The goal is to determine whether any of the near-earth asteroids contain water-bearing phyllosilicate (clay) minerals. If these minerals are present, they would provide a readily available source of water for propellant generation and use in life support systems. Telescopic detection of water on the near-earth asteroids is complicated because thermal emission from the asteroid itself masks the diagnostic absorption features for objects this close to the sun. Sophisticated thermal models are necessary to determine whether the absorption features are present. This year, development of these models was continued and more telescopic data to test the models was obtained

Lebofsky, L. A.

Nelson, M. L.

English

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©Abstract
ComPositions of Near-Earth Asteroids
L A. Lebofsky and M. L. Nelson
Lunar and Planetary Laboratory
The University of Arizona
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The goal of this study is to determine whether any of the near-earth asteroids contain
water-bearing phyflosiiicate (clay) minerals. If these minerals are present, they would provide a
readily available source of water for propellant generation and use in life support systems.
Telescopic detection of water on the near-earth asteroids is complicated because thermal emission
from the asteroid itself masks the diagnostic absorption features for objects this close to the sun.
$ophistica_ed thermal models will be necessary to determine whether the absorption features are
present. This year, we have continued development of these models and have obtained more
telescopic data to test the models.
Introduction
The CI1 and CM2 carbonaceous chondrtte meteorites are known to contain clay minerals in
their matrix. In fact, the Cll meteorites contained enough water that it was able to migrate in the
parentbody, dissolving minerals and redepositing them as evaporiteveins Asteroidal size bodies
with these compositions would prove rich sources of water and oxygen, and the resources might
bo easier to extrsct from clays than from other oxides. Reflectance spectra of these meteorites are
similar to telescopic spectra of some of the C-class asteroids in the main asteroid belt, Implying
similar mineralogy. Unfortunately, all of these asteroids are extremely dark, which makes it difficult
to measure accurate reflectance spectra. The dark components on their surface also mask the
presence of other components in their reflectance spectra because the darkest components
dominate the reflectance spectra of a mixture of minerals. In addition, the features diagnostic of
water occur in the 3-4/_ m wavelength region. In the main asteroid belt, this wavelength region is
still dominated by reflected light, although thermal emission makes a significant enough contribution
https://ntrs.nasa.gov/search.jsp?R=19910015073 2020-03-19T18:25:00+00:00Z
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that is mustbe modeledbeforethe reflectancespectrumcan be analyzed(Fig. 1). Careful
telescopic work has found water absorption features in the telescopic spectra of some of the
C-class asteroids, but not all. This makes it Impossible to assume that all C-class asteroids will
contain hydrated minerals. Recent work by Dan Britt has suggested that some of these asteroids
(the ones that do not show the water absorption features) may actually be similar in composition
to ordinary chondrites, which do not contain any hydrated phases.
The near-earth asteroids are much more accessible than the main belt asteroids, so it is
desirable to determine if any of the C-class objects in the near-earth population contain clays. This
is a difficult problem for two reasons. First, the near-earth asteroids are small, and the C-class
asteroids are dark, which makes them very difficult to observe telescopically. Second, the
near-earth asteroids are much nearer to the sun than the main belt asteroids, which makes them
significantly warmer. This shifts the peak in their thermal emission to much shorter wavelengths,
thus the thermal component of the asteroids dominates the spectrum in the 3-micron region where
the dlagnostio water absorption feature Is present. Simple thermal models are not accurate enough
to remove this thermal contribution.
Hapke has proposed the first model which treats the reflected and emitted radiation
simultaneously, which offers the best opportunity for modeling the spectra of the near-earth
asteroids. The model is an application of radiativetransfer theory to particulate surfaces. It includes
terms for the absorption and scattering properties of the component minerals, the photometric
geometry, and the physical properties of the surface. Reflectance spectra can be calculated from
the model and compared to measured spectra, or the modal can be fit to observational data,
making appropriate assumptions about the unknown parameters. Parts of this theory have been,
and are being tested extensively, but the thermal terms are new and relatively untested.
We have proposed to apply this theory to analyze telescopic spectra of C-class near-earth
asteroids to determine their water content. Before this is possible, we must test the theory to
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determineitsvalidityand sensitivity in better understood applications. This year, we have done
