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Title: NIR reflectance spectroscopy: The promises and challenges for future remote study of asteroids


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. 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.

3
Disciplinary dichotomy
  • Astronomical perspective To treat asteroids as
    objects to classify according to
    non-compositional characteristics. Advant
    age Simplifies the nature of asteroids. Disadvan
    tage 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. 19
    70s Visible-? 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.

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 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

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 25143 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 1. Low-Fe
    surface pyroxenes. 2. Olivine-bearing (i.e.,
    pallasites?). 3. Spinel-bearing (i.e.
    CAI-rich?). 4. 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 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.

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 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.

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.
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