Issue Date: January 2, 2006
Salvaging NASA's Crashed Genesis Mission
Four little sensors, installed upside down, caused the crash onto the Utah desert of a capsule of solar-wind samples collected by the National Aeronautics & Space Administration's Genesis spacecraft.
Most of the particle collectors aboard the craft were shattered, and all were contaminated in the September 2004 accident. But after a few weeks of sifting through the wreckage, Genesis scientists announced they believed they could salvage much of the project.
Since then, the analysis has been slow, involving painstaking clean-room examination and removal of surface contamination. Not only are scientists finding that they can indeed extract solar-wind particles from the mess, but they also have gotten some of their first scientific results, which they reported last month at the American Geophysical Union annual meeting in San Francisco.
"We are not giving up on measuring anything that we've planned from the beginning," said Donald S. Burnett, geochemistry professor at California Institute of Technology and principal investigator of the Genesis mission.
In addition to the preliminary work of cleaning samples and developing highly sensitive detectors, Genesis scientists have now shown that different types of solar wind appear to have similar noble gas isotope ratios and have possibly solved a long-standing puzzle from lunar sample studies about variations in solar-wind isotopic compositions.
The solar wind is a link in the chain of information that can reveal how and why the solar system formed. Scientists believe that the composition of the sun's surface matches that of the original, pre-solar-system nebula of gas and dust. Luckily, that material sails through space in the form of the solar wind: ionized particles, whipped up and spewed out from the sun's roiling surface. With a detailed description of relative abundances of elements and isotopes in the solar wind, scientists will be able to model the early chemistry of the solar system and the reactions that formed the planets and asteroids, according to Amy Jurewicz, a researcher at Arizona State University and former project scientist for Genesis at the Jet Propulsion Laboratory, Pasadena, Calif.
Several spacecraft—in particular, NASA's Advanced Composition Explorer—have traveled beyond Earth's interfering magnetic field to send back data on the solar wind. Astronauts on several Apollo missions planted solar-wind collectors on the moon; these were returned to Earth. But the samples returned to Earth by Genesis were to represent a new level of accuracy and completeness.
Not only did Genesis collect bulk solar-wind samples, but it also discerned and detected particles from several categories of solar wind, generated in various ways by three different solar surface processes.
After collecting samples for more than two years using foils, as well as crystalline wafers made of silicon, diamond, sapphire, and other materials, Genesis prepared to return to Earth. Braked by parachutes, the capsule was to have been snatched in midair by helicopters. Because of an error in the design, the sensors were installed upside down, and the capsule's parachute failed to deploy. The shattered capsule and its contents were carted off for cleaning. In the end, scientists determined they'd lost approximately 80% of the silicon collectors.
Adding to the already onerous cleaning tasks was a thin but persistent film that coated the collectors. Dubbed the "brown stain," this film was produced when silicon-based lubricants outgassed inside the craft were polymerized by ultraviolet light.
Scientists have found that, in some cases, washing the collector fragments with ultra-pure water cleans off much of the dirt. Other, more high-tech techniques, such as etching with oxygen, scour off more embedded contaminants, Burnett said at the meeting. Now, 15,000 samples are available for scientists to study.
The extremely low concentrations of solar-wind particles in the foils and wafers have spurred the development of a new generation of highly sensitive mass spectrometers, which several speakers described at the meeting. A new eight-channel mass spectrometer, designed to analyze noble gases, has a sensitivity several times greater than previous instruments, said Charles M. Hohenberg, physics professor at Washington University, St. Louis.
Hohenberg also unveiled new results showing that the different types of solar wind appear to have the same neon isotope composition. That simplifies the picture for future solar system modelers.
New studies of solar-wind particles in metallic glass targets on Genesis, from the lab of Ansgar Grimberg, a planetary scientist at the Swiss Federal Institute of Technology, Zurich, illustrate a curious effect, seen in some lunar samples: Isotope ratios of some elements differ in lower portions of a sample, even just a few nanometers down. Grimberg's group verified that the effect is simply a result of the way solar wind embeds into material, rather than from differences in the solar wind itself. "It solves a 15-year-old mystery," says Roger Wiens, scientific coinvestigator for Genesis and a planetary geologist at Los Alamos National Laboratory.
Genesis scientists at the meeting also reported that they have verified in some of the bulk solar-wind samples a key, long-known phenomenon: Elements such as magnesium and iron that are easily stripped of outer electrons—in other words, that have a low first ionization potential (FIP)—are more easily drawn into the particle streams, and so are enriched in the solar wind compared with other elements with higher FIPs. For example, the ratio of iron to helium is probably 10 times higher in the solar wind than in the sun, Burnett noted.
Jurewicz described initial investigations into another related but more subtle and controversial effect known as the first ionization time. It's hypothesized that in addition to FIP, the speed with which an element is ionized affects how easily it is incorporated into the solar wind. Her group is working on measuring sodium in the solar wind; sodium, like iron and magnesium, has a low FIP, but it has a very different first ionization time.
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