Near-Infrared Imaging Spectroscopy of 433 Eros: A Proposal for NEAR/NIS Team Membership
Jim Bell; July 7, 1994
Volume I. Investigation and Technical Plan
Abstract. This proposal defines the contributions that will be made by Jim Bell as part of the Near Earth Asteroid Rendezvous (NEAR) Near-IR Spectrograph (NIS) team. Dr. Bell has had substantial experience in the collection, reduction, calibration, and analysis of imaging spectroscopic data sets of solid surfaces, and would use this experience to help the NIS team devise the best possible data acquisition scenarios, reduction and calibration/validation procedures, and archiving techniques. Specific problems that the NIS team may expect to encounter are outlined, and proposed solutions based on the PI's previous experiences are presented. While the major thrust of this proposal concerns data collection, reduction, calibration, and validation issues, a set of focused, scientific analyses that the PI will conduct are also discussed. These analyses include identification of specific mineralogies on Eros, spatially mapping the asteroid's surface spectral variability, correlating observed NIS spectral variations with variations in morphology or chemistry observed by other instruments, relating the composition and mineralogy of the asteroid's surface to that of other asteroids, comets, and meteorites, and developing a chronology for the evolution of the asteroid's surface based on the stratigraphic implications of spectral variations.
A. Background and Description of the Investigation
This proposal defines the contributions that will be made by Jim Bell as part of the Near Earth Asteroid Rendezvous (NEAR) Near-IR Spectrograph (NIS) team. Dr. Bell has had substantial experience in the collection, reduction, calibration, and analysis of imaging spectroscopic data sets of solid surfaces, and would use this experience to help the NIS team devise the best possible data acquisition scenarios, reduction and calibration/validation procedures, and archiving techniques. Specific problems that the NIS team may expect to encounter are outlined, and proposed solutions based on the PI's previous experiences are presented. While the major thrust of this proposal concerns data collection, reduction, and validation issues, a set of focused, scientific analyses that the PI will conduct are also discussed.
2. Objectives and Significant Aspects
The goals of the NEAR mission are to determine the composition and physical properties of the near-Earth asteroid 433 Eros, and by inference to provide insights into the formation and evolution of primitive solar system bodies and the relationships between asteroids, comets, and meteorites. The NIS instrument will contribute to this goal by providing mineralogic and possibly compositional information for the asteroid's surface using spectroscopy from 0.8 to 2.6 µm. NIS is a 62 channel point spectrograph composed of two arrays covering the 0.8 to 1.6 µm region and the 1.4 to 2.6 µm region. NIS is converted into an imaging spectrograph using a scanning mirror along one axis and spacecraft roll along the other axis. The 0.8 to 2.6 µm wavelength range can allow for the detection and discrimination of a number of geologically important minerals that could be expected to exist on the surface of Eros. The recent review by Gaffey et al. (1993) summarizes the current state of knowledge of asteroid spectroscopy. Figure 1 and Table 1 (from Gaffey et al. 1993) summarize the spectra of the types of minerals NIS will most likely measure as well as the key mineralogic properties that can be derived. Except for the 3-µm hydration band and phyllosilicate fine structure, all the mineralogic properties in Table 1 will be derivable using NIS.
The objectives of the investigation proposed here can be outlined as follows:
(1) Assure the best possible NIS data collection procedures. These procedures include (a) Planning the timing of the observation sequences to maximize the amount of low phase angle global coverage and to permit the most useful sampling of the phase curves of a variety of different surface units; (b) Coordination of NIS sequences and the Multispectral Imaging System (MIS) observations. These complementary observations need to be coordinated both spatially (assure that specific regions imaged with MIS are also measured by NIS) and temporally (attempt to achieve similar phase coverage for the same regions in both NIS and MIS).
