1. 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

2. Context and considerations
Observations from a relatively new worker in the field 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.

3. 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.

4. 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 s – Visible-λ surveys; families; early taxonomies, meteorite spectra s – More taxonomies; detailed NIR asteroid spectral studies; VNIR surveys; OC controversy s – More taxonomies; 4 Vesta; OC controversy; NEAs and the emerging impact hazard.

5. 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.

6. 52-color survey: 1988

7. IRTF/SpeX: 2004

8. 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).

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

10. 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 Itokawa.

11. 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.

12. 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: Low-Fe surface pyroxenes Olivine-bearing (i.e., pallasites?) Spinel-bearing (i.e. CAI-rich?) Analogues to CV/CO chondrites.

13. 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.

14. 129 Antigone
Surface density ~4.4 g/cm3 based on Ostro et al. (1985).

15. 766 Moguntia
Pallasites as potential meteorite analogue?

16. 498 Tokio
Candidate for a CV/CO-type parent asteroid?
CV3 Allende

17. 347 Pariana
A potential CAI-rich asteroid, due to moderate albedos, that are not sampled in the meteorite record?

18. 516 Amherstia
Wo9-10Fs31
Calibrations suggest Amherstia has a single mafic silicate on its surface.
Spectra more similar to S-asteroids!

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

20. 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?

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

22. 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.