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A Guide to Asteroid-Meteorite Spectral Links 1 Introduction to Asteroid

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A Guide to Asteroid-Meteorite Spectral Links 1 Introduction to Asteroid 第 6 卷 第 5 期 深 空 探 测 学 报 Vol. 6 No. 5 2019年10 月 Journal of Deep Space Exploration October 2019 A Guide to Asteroid-Meteorite Spectral Links Daniel T. Britt （Department of Physics，University of Central Florida，Orlando 32816， USA） Abstract： There are 24 asteroid spectral types in the Bus-DeMeo spectral classification system and about 27 meteorite mineralogical types. The spectral links between asteroids and meteorites vary a lot in quality and reliability because of challenges with the lack of diagnostic features in some mineralogies and the effects of space weathering. I will review what can be learned remotely from visible and near-infrared spectroscopy about asteroid mineralogy and how to understand asteroid spectral identifications from a geologist’s point of view. Keywords：asteroids；spectroscopy；mineralogy；space weathering；classification Highlights： ● Asteroid spectral taxonomy does not imply a unique mineralogy for each class or subclass. ● Complicating any interpretation is space weathering，which is ubiquitous for asteroids， poorly understood for some asteroid mineralogies， and spectrally important. ● Spectral features only provide diagnostic identification of mineralogy for some asteroid types such as the S-complex and the A， K， L， Q， O， R， and V-types. Much of the taxonomic variation in these types of asteroids is probably caused by space weathering. ● The C and X-complexes and the D and T-types lack the diagnostic spectral information to unambiguously characterize their mineralogy. The low-albedo asteroids in these classes tend to be volatile-rich， moderate and high albedo asteroids tend to be anhydrous. ● Low albedo， weak， hydrated meteorites are probably very underrepresented in the meteorite collections relative to their actual abundance in the asteroid population. 中图分类号：P135+.3 文献标识码：A 文章编号：2095-7777（2019）05-0437-11 DOI:10.15982/j.issn.2095-7777.2019.05.004 引用格式：丹尼尔 T·布瑞特. 小行星与陨石的光谱联系[J]. 深空探测学报，2019，6（5）：437-447. Reference format: BRITT D T. A guide to asteroid-meteorite spectral links[J]. Journal of Deep Space Exploration， 2019，6（5）：437-447. 1 Introduction to Asteroid Spectroscopy What we know about the mineralogy of asteroids and Taxonomy comes largely from two sources: analysis of the meteorites that accumulate on Earth and the remote How are meteorites and asteroids related to each sensing of asteroids. The meteorite data has the other? Most meteorites come from asteroids (several advantage of state-of-the-art analysis techniques and is groups of meteorites do come from Mars and the extremely detailed and precise[3]. The remote sensing Moon). But which meteorites come from which data uses a wide range of wavelengths from visible, near- asteroids? For the popular Bus-DeMeo asteroid spectral classification system based on visible and near- infrared, and thermal. This, coupled with albedo can be [4] infrared spectroscopy[1-2] there are 24 spectral classes diagnostic of mineralogy in many cases . However, shown in Figure 1. For the meteorites that come from severalasteroid typesdo not have spectral features that asteroids there are 27 meteorite groups. How do these can provide diagnostic identification. In this paper we two classifications relate to each other? will review the data and techniques for characterizing 收稿日期：2018-09-17 修回日期：2018-12-17 深空探测学报 2019年 438 meteorites and asteroids, highlight the links between the minerals and identification of probable meteorite these types, and review the problems and uncertainties analogues. In fact, a number of asteroid spectral types in determining asteroid mineralogy from visible and likely have identical mineralogies and just reflect near-infrared spectroscopy. modest variation due to the factors mentioned above. The spectral classification of asteroids has evolved However, there are a number of asteroid spectral over several decades. With limited spectral information types and meteorite mineralogies that have no diagnostic classification started with three types: S (stony), C spectral features and any mineral identification is at best (carbonaceous), and M (metallic) [5-6]. With more and ambiguous. An example of this is shown Figure 2. The better spectral data the number of types has grown from spectra of a meteorite dominated by the iron-poor 3 to the Bus-DeMeo 24 but it is not clear how many of mineral enstatite, an iron-nickel meteorite, troilite (FeS) these classes correspond to unique mineralogies. The from the same meteorite, and a primitive volatile-rich Bus-DeMeo taxonomy specifically avoided the question carbonaceous chondrite are almost identical. The only of mineralogy by confining the taxonomy to spectral real spectral features are the increase