(2) Assure the best possible NIS data reduction, calibration, and validation. This goal can be achieved only if a substantial effort is made at understanding the instrument and calibration target characteristics prior to the flight and assuring that these characteristics can be monitored, documented, and understood during the mission. The PI's own experience with the French ISM instrument on the Phobos-2 spacecraft best illustrates this situation: The French/Soviet pre-flight ISM calibrations later proved to be incomplete and the instrument was eventually operated in space at a temperature that had not been studied in pre-flight. The result of this unfortunate circumstance was that the investigators eventually had to discard half the data set because it was impossible to calibrate and had to tie the calibration of the other half of the data almost entirely to groundbased observations (e.g., Murchie et al. 1991).
(3) Assure that the data are archived and distributed to the community in a timely and well-characterized fashion. The simplest and most cost-effective way to achieve this objective would be to use the existing Planetary Data System (PDS) infrastructure as well as the Internet. However, critical decisions will have to be made by the team regarding data compression, data storage formats, and data product reduction and calibration level for release.
3. Specific Talents and Technical Capabilities of Candidate Team Member
Expertise in the interpretation of reflectance spectroscopy of solid surfaces. The PI's primary experience is in the interpretation of Mars and Moon spectra; however, he also has experience in the interpretation of spectra of asteroids. For Mars, the primary issues have been interpretation of ferric- and ferrous-bearing mineralogic absorption features (e.g., Bell 1992; Bell et al. 1990a,b, 1992a) and secondary issues have been the interpretation of weaker absorption features in the near-IR possibly indicative of carbonates, sulfates, or phyllosilicates (e.g., Bell and Crisp 1993; Bell et al. 1994a). For the Moon, interpretations have concentrated on absorptions due to iron- and titanium-bearing basaltic minerals (Bell and Hawke 1994; Campbell et al. 1992). In asteroid spectroscopy, the PI's experience has been in the interpretation of enigmatic absorptions detected in the near-IR spectrum of 5145 Pholus (Cruikshank et al. 1993, 1994) and in the 3 to 4 µm spectrum of 1 Ceres (Cruikshank et al., unpublished manuscript).
Experience and expertise in the collection, reduction, and calibration of imaging spectroscopic data sets. The PI has been a leading advocate of the collection of groundbased imaging spectroscopy data sets for solar system objects. He has been eager to utilize newly available near-IR and mid-IR array detector technologies (e.g., IRTF's ProtoCAM and NSFCAM; UKIRT's CGS4; KAO's mid-IR array camera) to obtain unique new planetary data sets (e.g., Bell and Crisp 1994; Bell et al. 1990a, 1994a,b). In most cases the lack of previous observations of this type has meant that the PI has had to develop new data reduction and calibration tools for processing of the measurements (e.g., Bell et al. 1992a; Bell and Crisp 1993). Because imaging spectroscopy produces large volumes of data, most of the tools developed by the PI are highly automated in order to allow for efficient and timely data processing. While these tools are not identical to what will be needed for NIS data reduction, they are similar in outline to the types of "standard" data reduction procedures that will need to be applied.
Proficiency with programming and workstation system administration. The PI has proficiency in Fortran and of IRAF and IDL data analysis packages for reflectance and image cube data sets. The PI is the system administrator for his own Unix (Sun/SPARC 2) workstation and co-administrator of one of the Ames Space Science Division's Sun/SPARC 10 workstations. These abilities may be useful to the team in terms of developing the NIS data reduction, calibration, and analysis software and interacting with instrument developers and flight controllers using whatever networking and/or workstation environment that the mission eventually chooses.
Experience in working with a team in the successful planning and collection of imaging and spectroscopic data sets. Over the past 8 years the PI has been involved with numerous spectroscopy projects utilizing laboratory and telescopic spectrophotometers, InSb and PbS line arrays, and imaging near-IR and CCD cameras. He has more than 50 nights observing experience on 0.6 to 3.8 m telescopes at Mauna Kea, Wyoming, Lick, Lowell, and Pic du Midi Observatories. All of these efforts have involved teams of people assembled to achieve a common goal: collect the best possible data and ensure its timely and accurate reduction and calibration. The PI was the leader of many of these teams and his subsequent publications record testifies to the success of these efforts and the ability of the PI to work with and forge links among and between astronomers, geologists, spectroscopists, theorists, and instrumentalists. Because the NEAR science teams will be small, the ability to work well with persons of widely varying interests and abilities will be a critical requirement for NIS team members.
