Earth’s companion in space for the last 4.5 billion years, the Moon stands as the only celestial object whose features are visible from Earth with the naked eye. Like the Earth and neighboring planets Mercury, Venus and Mars, the Moon is geologically differentiated, possessing a core, mantle and crust. In legend and folklore, it evokes mystery, strange powers — and wisdom. Its tidal forces govern humankind’s relationship with our planet’s vast seas. We measure time and planting seasons by the Moon’s phases, and its reflective light illuminates our otherwise dark nights. In many cultures, it has been identified with deities.
Earthly humanity and the Moon are bound together, but let us imagine the Moon as it was before any human existed to question its origins.
Go back 4 billion years. Asteroids and meteors crash into the surface of the Moon. Volcanoes erupt and molten lava flows across the moonscape. Immense impact basins are created, surrounded by mountainous rings. Material dispersed by the impacts forms lumpy deposits and blankets of particulate matter. One of the enormous impact basins will later be called Tranquillitatis, and humans will eventually walk there. The largest and oldest recognizable impact structure will become known as the South Pole–Aitken basin.
Millions of years pass. The bombardment tails off, yet smaller objects continue relentlessly to strike the Moon. Basaltic lava flows into the basins, creating smoother areas that humans more than 3 billion years in the future will call “seas” or “maria.” Whether due to crustal effects of massive impacts or the gravitational pull of the Earth, volcanism is more extensive on the Moon’s Earth-facing side, nearly a third of which is covered by flood basalts. A few silicic volcanoes form in this region, known broadly as Oceanus Procellarum. On the far side, where the crustal rocks are thicker, the surface remains ruggedly cratered, with relatively few and small patches of maria.
The Earth continues to change; volcanism and plate tectonic activity slowly erase outward signs of the great impacts. At the same time, the much smaller Moon begins to cool. Volcanism slows. The maria become pockmarked and surface boulders are weathered down by constant impacts.
And on the Moon’s far side, perhaps as recently as a billion years ago, a volcanic complex forms above a rare silicic shallow intrusion, awaiting discovery.
As a member of NASA’s Lunar Reconnaissance Orbiter Camera science team, geologist Bradley Jolliff, PhD, the Scott Rudolph Professor of Earth & Planetary Sciences, keeps a close eye on the Moon from his office in Rudolph Hall. (The Earth & Planetary Sciences building was recently dedicated Scott Rudolph Hall. Visit 15 Secrets of Rudolph Hall to learn more about this fascinating building and its recent dedication.)
In 1998, the Lunar Prospector Gamma Ray Spectrometer detected an isolated concentration of the radioactive element thorium in the region of the Compton and Belkovich craters on the Moon’s far side. [Compton Crater was named after Arthur Holly Compton (Washington University chancellor, 1945–53, and recipient of the Nobel Prize for Physics in 1927) and his brother, Karl T. Compton.] Similar “hot spots” exist on the Moon’s near side where upwellings of magma rich in silica occurred, but the Compton-Belkovich Thorium Anomaly appears to be uniquely situated. [Incidentally, the anomaly was first discovered in 2000 by David Lawrence, PhD ’96 (physics), who was a member of the Lunar Prospector Gamma Ray Spectrometer team, and who continues to collaborate with Jolliff.]
When the Lunar Reconnaissance Orbiter (LRO) was launched in 2009, Jolliff and his team finally had an opportunity to analyze the anomaly. The Lunar Reconnaissance Orbiter’s Narrow Angle Cameras provide high-resolution images of the lunar terrain from multiple angles and illumination geometries, while its Diviner instrument provides data to help map compositional variations on the surface and derive subsurface temperatures. Jolliff’s team used three-dimensional digital terrain models from the camera images to determine whether the domes associated with the thorium signature indeed had volcanic origins.
“The combination of LRO Camera and Diviner data makes a compelling case for Compton-Belkovich being the only lunar far-side silicic volcanism,” says Benjamin Greenhagen, MA ’05, PhD (UCLA), research scientist at NASA’s Jet Propulsion Laboratory and deputy principal investigator on LRO’s Diviner Lunar Radiometer Experiment.
“The density of impact craters there is very low,” Jolliff says, “and that leads us to suspect that it may be much younger than most of the other lunar volcanism. If that’s the case, then it really is a very special geologic place on the Moon, one where not only was there unusual volcanism — unusual in composition — but volcanism that perhaps occurred much later in lunar history than we thought possible. So that forces us to rethink how the Moon evolved thermally.”
