NIR reflectance spectroscopy: The promises and challenges for future remote study of asteroids Dr. Paul S. Hardersen University of North Dakota GSA Fall meeting 2006, Philadelphia G.K. Gilbert Award Session Tuesday, October 24, 2006

Context and considerations Observations from a relatively new worker in the field. 1. Asteroid studies are inherently multi-disciplinary: -- Mineralogy (geology). -- Partial/complete melts (geology). -- Analogue materials (meteoritics). -- Colors and classes (astronomy). -- Orbital characteristics (dynamics). -- Collisional properties/impacts (geophysics). Goal: Combine data from all fields into a comprehensive understanding of asteroids.

Disciplinary dichotomy Astronomical perspective: To treat asteroids as objects to classify according to non-compositional characteristics. Advantage: Simplifies the nature of asteroids. Disadvantage: Simplifies the nature of asteroids. Geologic perspective: To treat asteroids as complex, geologic bodies that evolve based on their original compositions, changing thermal regimes, and collisional histories. Consider with meteorites as a way to constrain early solar system nebular and thermal environment.

The past: ~1970 to 2000 Theory – Developed by Roger Burns. Explains near-infrared (NIR) absorption features via quantum mechanics and crystal field theory. Practice – NIR spectroscopy lagged behind astrometric, photometric research. 1970s – Visible-λ surveys; families; early taxonomies, meteorite spectra. 1980s – More taxonomies; detailed NIR asteroid spectral studies; VNIR surveys; OC controversy. 1990s – More taxonomies; 4 Vesta; OC controversy; NEAs and the emerging impact hazard.

The recent past: 2000 to 2006 Instrumentation: IRTF SpeX spectrograph offers dramatic improvements in sensitivity and quality. Provides low-resolution observations necessary for asteroid NIR spectral studies.

52-color survey: 1988

IRTF/SpeX: 2004

However… The quality of a given data set is dependent upon the quality of the observations and the data reduction protocols. From Sasaki et al. (2004).

Spectral calibrations Currently, three types of interpretational methodologies: 1. Taxonomies. 2. Spectral curve matching. 3. Quantitative mineralogical analysis. a. Pyroxene group – best calibrated b. Olivine group – less constrained c. Spinel group – improved constraints

Near-Earth Asteroids (NEA) studies Physical characterizations for impact assessment. Continual discovery of binary NEAs and MBAs. NEAR-Shoemaker mission to 433 Eros. JAXA mission to 25143 Itokawa.

Ordinary chondrites/S-asteroids Ordinary chondrites – Most common meteorite type in terrestrial collection. S-asteroids – “Silicaceous” asteroid taxonomic group. OC = S-asteroid  but with spectral irregularities. Primitive asteroid belt? Thermally evolved asteroid belt? Survey S-asteroids to identify OC candidates.

M-asteroids Canonical knowledge: M-asteroids exhibit featureless spectra in the VNIR spectral region (pre-2000). Emerging knowledge: IRTF/SpeX observations allow detections of NIR absorption features as weak as ~1%. M-asteroids emerging as a mineralogically diverse group of objects: 1. Low-Fe surface pyroxenes. 2. Olivine-bearing (i.e., pallasites?). 3. Spinel-bearing (i.e. CAI-rich?). 4. Analogues to CV/CO chondrites.

16 Psyche Surface density estimate of ~3.75 g/cm3 (from Ostro et al. 1985). Spectral and radar data suggest a mixture of metal, pyroxene – with significant porosity.

129 Antigone Surface density ~4.4 g/cm3 based on Ostro et al. (1985).

766 Moguntia Pallasites as potential meteorite analogue?

498 Tokio Candidate for a CV/CO-type parent asteroid? CV3 Allende

347 Pariana A potential CAI-rich asteroid, due to moderate albedos, that are not sampled in the meteorite record?

516 Amherstia Wo9-10Fs31 Calibrations suggest Amherstia has a single mafic silicate on its surface. Spectra more similar to S-asteroids!

Complementary research Complementary research that aids NIR asteroid interpretations: 1. Radar studies (Arecibo): Can derive asteroid surface radar albedo and likelihood of significant surface metal content (i.e., 16 Psyche). 2. 3 μm spectral studies: Can *potentially* identify phyllosilicate minerals. -- Difficult spectral region to observe. -- Features are not diagnostic.

Applications – Thermal history What was the cause of the early solar system heating event? 26Al radionuclides? T Tauri induction heating? Can we determine the heating pattern from NIR spectra?

Future needs More spectral calibrations will improve ability to interpret asteroid NIR spectra: 1. Clinopyroxene/Orthopyroxene/Olivine mixtures. How does varying combinations affect spectral properties? 2. Refine and improve the olivine calibration. 3. Realistic space weathering research. 4. Method to diagnostically identify phyllosilicates spectrally.

Conclusions Asteroid research still a relatively small field. More students needed! Significant discoveries likely in NIR spectral research. Collaborations between meteoritics and spectroscopy community a must. Acknowledgements: NASA Planetary Astronomy Program.