N. L. CHABOT ET AL . Planetary Materials Research at APL Nancy L. Chabot, Catherine M. Corrigan, Charles A. Hibbitts, and Jeffrey B. Plescia lanetary materials research offers a unique approach to understanding our solar system, one that enables numerous studies and provides insights that are not pos- sible from remote observations alone. APL scientists are actively involved in many aspects of planetary materials research, from the study of Martian meteorites, to field work on hot springs and craters on Earth, to examining compositional analogs for asteroids. Planetary materials research at APL also involves understanding the icy moons of the outer solar system using analog materials, conducting experiments to mimic the conditions of planetary evolution, and testing instruments for future space missions. The diversity of these research projects clearly illustrates the abundant and valuable scientific contributions that the study of planetary materials can make to Pspace science. INTRODUCTION In most space science and astronomy fields, one is When people think of planetary materials, they com- limited to remote observations, either from telescopes monly think of samples returned by space missions. Plan- or spacecraft, to gather data about celestial objects and etary materials available for study do include samples unravel their origins. However, for studying our solar returned by space missions, such as samples of the Moon system, we are less limited. We have samples of plan- returned by the Apollo and Luna missions, comet dust etary materials from multiple bodies in our solar system. collected by the Stardust mission, and implanted solar We can inspect these samples, examine them in detail wind ions collected by the Genesis mission. However, in the laboratory, and try to unravel and interpret their samples from other bodies in our solar system are also history. By understanding these samples, we can under- regularly delivered to Earth in the form of meteorites stand the history of our solar system. and cosmic dust. Meteorites sample a diverse range of 156 JohnS HOPKINS APL TEchnIcal DIGEST, VolUME 27, NUMBER 2 (2006) PLANETARY MATERIALS RESEARCH AT APL planetary materials from many more planetary bodies igneous rocks that have undergone partial melting and than just the few that have been visited by spacecraft— crystallization, often accompanied by chemical changes. from remnants of the earliest solar system, to the deep Iron meteorites lie at the opposite extreme, being com- interiors of evolved asteroids, to the only samples we posed of iron-nickel metal with the occasional silicate have from the planet Mars. In addition, we have unpar- or oxide grain. Stony-iron meteorites fall somewhere alleled access to abundant samples from one planet in between, containing approximately equal amounts in our solar system: Earth. Though often overlooked of metal and silicate material. Often, stony meteorites when thinking about planetary materials, the study of represent the crust and mantle of a planetary body, iron Earth is fundamental not only for learning about our meteorites represent the core of a differentiated plan- own planet but also for interpreting the materials from etary body, and stony-iron meteorites may represent all the other solar system bodies. On Earth, field studies material found at the boundary of these regions. can be conducted to examine a wide range of geologic Most meteorites are samples from asteroids, but a few processes in person and in great detail. Numerous ter- have been identified as originating from the Moon or restrial samples can be used as analogs for understand- Mars. The meteorites from the Moon have chemical ing other solar system bodies. Well-characterized Earth similarities to lunar samples returned by the Apollo materials can be used to test instruments for future program and so are classified as having come from space missions as well. the Moon. Some of the meteorites from Mars contain Scientists at APL are actively involved in the field trapped gas in parts of the rock; the composition of of planetary materials research. Here we describe some that gas is identical to the composition of the atmo- of these research projects; their diversity illustrates the sphere of Mars as measured by the Viking landers, large range of information that can be learned through indicating that these meteorites are from the surface the study of planetary materials. of Mars. Martian meteorites currently represent our only “returned” samples of the surface of Mars, and as a METEORITES NASA sample return mission to Mars is not slated to After traveling incredible distances through our solar occur within the next 20 years, they will likely continue system from their parent bodies, “meteorites” are the to be our only source of ground truth for the planet for rocks that have safely made the passage through Earth’s some time. Studies of Martian meteorites have taught us atmosphere and fallen to the surface. (“Meteor” refers to a great deal about the processes that have affected the the material as it