A number of Earth & Planetary Sciences students, both graduate and undergraduate, work in Jolliff’s labs, trying to unravel the hot spot’s history.
“They’re working with me on how its materials are distributed and how the surface has evolved over time,” Jolliff says. “Mike Zanetti helps immensely by working with the undergrads, actually supervising some of the work that they do. It’s nice having such a wonderful team.”
Graduate student Zanetti also works with LROC images to put features into geologic context. Graduate student Ryan Clegg analyzes the blast zone effects of the Apollo landers, using LROC to help determine surface properties of the soil and analyze photometry. Graduate student Steve Seddio studies rare lunar granites in Apollo samples. Junior Katherine Shirley worked with Zanetti to determine crater distributions, while senior Natalie Accardo researched the distribution and weathering of boulders to help understand both the age of the area and the composition of the rocks.
“I am primarily focused on impact craters, and I use LROC images to study their morphology and compare them to impact craters on Earth,” Zanetti says. “This is a two-way street: We need to study the lunar craters because they are pristine, but we need to look at Earth craters because we can visit them and examine what happens to the rocks.”
Jolliff, according to Zanetti, lets him control his projects, which is very important to training an independent scientist. “But,” Zanetti stresses, “he continually challenges me to provide more evidence.”
Greenhagen joined the LRO Diviner team as deputy principal investigator after finishing his PhD in 2009. He credits earning this position so early in his career to working with Jolliff on his master’s while at Washington U.
“Brad taught me how to think like a scientist — that is, how to identify a question and develop a rigorous methodology to solve it,” Greenhagen says. “Working on LRO enables me to continue collaborating with Brad, who is one of the most productive members of the LROC science team.”
According to Greenhagen, Jolliff is an expert at organizing researchers around a collaborative task. “He has the expertise to know all the right questions to ask and the willpower to get each task completed in a timely manner,” Greenhagen says.
As enlightening as all of the LRO data have been, Jolliff would particularly relish getting his hands on actual far-side lunar samples for study.
This goal was central to the proposed MoonRise mission to the South Pole–Aitken basin, a finalist in NASA’s New Frontiers program for which Jolliff was the principal investigator. Although the MoonRise mission was not selected for funding, the lunar sample return remains a priority and will most likely be proposed again when NASA opens its next New Frontiers competition.
As the largest and possibly oldest impact basin on the Moon, the South Pole–Aitken basin is a tantalizing area for exploration. Collecting samples of material — whose internal geologic clock was reset when the impact occurred — would be invaluable to determining its actual age.
“The mission is inherently risky, because you have to deal with a lot of elements,” Jolliff says. “You have to launch a spacecraft and land it on the Moon. Because you’re working on the far side, you can’t have direct communications; you need a relay communications satellite. Then you have to collect the sample, put it in a canister, lift off, and return safely to Earth.”
The Moon’s poles also contain deposits of tremendous scientific interest. “Certain places on the poles exist in permanent shadow, which makes them among the coldest places in the Solar System,” Jolliff says. “They’ve been sitting there cold for perhaps two billion years and collecting what we call volatile elements — things like water and other hydrogen-bearing compounds that have condensed in that cold but were boiled off or affected by the Sun elsewhere. We’d really like to know the history of the volatile elements in the Solar System.”
Jolliff foresees that one day we will have a Moon base from which to launch deeper space exploration. Within a couple days’ travel of Earth but outside low Earth orbit, the Moon is an ideal place to study how humans might survive the hazards of space beyond the Earth’s protective magnetic field before undertaking manned missions to Mars or elsewhere. The presence of oxygen, hydrogen and other elements needed for both life support and fuel, combined with low gravity, makes the Moon an ideal fueling station.
“Understanding how lunar materials got distributed the way they did is important to me, especially if we are ever to use the Moon as a resource for further exploration,” Jolliff says. “The Moon is still a hard place to get to. We tend to get seduced, by all of the science fiction that goes on and all of NASA’s wonderful successes with Mars, into thinking that doing things in space is routine, but it’s not.”
Jolliff says that we are no longer a spacefaring nation: “We’re a spacefaring humanity, a spacefaring species,” he says. “It’s our destiny to go out into space, and our next nearest neighbor, the Moon, has got to be part of that.”
Terri McClain is a freelance writer based in St. Charles, MO.