Revealing Secrets of our Solar System
By David Brill
It’s unlikely that any UT faculty member faces a longer commute than Harry (Hap) McSween, Chancellor’s Professor and Distinguished Professor of Science in the Department of Earth and Planetary Sciences.
For instance, he had to travel roughly 150 million miles over nearly four years to reach his latest research destination, Vesta, an asteroid 325 miles in diameter that circles the sun as part of the asteroid belt between the orbits of Mars and Jupiter. Although McSween helped pick the destination, he didn’t do the driving. That task belonged to NASA’s engineers at the Jet Propulsion Laboratory in Pasadena, which manages the Dawn mission.
The Dawn spacecraft blasted off from the Kennedy Space Center in 2007 and entered Vesta’s orbit on July 15, 2011, where it began a yearlong geologic survey of the asteroid. Dawn is part of NASA’s Discovery Program. It marks the agency’s first mission to extensively study an asteroid whose geology and structure can provide a glimpse into the solar system’s earliest epoch and reveal how Earth and other planets formed.
The Dawn project engages an international team of about forty scientists—including co-investigator McSween—charged with analyzing the data that streamed from the spacecraft during its orbit of Vesta, the only asteroid visible from Earth by the naked eye.
McSween, a former US Air Force pilot whose fascination with rocks and minerals traces to his boyhood, serves as lead scientist for Dawn’s surface composition working group. He and his team are responsible for interpreting data from key components of the spacecraft’s scientific payload.
Dawn is essentially a geology laboratory in a box about the size of two large refrigerators placed back to back, and features a communications dish and twin solar arrays that extend 65 feet. By tracking the spacecraft’s orbit and speed, which were influenced by Vesta’s gravity, scientists were able to calculate the asteroid’s mass and gravity and confirm the presence of a dense concentration of matter—or core—at the asteroid’s center.
A Gamma Ray and Neutron Detection (GRaND) instrument allowed scientists to determine the geochemical makeup of Vesta’s surface, including the abundance of some major elements.
Dawn’s visible and infrared (VIR) mapping spectrometer, which detects wavelengths of reflected and emitted radiation, revealed a highly variable crust composed primarily of basalts and pyroxene cumulates—minerals formed from molten rock.
A framing camera created a detailed topographic map that displays a diverse mineralogy. The map highlights the effects of space weathering and impact excavation that brought underlying mineral layers to the asteroid’s surface. The number of impact craters revealed by the camera serves as a sort of clock, placing the asteroid’s age at 4.5 billion years—nearly as old as our sun.
McSween’s research on Vesta has yielded a wealth of data. In fact, he has co-authored six articles that appeared in the May 2012 edition of the journal Science, and more are in the works.
Among the mission’s key findings, Dawn’s assay of the composition of the asteroid’s surface closely matches the mineralogy of a large class of meteorites that have fallen to Earth and confirms Vesta as the source.
“Vesta was such an enticing target because we already had samples from the asteroid,” says McSween, who was recently awarded the National Academy of Sciences J. Lawrence Smith Medal for his pioneering studies on meteorites and the geologic history of Mars.
The unique geologic signature of howardite-eucrite-diogenite (HED) meteorites from Vesta suggests its crust was formed through a process of melting. The magma ocean model hypothesizes that, in the earliest days of our solar system, Vesta contained rapidly decaying radioactive isotopes that generated sufficient heat to melt the asteroid. The magma cooled and crystallized, creating Vesta’s differentiated structure—which features a core, mantle, and crust—similar to Earth’s.
“The map of Vesta’s mineralogy shows a surface dominated by minerals that spectrally look like eucrite (the E in HED),” says McSween.
The massive Rheasilvia Crater on Vesta’s southern hemisphere—whose diameter is nearly as wide as the asteroid itself—suggests an impact sufficient to eject debris into space, and is the likely source of meteorites from Vesta. The impact, which occurred fairly recently—about one billion years ago—also stripped away some of the asteroid’s upper crust, revealing what McSween surmises is a mantle layer of diogenite (the D in HED), which crystallized slowly underground.
Howardites (the H in HED) are composed of fragments of eucrite and diogenite that have been pulverized by meteor impacts to form a soil on Vesta, much like on the moon, and then cemented into hardened rocks.
Study of the Core
Vesta is what’s known as a protoplanet or planetary embryo. According to the theory of planet formation, during the first ten million years of our solar system—a mere blink in geologic time—orbiting bodies such as Vesta collided with other orbiting objects and accreted to form larger and larger masses. Some of those masses became the planets that now make up our solar system. Earth continued to accrete and gain mass for nearly fifty million years; for Vesta, the process lasted less than a few million years.
Vesta is a leftover planetary building block whose evolution toward planethood was thwarted by the formation of massive Jupiter. It may not have made the final planetary cut, but Vesta is one of only three known giant protoplanets to survive billions of years of cosmic bombardment and remain largely intact. Another is Ceres, Vesta’s larger and vastly different sibling (Ceres contains water ice, while Vesta is dry). Dawn is scheduled to arrive at Ceres in February 2015 to conduct a similar geologic study.
Impacts powerful enough to form Vesta’s Rheasilvia Crater would have pulverized less sturdy bodies, says McSween, who suspects that Vesta’s dense iron core may have served as a protective skeleton, allowing the asteroid to absorb the impact without breaking apart.
Education and Inspiration
The subjects of McSween’s research may be situated millions of miles away, but the data they, and NASA’s other exploratory missions produce, have provided a myriad of benefits here on Earth, in addition to helping us understand how our planet came to be.
Though each venture into space requires unique and highly specialized tools, the knowledge gain is cumulative. “With each mission,” says McSween, “our ability to build clever instruments races ahead.”
Beyond the technological products that have resulted from a half-century of NASA research—sophisticated computer programs and processors, ion propulsion systems, and mechanized rovers—space exploration is inspiring science’s next generation, says McSween.
Then, of course, there’s the small matter of saving civilization.
“Our purely scientific study of asteroids has indicated that we live in the fast lane—a swarm of near-Earth objects,” says McSween, adding that many of those objects have erratic orbits that are “fiendishly” difficult to calculate. “Sooner or later, a big object, like the one that ‘did in’ the dinosaurs, is going to target Earth, and our ongoing research on asteroids might provide the tools to avert disaster.”
Now, with Dawn on its way to Ceres, McSween has settled in for another long interplanetary commute. With the spacecraft’s arrival at the asteroid, nearly three years hence, McSween and his colleagues will snap to work, delving ever deeper into the solar system’s longest-held secrets.