in reflectance with “features” analyzed in principle components space[1-2]. longer wavelength, a so-called “red slope” and a The first two principal components of asteroid spectra difference in brightness. In this case, lacking additional are typically the spectral slope and the strength of major information, no spectral identification can be made and asteroids that may have wildly different mineralogies mafic absorption bands [1-2,18]. In principle components may share the same spectral class. space there are few natural divisions between the major asteroid spectral classes and typically all the classes blend into each other. Complexes and subtypes tend to occupy overlapping zones in principle components space. This feature-based approach only considered the variation seen in absorption bands and spectral slopes. Some of the variation is probably not the result of mineralogy, but of differences in surface texture, degree of space weathering, other unknown effects of the space environment, and variation or error in the observational techniques used[2,7]. While all of these factors add uncertainty to mineralogical identification, in many Fig. 2 The spectra of four featureless meteorites. These meteorites have wildly different mineralogies， but similar spectra because of the cases they do not preclude reasonable interpretation of lack of strong absorptions in the visible and near-infrared 1.1 Meteorites and Bias It is useful to remember that the meteorite collection is profoundly biased and cannot represent the diversity or relative abundance of asteroids. There are a number of factors that introduce bias and I will briefly review the major factors. ① The distribution of asteroid types in the solar system. The asteroid belt appears to be zoned with volatile-rich, hydrated, carbonaceous asteroids less abundant in the inner asteroid belt and less abundant near resonances that supply material to Earth- crossing orbits in the inner solar system[8-9]. ② The process of ejection of meteoroids. Ejecting boulders Fig. 1 The Bus-DeMeo asteroid spectral taxonomy [1] from asteroids is probably a mix of collisional processes 第 5 期 丹尼尔T.布瑞特：小行星与陨石的光谱联系 439 and YORP effect spin-up. Survival from the shock about 150 years, so the recovered numbers are still quite and acceleration would bias the data toward strong modest. This makes the analysis of many meteorite types materials[10]. ③ The collisional evolution of meteoroids. and subtypes more difficult because of the statistics of There is the potential for meteoroids to collide during small number. their evolution toward Earth-crossing orbits. Again, 1.2 The Basic Principles of Meteorite and collisions and shock will select against weaker materials Asteroid Spectroscopy since these materials are more likely to fragment In all spectroscopy photons interact with the completely. ④ The process of a meteoroid’s surface mineralogy of asteroids. Reflected and emitted atmospheric entry at Earth. The process of deceleration photons carry wavelength dependent information about in the atmosphere imposes substantial stresses on asteroid mineralogy. The fundamental quantum- themeteoroid and strongly select against weak materials. mechanical interactions between photons and the Any fractures or imperfections in the structure with mineral crystal structure result in wavelength-dependent absorptions or emissions that are diagnostic of the strengths below this limit will fragment, as reflected in mineralogy. For asteroids in the visible and near the observations of bolides. ⑤ The final bias is the infrared, some of the major optically active minerals are collection of the meteorite. The collection of meteorites shown in Figure 3 and examples of meteorite spectra are is a very human process and is affected by a range of shown in Figure 4. These include olivine which has a factors that include the local laws, the geology and fauna broad absorption centered at 1 micron, pyroxene with of the fall location (meteorites are difficult to find in absorptions at 1 and 2 microns, and feldspar with a weak forests, jungles or rocky terrain), the climate (wet absorption at 1.3 microns. These absorptions are related climates rapidly degrade meteorites), population density the interaction of iron in the crystal structure of the (more people the more likely to find a meteorite), the minerals. Other absorptions are related to vibrational local weather, and a range of other factors.Meteorite fall modes excited by photons in hydroxyls (OH) and water and recovery has only been a scientific enterprise for (H2O) that is either bound in the crystal structure of Fig. 3 Spectral absorption features of major asteroidal minerals 深空探测学报 2019年 440 1.3 Complications： Space Weathering In addition to the problems with opaques and optically inactive minerals, the surfaces of all asteroids have been to a greater or lesser extent
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