4. Investigation Approach
a. ConceptThe primary aim of the PI's involvement in the NIS team will be to assist the Team Leader (TL) and other team members in the following tasks:
(1) Define data acquisition sequences based on a prioritized set of science goals determined in advance by the team
(2) Perform instrumental corrections on the raw data using pre-flight and in-flight calibration information
(3) Calibrate the reduced data into standard flux and/or reflectance units and assemble the individual spectra into spatially contiguous 3-D image cubes
(4) Distribute and archive the calibrated data to the scientific community and general public
(5) Perform the initial scientific analysis of the NIS spectra and communicate the initial results to the scientific community and general public
b. Detailed Methods and ProceduresEach of the tasks listed above is now explained in detail:
(1) Define Data Acquisition Sequences. It will be the responsibility of the NIS team to insure that spectroscopic observations are obtained at appropriate times during the mission. For the mission as currently defined, the most critical time period for NIS measurements will be the approach and initial flyby (days 1 to 39), when the phase angle of the observations is low and thus the effects of shadowing are minimized. These observations will allow for maximum surface area coverage as well as for the measurement and discrimination of highly phase-dependent units such as crater rays and steep slopes.
During the rest of the mission, when MIS is obtaining images optimized for morphologic mapping or XGRS is obtaining close-pass compositional information, it will be possible to use NIS for targeting of specific morphologic features or units. These observations will have albedo and spectral slope artifacts introduced because of phase angle variations; however absorption features should still be detectable. By comparison with the low phase angle NIS and (possibly) MIS measurements of the same regions and by collecting additional data of these regions at a variety of phase angles, the phase curves (as a function of wavelength) of these materials can be determined. This information can be used in several important ways: first, knowledge of the phase behavior of surface materials can allow for "correction" of the albedo and spectral slope of materials so that accurate band center positions and depths can be derived from higher-phase measurements. And second, knowledge of the phase behavior can allow, though modeling (e.g., Hapke 1981; 1986), estimates of surface physical parameters such as effective particle size, macroscopic slope, and porosity.
(2) Perform Instrumental Corrections. The raw data delivered to the team will need to have several instrumental corrections applied. These include removal of an offset voltage and possibly also of a dark current, correction for detector-to-detector response nonuniformities (flatfielding), and possibly also linearization of the data values if they are not saturated but fall outside of the detectors' nominal linearity range. An additional correction which will need to be applied is merging of the two spectral segments from 0.8 to 1.6 µm and 1.4 to 2.6 µm, using the 1.4 to 1.6 µm overlap region as a guide. All of these corrections can be applied using automated software; in fact, because of the large data volume expected (nominal estimates range from 5400 to 86000 spectra per day) these corrections must be done automatically given the limited personnel and fiscal resources available for the mission.
The PI has substantial experience with the reduction of computationally large imaging and spectroscopic data sets such as the one expected from NIS (e.g., Bell 1992, Bell et al. 1991; 1992a; 1994a; Bell and Crisp 1993). He has developed a set of data reduction and analysis tools (written in Fortran, currently being converted to IDL format) that perform the instrumental correction procedures discussed above on telescopic imaging and spectroscopic data sets. These tools, and others available from other researchers using IDL, IRAF, ISIS/PICS, or other data reduction packages, can easily be modified to accommodate the expected format of NIS spectra.
