TY - JOUR
T1 - Infrared Spectroscopy of Large, Low-Albedo Asteroids
T2 - Are Ceres and Themis Archetypes or Outliers?
AU - Rivkin, Andrew S.
AU - Howell, Ellen S.
AU - Emery, Joshua P.
N1 - Funding Information:
A. S. R. acknowledges long-standing support from the NASA Planetary Astronomy program over the time frame that these observations and analyses have been done, including Grants NNX14AJ39G, NNX09AB45G, NNG05GR60G, and NAG5-10604. This work, and companion papers past and future, would not be possible without such steady support. Grant SOF 04-0050 from SOFIA has also provided partial support for this work for A. S. R. J. P. E. acknowledges SSO Grant NNX16AE91G and generous support of the L.A. Taylor Endowment. The raw telescopic data that were used in this paper are held at the Caltech/IPAC Infrared Science Archive and publicly available as of June 2019. The reduced spectra used in the figures are available at the Johns Hopkins Applied Physics Laboratory Data Archive at http://lib.jhuapl.edu/papers/infrared-spectroscopy-of-large-low-albedo-asteroid/ and are being submitted to the Planetary Data System Small Bodies Node. All authors are Visiting Astronomers at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration. We acknowledge the sacred nature of Maunakea to many Hawaiians and our status as guests who have been privileged to work there. Many thanks to the stalwart telescope operators of the IRTF who were instrumental in taking these data through the years and to Bobby Bus and Eric Volquardsen for developing “the ATRAN part” of the data reduction. Thanks to Driss Takir for sharing his spectra with us for analysis. Thanks to Beth Clark and an anonymous reviewer who outed himself as Ralph Milliken for helpful reviews.
Funding Information:
A. S. R. acknowledges long‐standing support from the NASA Planetary Astronomy program over the time frame that these observations and analyses have been done, including Grants NNX14AJ39G, NNX09AB45G, NNG05GR60G, and NAG5‐10604. This work, and companion papers past and future, would not be possible without such steady support. Grant SOF 04‐0050 from SOFIA has also provided partial support for this work for A. S. R. J. P. E. acknowledges SSO Grant NNX16AE91G and generous support of the L.A. Taylor Endowment. The raw telescopic data that were used in this paper are held at the Caltech/IPAC Infrared Science Archive and publicly available as of June 2019. The reduced spectra used in the figures are available at the Johns Hopkins Applied Physics Laboratory Data Archive at http://lib. jhuapl.edu/papers/infrared‐ spectroscopy‐of‐large‐low‐albedo‐ asteroid/ and are being submitted to the Planetary Data System Small Bodies Node. All authors are Visiting Astronomers at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration. We acknowledge the sacred nature of Maunakea to many Hawaiians and our status as guests who have been privileged to work there. Many thanks to the stalwart telescope operators of the IRTF who were instrumental in taking these data through the years and to Bobby Bus and Eric Volquardsen for developing “the ATRAN part” of the data reduction. Thanks to Driss Takir for sharing his spectra with us for analysis. Thanks to Beth Clark and an anonymous reviewer who outed himself as Ralph Milliken for helpful reviews.
Publisher Copyright:
©2019. American Geophysical Union. All Rights Reserved.
PY - 2019/5
Y1 - 2019/5
N2 - Low-albedo, hydrated objects dominate the list of the largest asteroids. These objects have varied spectral shapes in the 3-μm region, where diagnostic absorptions due to volatile species are found. Dawn's visit to Ceres has extended the view shaped by ground-based observing and shown that world to be a complex one, potentially still experiencing geological activity. We present 33 observations from 2.2 to 4.0 μm of eight large (D > 200 km) asteroids from the C spectral complex, with spectra inconsistent with the hydrated minerals we see in meteorites. We characterize their absorption band characteristics via polynomial and Gaussian fits to test their spectral similarity to Ceres, the asteroid 24 Themis (thought to be covered in ice frost), and the asteroid 51 Nemausa (spectrally similar to the CM meteorites). We confirm most of the observations are inconsistent with what is seen in meteorites and require additional absorbers. We find clusters in band centers that correspond to Ceres- and Themis-like spectra, but no hiatus in the distribution suitable for use to simply distinguish between them. We also find a range of band centers in the spectra that approaches what is seen on Comet 67P. Finally, variation is seen between observations for some objects, with the variation on 324 Bamberga consistent with hemispheric-level difference in composition. Given the ubiquity of objects with 3-μm spectra unlike what we see in meteorites, and the similarity of those spectra to the published spectra of Ceres and Themis, these objects appear much more to be archetypes than outliers.
AB - Low-albedo, hydrated objects dominate the list of the largest asteroids. These objects have varied spectral shapes in the 3-μm region, where diagnostic absorptions due to volatile species are found. Dawn's visit to Ceres has extended the view shaped by ground-based observing and shown that world to be a complex one, potentially still experiencing geological activity. We present 33 observations from 2.2 to 4.0 μm of eight large (D > 200 km) asteroids from the C spectral complex, with spectra inconsistent with the hydrated minerals we see in meteorites. We characterize their absorption band characteristics via polynomial and Gaussian fits to test their spectral similarity to Ceres, the asteroid 24 Themis (thought to be covered in ice frost), and the asteroid 51 Nemausa (spectrally similar to the CM meteorites). We confirm most of the observations are inconsistent with what is seen in meteorites and require additional absorbers. We find clusters in band centers that correspond to Ceres- and Themis-like spectra, but no hiatus in the distribution suitable for use to simply distinguish between them. We also find a range of band centers in the spectra that approaches what is seen on Comet 67P. Finally, variation is seen between observations for some objects, with the variation on 324 Bamberga consistent with hemispheric-level difference in composition. Given the ubiquity of objects with 3-μm spectra unlike what we see in meteorites, and the similarity of those spectra to the published spectra of Ceres and Themis, these objects appear much more to be archetypes than outliers.
KW - Ceres
KW - asteroid composition
KW - meteorites
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U2 - 10.1029/2018JE005833
DO - 10.1029/2018JE005833
M3 - Article
AN - SCOPUS:85066502602
SN - 2169-9097
VL - 124
SP - 1393
EP - 1409
JO - Journal of Geophysical Research: Planets
JF - Journal of Geophysical Research: Planets
IS - 5
ER -