arizona State University
› Larger image
A pit in the Marius Hills region, previously discovered by the JAXA SELENE/Kaguya mission, is thought to be a skylight into a lava tube in the rille-riddled region. Similar to the Marius Hills pit, the pit in Mare Ingenii is probably the result of a partially collapsed lava tube. However, the numerous volcanic features of the Marius Hills (such as the prominent rilles and domes) are not found in Mare Ingenii.


The Earth From The Moon


LRO mosaic image of the Earth> View larger image

The Earth as seen from the Moon! LROC NAC mosaic of images snapped on 12 June 2010 during a calibration sequence (Images E130954785L and E130954785R). Credit: NASA/Goddard/Arizona State University
All cameras are susceptible to scattered light. You may have seen scattered light in pictures you have taken looking towards the Sun. Sunlight reflects off the optics and sometimes off the structure of the lens, and often appears as a gradient of brightness across the image. Attaching a baffle to your camera, like we did with the LROC Wide and Narrow Angle Cameras, can minimize this effect. More subtle effects are often present but usually you simply just don’t notice artifacts because of strong color contrasts in the scene. Since the Moon has only very small color contrasts, the LROC team must characterize even subtle scattered light effects within the 7-color Wide Angle Camera (WAC) images. Changes in composition (rock types) result in subtle differences of color, typically about 10% or less. For scientists to make accurate interpretations of WAC color maps, the amount of scattered light must be quantified (and preferably corrected). One way of measuring scattered light is imaging a bright object against a dark background. From the Moon, the Earth serves that function well. While a series of WAC calibration images of the Earth were being acquired, the Narrow Angle Camera (NAC) was shuttered to capture this spectacular Earth view. The bottom of the Earth was clipped because the prediction of the exact time when the cameras’ fields of view would cross the Earth was off by a few seconds.
Since the NAC acquires only one line of a picture at a time, the spacecraft had to be nodded across the Earth to build up the scene. The NAC Earth view is actually a mosaic of NAC-Left and NAC-Right images put together after calibration. The distance between the Moon and the Earth was 372,335 km when the picture was taken, with a pixel scale of about 3.7 km, and the center of this view of Earth is 25°N latitude, 114°E longitude (a few hundred kilometers north of Hong Kong).

LRO mosaic image of the Earth annotated> View larger image

AP: Arabian Peninsula; CS: Caspian Sea; H: Himalayan Mountains; L: Lena River; I: Indian Ocean; A: Australia; J: Japan; P: Pacific Ocean; large yellow arrow indicates approximate position of the North Pole. Credit: NASA/Goddard/Arizona State University
It was a beautiful clear summer day over the North Pole. You can see ice covering most of the Arctic Ocean with a few leads of open water (dark) starting to open up. If you look very close you can follow the Lena River upstream from the Arctic Ocean all the way to Lake Baikal. Much of the Middle East was clear and you can trace spectacular swirl patterns of folded rock layers through Iran, Afghanistan, and Pakistan. These mountains formed as the Eurasian and Arabian tectonic plates collided.