It has been the PI's experience that the successful application of automated data reduction routines requires a thorough knowledge of the instrumental behavior over (and beyond) the expected range of operating conditions. Thus, it will be critical to incorporate spacecraft engineering information into the data reduction procedure. Examples include (a) Temperature, because the flatfield response of the detectors will vary with temperature; (b) Spacecraft orientation relative to Eros and the Sun, because the effects of scattered light from off-axis scattering will need to be understood and removed from the spectra (the PI was involved with the characterization of scattered Mars light during ISM Phobos observations; Murchie et al. 1991); and (c) Surface coordinates of each measured spectral track, so that the phase angle of the observations can be determined and so that the spectra can be properly mosaicked into an image cube (spatial¥ spatial ¥ spectral) and directly compared to MIS morphology images.
(3) Calibrate Reduced Data and Assemble Image Cubes. Spectra that have had instrumental corrections applied need to undergo a well-documented absolute calibration into flux units so that parameters such as radiance factor and albedo can be derived. As a NIS TM, the PI will press to have this calibration performed in a number of different, independent ways. Possibilities include observations of a well-characterized calibration target on the spacecraft or within the instrument, observations of standard stars during the mission, observations of standard lunar regions during the Earth swingby maneuver, or comparison to simultaneously-obtained groundbased full-disk spectrophotometric observations of the asteroid. These possibilities are discussed in more detail in Section B below.
Again, critical to this calibration exercise will be a detailed knowledge of the instrumental response as a function of time and environmental conditions, and also of the behavior of any calibration targets used as a function of exposure time to the space environment. It will be the responsibility of the team to assure that as much of this knowledge as possible is secured prior to launch, and that efforts are made to characterize the calibration stability during the mission.
Assembly of the individual spectra into 3-D image cube data sets will be a particularly onerous task for the team, based on the PI's past experience with similar telescopic and spacecraft data sets. NIS will build up one spatial axis of information using an internal scan mirror, and the second axis will be collected using spacecraft rotation orthogonal to the scan mirror direction. This data collection technique will cause numerous problems when the spectra are assembled into image cubes: First, spacecraft motion during the scanning of the internal mirror along one spatial axis will cause the spatial location of the spectra to drift at an angle across the surface (Figure 2a). Second, rotation of the spacecraft to build up the second spatial axis will unavoidably result in small gaps in spatial coverage because of irregularities in the shape of Eros or small errors in the pointing capabilities of the spacecraft (Figure 2b). The result will most likely be a "patchy" image cube similar to what was obtained by the Galileo NIMS instrument for the Moon (McCord et al. 1994). This situation can be alleviated somewhat by a careful choice of spatial over-sampling in either the mirror scan rate or spacecraft rotation rate. The best image cube assembly technique will be to use the engineering information to determine the inertial position of each spectrum, and then to combine that information with shape model data derived from MIS images or (possibly) LIDAR data to determine the effective latitude and longitude of the spectrum. Spectra thus mapped can be placed onto a standard projection map of Eros and overlapping pixels from observing sequences close in time can either be coadded or averaged to increase signal to noise. The PI's experience indicates that this procedure will be software and personnel intensive, and is thus likely to be the last step performed prior to release of the final data set.
(4) Distribute and Archive Calibrated Data. Individual NIS spectra and calibrated NIS image cubes should be made available to the scientific community and general public as soon as possible and at various levels of processing. This can best be achieved in two ways: (1) Maintaining a publicly-accessible Internet archive of the currently best available version of the data in a standardized, easily accessible format (such as tables of ASCII spectra or FITS/GIF images). A similar procedure was used during the Clementine mission for a subset of their data, and proved to be an important educational and public relations success in that many people truly felt as if they were a part of the mission as it went on. (2) Assuring that the final, calibrated data set and detailed calibration and engineering information records are permanently stored in the NASA PDS and NSSDC archives in standard formats.
The PI has experience in all these areas, as he has been an extensive user of PDS and NSSDC data sets and is familiar with the various format options available. Also, he has led the effort in establishing an Internet communications system among amateur and professional Mars researchers that will shortly include a World-Wide Web (WWW) Internet site for the archiving and distribution of Mars images and spectra. A similar procedure could be used to make NIS (and other) data from NEAR accessible to the public as well as to interested researchers.
