3-μm SPECTROSCOPY OF WATER-RICH METEORITES AND ASTEROIDS:NEW RESULTS AND IMPLICATIONS
D. Takir1, J. P. Emery2, C.A., Hibbitts3, 1SETI Institute, Mountain View, CA 94043, USA;
2
University of Tennessee, Knoxville, TN 37996, USA; 3JHU Applied Physics Laboratory,Laurel, MD 20723-6005, USA.
Introduction: Ground-based observations of water-rich asteroids are important toconstrain many questions related to the abundance and distribution of volatiles in the earlySolar System, and to the evolution of many diverse Solar System bodies. Recent studies
have revealed that several airless bodies, including the Moon and asteroid Psyche ([1], [2],[3], [4]), show spectral indications of hydration on their surfaces. Absorption features at∼3.0 µm are particularly indicative of aqueous alteration. These absorptions are likely due
to hydroxyl and/or water- bearing materials (OH/H2O) (e.g., [5], [6]), but could also be due
to surficial OH implanted from solar wind or exogenic sources like those seen on Vesta[7]. We are observing and studying 60 new water-rich asteroids in the 3-µm region using
NASA Infrared Telescope Facility (IRTF) and Gemini North telescopes, in addition to the50 water-rich asteroids previously observed by [6] and [8]. We are also studying additional
carbonaceous chondrite meteroites (e.g., CI, CM, CO, CV, CR, CH, and CB) by measuring
their 3-µm spectra under asteroid-like conditions to determine their mineralogical andspectral indicators. Applying these spectral analyses of carbonaceous chondrites to waterrich asteroids in the 3-µm region has been challenging because chondrite spectra havegenerally been acquired in ambient terrestrial environments, and hence are contaminatedby atmospheric water. In this work, however, chondrite IR reflectance spectra are measured
under asteroid-like conditions (i.e., vacuum and elevated temperatures) to eliminate theadsorbed water that affected previous analyses. This investigation is important for
matching the observed water-rich asteroids to specific chondritic groups for better
understanding of the origin and evolution of our Solar System. Results from this work will
also help analyze and characterize the returned carbonaceous samples from asteroids Bennu (OSIRIS-REx’s target) and Ryugu (Hayabusa2’s target), putting these returned samples
into a wider perspective and broader Solar System context.
Methodology: Ground-based spectra of water-rich asteroids were measured using the longwavelength cross- dispersed (LXD: 1.9-4.2 µm) mode of the SpeX spectrograph/imager at
the NASA IRTF and the cross-dispersed mode of the Near InfraRed Spectrograph (GNIRS)
spectrometer at Gemini North, following the methodology of [6]. New 3-µm bi-directionalreflectance spectra (incidence = 15o, emission = 45o, phase angle = 60o) of carbonaceouschondrites (CVs, COs, and CIs) have been collected at the Johns Hopkins University
Applied Physics Laboratory (JHU APL) under vacuum-desiccated conditions, following
the methodology used in [9].
Results: [8] found that CM (and CI) chondrites are possibly the meteorite analogs forwater-rich asteroids with the sharp 3-µm band, attributed to phyllosilicates. The sharpspectral group contains asteroids that are located in the 2.5 < a < 3.3 AU region. No
meteorite match was found by [8] either for the rounded group, Ceres-like group, orasteroid Europa-like group. These three spectral groups are located farther from the Sun(3.0 < a < 4.0 AU). Here we present new 3-µm spectra of water-rich asteroids, including
212 Medea, 690 Wratislavia, 372 Palma, 259 Aletheia, 114 Kassandra, 701 Oriola, 360
Carlova, 747 Winchester, 386 Siegena, 356 Liguria 233 Asterope, 135 Hertha, 87 Sylvia,
and 142 Polana. Asteroid Polana, which is the main asteroid in the New Polana Family[10], is thought to be the probable source of primitive near-Earth asteroids including Bennu
and Ryugu [11]. The new 3-µm spectra are grouped according to the classification schemeof [6]. We will also present new 3-µm spectra of carbonaceous chondrites, which will be
used to interpret ground-based, and eventually space-based, spectra of water-rich asteroids.
We will then compare the new spectra of meteorites and asteroids to determine new
possible matches on the basis of the 3-µm band.
References: [1] Clark (2009) Science 5952:552. [2] Pieters et al. (2009) Science 5952:568.
[3] Sunshine et al. (2009) 5952:565. [4] Takir et al. (2017) ApJ 153:1. [5] Rivkin et al.
(2002) In Univ. Of Arizona Asteroids III 235- 253. [6] Takir and Emery (2012) Icarus
219:641-654. [7] Reddy et al. (2012) Icarus 221:544-559. [8] Takir et al. (2015) 257:185193. [9] Takir et al. (2013) 48:1618-1637. [10] Walsh et al. (2013) Icarus 225:283-297.
[11] Bottke et al. (2015) Icarus 247:191-217.