traverses the atmosphere; most mete- surface of Mars over the past 4.5 billion years. The ages ors do not make it to Earth’s surface to become meteor- of these meteorites range from ≈150 million years in the ites.) The meteorites we have in our collections today, case of the group called the shergottites, which probably which come in all shapes and sizes, represent material represent surface lava flows, to 4.5 billion years in the from numerous solar system bodies. As sample return case of Allan Hills (ALH) 84001, which represents a missions to planetary bodies are sparse, meteorites offer rock formed farther below the surface of the planet, pre- researchers a valuable opportunity to learn a great deal sumably from the ancient highlands. However, although about the history of the solar system and the processes we know these samples are from Mars, we don’t know that have shaped it.1 where on the planet they originated. Meteorites are found worldwide and are named for ALH 84001, as shown in Fig. 1a, is the famous Mar- the location where they fall or are found. Currently, the tian meteorite that reportedly contained fossilized most productive locations for finding meteorites are the evidence of Martian nanobacteria (i.e., life on Mars). world’s deserts, namely, Antarctica and the Sahara, as Though this debate rages on, most researchers concur they provide arid environments that promote the pres- that the McKay et al.2 study was inconclusive and that ervation of meteoritic materials by limiting their expo- incontrovertible evidence for life on Mars remains to sure to liquid water. be discovered. Aside from the life on Mars debate, Meteorites are classified into different types based ALH 84001 offers a unique opportunity to study a rock on their composition. The two primary types are stony from another planet that is nearly as old as the solar and irons. The stony meteorites include chondrites system and older than any rocks preserved on Earth’s and achondrites and are composed of silicate and oxide surface. ALH 84001 contains secondary minerals (Fig. minerals, with an occasional metal grain. Chondrites, 1b) that were not present when the rock first crystal- the most common type, are essentially rocks made up lized but formed during subsequent alteration processes of early solar system materials, including a common such as during impact events or by exposure to water. component called chondrules, small blobs of primi- APL scientists led a study that examined carbon- tive material that have undergone very little chemical ate minerals in ALH 84001 and found that multiple change since their formation. Achondrites, on the other generations of carbonate and other secondary miner- hand, lack these little blobs and are early solar system als were produced in the rock.3 It was concluded that JohnS HOPKINS APL TEchnIcal DIGEST, VolUME 27, NUMBER 2 (2006) 157 N. L. CHABOT ET AL . Terrestrial field studies have focused on a variety of geologic processes ranging from cratering, to volcanic and hydrothermal activity, to fluvial erosion, to desert processes and aeolian transportation of material. Field work at APL has focused on aspects of the cratering pro- cess4–7 and the role of volcanic hydrothermal systems as niches for extremophile organisms and as an analog for possible Martian hydrothermal systems.8 Since reflectance spectroscopy, at UV to IR wave- lengths, is commonly used to determine the mineralogy of surface materials, and as it has the potential to detect the presence of some forms of living organisms through the proxy of various pigment molecules (e.g., chloro- phyll), both of which are important planetary objec- tives, the potential value of reflectance spectroscopy to detect hydrothermal systems has been tested by exam- ining such environments in Yellowstone National Park and the Great Basin of Nevada. Hydrothermal systems are typically characterized by the flow of hot water sat- urated with silica and the production of siliceous sinter (amorphous silica [SiO2]). Silica is dissolved in the heated water as it travels through the subsurface and is then redeposited when the water emerges onto the sur- face and cools (Fig. 2). These hydrothermal systems are also hosts to a variety of organisms that can survive in extreme environments (“extremophiles”), in this case at high temperatures (close to boiling) and high acid- ity (pH ≈2). As these extremophile organisms live and grow in the silica-rich waters they become entombed Figure 1. (a) The meteorite ALH 84001 is a sample from Mars. as sinter is deposited around them. Several questions (Photo courtesy of NASA/JSC.) (b) By studying the detailed have been examined: Do these extremophile commu- mineralogy of ALH 84001, insight can be gained into the past nities have diagnostic reflectance spectra and, if so, do history of Mars. The carbonates identified here indicate that these spectral signatures persist after the organisms the sample has been altered by water (Cb = carbonates, Fs = are entombed and die? Can this signature be detected feldspathic glass, Opx = orthopyroxene).