(5) Initial Scientific Analyses and Communication of Results. It will be the responsibility of team members to assist the team leader in the initial scientific validation and analyses of the NIS data. In this regard, the PI would contribute in two specific ways: First, he has had much experience in the analysis and interpretation of near-IR spectra of solid surfaces in the solar system. This experience has primarily been for Mars and the Moon, but he has also been involved in projects dealing with asteroids and other small bodies (e.g., Bell and McCord 1991; Murchie et al. 1991; Bell et al. 1992b; Cruikshank et al. 1993; 1994) and is thus familiar with the types of spectra expected to be seen at Eros. Second, the PI has specific research interests of his own that he would seek to pursue as a member of the NIS team. These research areas include (a) Using innovative techniques such as image-oriented spectral mixture modeling (Adams et al. 1993; Bell and Hawke 1994) to maximize the amount of scientific return from the NIS data set; (b) Specific spectral/geologic questions such as mapping the degree of crustal excavation by impact craters on Eros and the distribution of impact crater ejecta types across the surface (e.g., Bell and Hawke 1991; Campbell et al. 1992); and (c) Quantifying the degree of spectral heterogeneity of the surface of Eros and making comparisons to other small bodies, such as Gaspra, Ida, and Phobos. More details of these proposed analyses are discussed below.
B. Instrument Calibration Requirements
1. Instrument Calibration Options
Calibration of the returned NIS spectra represents the single most important task to be performed by the NIS team. As such, several different and independent calibration techniques should be devised so that the calibration can be verified and systematic errors in the various calibration techniques can be estimated.
As part of the NIS team, the PI would advocate such a multiple-method calibration technique and assist the team leader in designing and implementing it. The most feasible and realistic calibration techniques include: (a) observations of a well-characterized calibration target on the spacecraft or within the instrument; (b) observations of standard stars during the mission; (c) observations of standard lunar regions during the Earth swingby maneuver; and (d) comparisons between NIS spectra and simultaneously-obtained groundbased spectrophotometric observations. Of these, the most reliable technique would be to use one or more calibration targets on the spacecraft. These targets would be composed of highly reflective, standardized, well-characterized material like Spectralon which could be positioned (either internally or externally) so as to reflect sunlight back to the instrument. Spectralon is an example of a polytetrafluoroethylene compound that has been well characterized chemically as well as spectrally from the near-UV through the near-IR (e.g., Stiegman et al. 1993; Bruegge et al. 1993), and is being developed for use on a number of NASA Earth-orbiting satellite spectroscopy missions, including MISR, MODIS, and SeaWiFS. While the AO did not provide enough information about the design of NIS and NEAR to determine specific placement options for calibration targets, it is hoped that there will be enough flexibility in the spacecraft and instrument design to accomodate such targets.
2. Ground Calibration Operations
Primary ground calibration tasks that the PI would participate in would be:
(1) Instrument Characterization. This involves the thorough and detailed testing of the instrument under the widest possible range of environmental and operational conditions. It also involves the measurement in the lab of a variety of materials and mixtures of materials using the instrument prior to flight so that the team develops an understanding of the expected limits of the data. Critical parameters to establish include: (a) instrument S/N; (b) stability of flatfield and dark current responses as a function of temperature and signal; (c) degree of off-axis scattering; (d) detectability of different minerals, rocks, and other substances given the specific spectral resolution, spectral sampling, and spatial resolution of NIS.
(2) Calibration Target Selection and Characterization. If it is feasible to place a calibration target onboard the spacecraft or to incorporate it within NIS, then the material chosen as the target must be well characterized and its calibration stability as a function of space exposure age must be well understood. As mentioned above, the best choice of materials would probably be the diffuse reflectance standard Spectralon, but this material must be properly "baked" and otherwise space-hardened to guarantee optimum performance (e.g., Bruegge et al. 1993). In addition, because Spectralon has such a high reflectance and the albedo of Eros is only Å 0.2, the team should consider using a variety of "gray" reflectance standards composed of darkened Spectralon. This will allow for maximum S/N in simultaneous observations of Eros and the calibration target, should such observations be feasible (otherwise, observations optimized for Eros would saturate the calibration target, and observations optimized for the bright calibration target would not allow the maximum possible DN range for the Eros data).
(3) Alternate Calibration Techniques Selection and Characterization. Three possible alternate calibration techniques were mentioned above: observations of standard stars during the mission, observations of standard lunar regions during the Earth swingby maneuver, or comparisons between NIS spectra and simultaneously-obtained groundbased spectrophotometric observations. Depending on the final mission profile and instrumental characteristics, the first two of these possibilities may or may not be possible or feasible. It is assumed that the spacecraft will have accurate pointing capabilities, and thus the acquisition of bright standard stars may be possible. If so, then solar type and infrared standard stars could be observed and compared to groundbased and/or theoretical expectations of the stars' spectra to serve as a bootstrap calibration technique. If observations of the Moon are possible during the Earth swingby, then NIS spectra obtained in standard mare regions (e.g., Mare Serenitatis, Mare Humorum) or of the Apollo landing sites could be used again as a way to bootstrap the calibration by relating the NIS observations to absolute reflectance using spectra of returned Apollo lunar samples. This technique works surprisingly well for lunar telescopic observations, and it was used extensively as a calibration aid for the Galileo lunar NIMS spectra and SSI multispectral images (e.g., Belton et al. 1992; McCord et al. 1994). The third possibility, simultaneous groundbased and NIS observations during those times when the asteroid is viewable from Earth, would provide an important confirmation of calibration results obtained by other methods. These observations would need to be coordinated with the mission profile to insure simultaneous temporal coverage. An alternate possibility would be to have NIS and groundbased observers simultaneously measure standard stars (or other bright point sources) at different times during the mission in order to monitor the calibration stability. In the end, as many of these alternate calibration techniques as is possible should be attempted in order to most fully understand the calibration uncertainties inherent in the NIS spectra.
(4) Data Reduction and Calibration: Software Development. The data reduction and calibration software must be fully tested and documented well before launch. This can best be achieved using actual NIS spectra obtained during pre-flight testing operations. As in the other procedures discussed above, the software must be rigorously tested under the widest possible range of mission scenarios. This is especially critical if any of the software will be stored onboard the spacecraft. Examples of onboard procedures might include bias removal or data compression. In these cases, critical systems testing must be performed to insure that the software functions properly after integration with other flight systems and the main onboard CPU.
3. Flight Calibration Operations
Primary calibration tasks that the PI would participate in during the mission would be:
(1) Application and Refinement of Automated Data Collection and Reduction Routines. Once data begin to flow to the NIS team, they will need to be piped through a standard set of reduction and calibration tools developed in advance (see above). The PI proposes to take a very active role in this process which will, at least initially, need to be supervised in order to accommodate any unexpected contingencies or to modify the procedures to take advantage of unexpected opportunities.
(2) Absolute Flux Calibration of the Spectra. Once primary and alternate calibration schemes (discussed above) have been identified by the team, the PI proposes to incorporate these schemes into the tail end of the automated data reduction algorithms. Thus, if all proceeds nominally, the conversion from instrumental voltage or DN to absolute units like W/cm2/µm or radiance factor should be a one-step process. For example, if a calibration target is measured occasionally, then a simple conversion of DN to flux can be achieved using the calibration target observation closest in time to the Eros observation being calibrated. A similar technique would be used if standard stars are the calibration source.
(3) Data Archiving and Distribution. Data streaming to the team will need to be intelligently archived and indexed in order to keep track of progress on mission goals and in order to be able to communicate and share results with other researchers and the public as efficiently as possible. Possible archiving techniques include magnetic tape, CD-WORM, and large capacity hard disk. Using estimates provided in the AO for daily NIS downlink allocations from the DSN, a reasonable maximum expectation would be 800 dabps, or Å 86400 spectra per day. This corresponds to roughly 20 Mbytes of NIS data per day in this example, or just under 1 Gbyte of NIS data if this example situation existed throughout the entire approach and initial flyby phase. While this is a large data volume, it is not beyond the capacity of current hard disk technology, which will certainly be far more advanced by 1999. Thus, it is likely that much of or at least large portions of the NIS data set could be archived "on line" on the Science Data Server in various stages of reduction for access by the community or public at large. The Internet would provide the simplest data distribution technique, and a WWW home page site could easily be established to provide examples of the most interesting subset of the NIS spectra to interested persons. For more permanent and formal data archiving and distribution, the final NIS data set would be converted to standard PDS data record format and sent to the PDS and NSSDC for transferal to CDROM or other appropriate media.
C. Initial Scientific Analyses
Like other missions, it is anticipated that initial scientific analyses of these data will be reported in the literature in a timely fashion. Because of the PI's background in the interpretation of reflectance spectra of solid surfaces, he would imagine taking a direct role in the initial scientific analyses of the NIS spectra. These analyses would include:
(a) Identification of specific mineralogies on the surface of Eros. Specific examples include spectral searches for pyroxene and determination of pyroxene chemistry based on positions and relative strengths of the 1- and 2-µm absorption features (e.g., Adams 1974); searches for olivine and determination of olivine Fe/Mg content based on absorption features in the 0.9 to 1.5 µm region (e.g., Burns 1970); searches for and identification of hydrated silicate minerals (or ice) based either on the presence of specific absorptions characteristic of OH- itself near 1.4 and 1.9 µm or of bands characteristic of cationOH absorption from 2.2-2.4 µm (e.g., Bell et al. 1994a).
(b) Spatially mapping the spectral heterogeneity of the asteroid's surface. This will be achieved using proven techniques like ratio images, band depth maps, and principal components analysis (e.g., Bell 1992; Bell and Crisp 1993) in order to show the spatially contiguous structure of spectral parameters. Also, newer techniques like image-oriented spectral mixture modeling (Adams et al. 1993) will be used to try to better separate instrumental from asteroidal effects in the data. Mixing models also allow for a logical and methodical determination of the causes of spectral variance within an imaging data set, and the PI's experience is that they also provide an elegant and intuitive (imaging) way to search for unforseen absorption features using the model residuals.
(c) Correlating observed NIS spectral variations with variations in morphology or chemistry observed by other instruments. This will be achieved by effectively using the MIS images as a visible wavelength image cube that effectively extends the NIS spectral coverage. Enigmatic units measured by MIS will be targeted for specific examination by NIS (and vice versa). A similar procedure will be followed for the spatially-resolved XGRS measurements.
(d) Using the above investigations to help to team leader relate the observed composition and mineralogy of Eros' surface to that of other S class asteroids, comets, and meteorites.
(e) Finally, assisting the team in developing a chronology for the evolution of the asteroid's regolith (if any) based on the observed surface and (hopefully through spectral analyses of impact craters and their ejecta blankets) subsurface composition.
More detailed analysis studies would be proposed by the PI as part of a possible post-mission NEAR Data Analysis Program.
D. Data Reduction and Analysis Requirements
(1) Hardware. All of the data reduction and analysis procedures proposed above will be conducted on Unix workstations (e.g., Sun/SPARC or their late 1990s equivalent). The PI has substantial experience with the Unix operating system and with System Administration procedures and thus will not require additional support in this regard. It is anticipated that most of the interactions between team members during the Science Support phase will involve software development and mission sequencing tasks. Much of this interaction can take place over the Internet using state of the art networking and remote task management tools (such a system was successfully established for the Mars Observer mission prior to the loss of the spacecraft). During the Mission Operations/Data Analysis and Reduction Phase of the mission, the Science Support workstations should be upgraded to the fastest available in order to most efficiently process the large volume of expected data. Also during that time, additional hard disk and data archiving peripherals will be needed to store the data products at various levels of reduction (each level of data reduction saved will double the needed data storage volume).
(2) Software. It is expected that a large fraction of the PI's effort during the Science Support Phase will involve development and testing of data reduction and analysis software. Two main options exist for the software development: (1) use a reliable, well-known data reduction/analysis package, or (2) develop a unique software package specific to NIS applications. For the first option, it would be possible to adapt any of a number of standard packages to automate the NIS data reduction stream. Examples of possible packages include IDL (commercial), IRAF (NOAO), or PICS/ISIS (USGS). For the second option, the PI's experience is that development of unique NIS data reduction routines would be straightforward using Fortran or C programs (this has been the path followed by the PI for his own imaging spectroscopy data reduction and analysis research). Both options probably involve comparable amounts of time and effort (the packages would need to be programmed to handle NIS data). Option (1) may involve a steeper initial learning curve for the team depending upon which package is chosen but it may allow easier interactions among teams if one standard package is chosen for the mission as a whole; option (2) may be "safer" in that it does not rely on a commercial or government package that may change with time. This decision will need to be made as early as possible by the Team Leader and PSG.
E. References Cited
Adams J.B. (1974) Visible and near-infrared diffuse reflectance spectra of pyroxenes as applied to remote sensing of solid objects in the solar system. J. Geophys. Res., 79, 4829-4836.
Adams J.B., Smith M.O., and Gillespie A.R. (1993) Imaging spectroscopy: Interpretation based on spectral mixture analysis. In Remote Geochemical Analysis: Elemental and Mineralogical Composition , ed. C. Pieters and P. Englert, pp. 145-166. Cambridge: Cambridge Univ. Press.
Bell III, J.F. (1992) Charge-Coupled Device Imaging Spectroscopy of Mars. 2. Results and Implications for Martian Ferric Mineralogy, Icarus, 100, 575-597.
Bell III, J.F. and D. Crisp (1993) Groundbased Imaging Spectroscopy of Mars in the Near-Infrared: Preliminary Results, Icarus, 104, 2-19.
Bell, J.F., III, and B.R. Hawke (1991) CCD Narrowband Filter Imaging of Lunar Crater Rays, Lunar and Planetary Science XXII, 75-76.
Bell III, J.F. and B.R. Hawke (1994) Compositional variability of the Serenitatis/Tranquillitatis region of the Moon from telescopic multispectral imaging and spectroscopy, submitted to Icarus.
Bell III, J.F. and T.B. McCord (1991) A search for spectral units on the Uranian satellites using color ratio images, Proc. Lunar Planet. Sci. XXI, 473-489.
Bell III, J.F., T.B. McCord, and P.G. Lucey (1990a) Imaging Spectroscopy of Mars (0.4-1.1 µm) During the 1988 Opposition, Proc. Lunar Planet. Sci. Conf. XX, 479-486.
Bell III, J.F., T.B. McCord, and P.D. Owensby (1990b) Observational Evidence of crystalline iron oxides on Mars, J. Geophys. Res., 95, 14447-14461.
Bell III, J.F., D. Crisp, P.G. Lucey, T.A. Ozoroski, W.M. Sinton, S.C. Willis, and B.A. Campbell (1991) Spectroscopic Observations of Bright and Dark Emission Features on the Night Side of Venus, Science, 252, 1293-1296.
Bell III, J.F., P.G. Lucey, and T.B. McCord (1992a) Charge-Coupled Device Imaging Spectroscopy of Mars. 1. Instrumentation and Data Reduction/Analysis Procedures, Experimental Astronomy, 2, 287-306.
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