much to advance our understanding of the theory.
Results for This Year
Software Development. This year Marcia Nelson has ported a major program to the Planetary
Image Resource Center computers at the Lunar and Planetary Laboratory. This program calculates
many of the terms in the published Hapke theory, and fits the equations to reflectance spectra to
determine parameters in the theory of interest, such as the grain size and abundance of component
minerals.
Model/ng. Last year, Marcia Nelson worked on three modeling applications to improve our
understanding of the Hapke model. In the first project, she showed that the theory can calculate
spectra of mineral mixtures directly from the optical constants of the components, and given the
end members, the mineral abundances can be determined fairly accurately from the optical
constants. In the second project, she used the theory to determine the modal composition of the
surface of Vesta, assuming end members from the pre-existing literature. In the third project, she
calculated spectra of some lunar analogue mineral mixtures to study the mixture systematlcs.
This year, she has been continuing to analyze her previous results. The previous Vesta
analysis was adequate, but not as good as had been hoped. Initially, the problems had been
attributed entirely to the deficiencies in the mineral chemistry of the end members Nelson was able
to obtain from the literature and the lack of information on the grain size. The differences in mineral
chemistry definitely contributed to the problems, but on closer examination of the initial failed
attempts, she realized that the uncertainty In the grain size was also a major contributor. No
information was available on the grain sizes of the minerals chosen as end members. A uniform
grain size was assumed for all components which was reasonable for a regoUth,assuming asteroid
regoliths are elmilar to that of the Moon. This year, it was realized that the grain size of one of the
components was probably much larger than the one assumed, because the laboratory spectrum
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wasunusuallydarkforthatmineral.Thisdistortedthecalculatedabundancesbecausethefitting
programwasforcedto addexcessiveamountsof the bdghter components to match the continuum
reflectance.
Further analysis showed that in all three of these applications, the grain size had a major effect
on the model. This is a parameter which had not really been examined systematically in the theory.
We didnt have a laboratory available to create an ideal test set of spectra, so she found ways to
learn as much about this problem as possible with the spectra obtained for the other projects.
We had all of the necessary data for a complete analysis of the single grain size fraction used
in creating the samples for the first project. Nelson measured the actual grain size of the samples
from a number of scanning electron microscope images of the samples and used these to calculate
the average grain size. This had been used in the initial calculation of the mineral abundances. She
then used the software ported this year to fit the laboratory spectra to determine what grain size
provided the best model fiL The grain size determined by the fit differed insignificantly from that
determined by direct measurement. She next made histograms of the actual grain size
measurements. The theory assumes sphedcal grains, but the actual measurements of the length
and width showed a bimodai distribution. She determined the relative areas under the two peaks
and calculated spectra for mixtures of grains of sizes corresponding to the two peaks and
abundances proportional to their areas. These spectra were virtually indistinguishable from those
calculated using a single average grain size (Figures 2 and 3).
This work has shown several things. Rrst, it has proven that the theory is using the actual
average grain size, and not some imaginary grain size determined by the mean optical pathlength
of the light in the grains. It has also shown that an average grain size is sufficient to obtain valid
results for a narrow grain size fraction. And it has shown that the theory results are extremely
eensitive to the grain size. This means that a reasonable range of grain sizes will have to be used
to create a sedes of possible results when the model is applied to real surfaces where the grain size
is unknown. Nelson is in the process of doing this with the Vesta model at this time.
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Observing. This year, we have had four observing runs (two at the MMT and 61" near Tucson
and two at the IRTF In Hawaii), which were partially funded through NASA Planetary Astronomy.
Useful data were obtained on half of these runs. Telescopic spectra of several asteroids were
obtained over 1-3.5/J m wavelength range. These data will be used to test the models we are
developing. The Vesta data will be used to test the complete theory next year.
P_blicatlons
Gaffey, M. J., L. A. Lebofsky, M. H. Nelson, and T. D. Jones. Asteroid surface compositions from
earthbased reflectance spectroscopy (submitted for Inclusion in conference volume edltted by
Carle Pleters).
Howetl, E. S. and L A. Lebofsky. Bull. Amer. Astron. Soc., 22, 1122-1123, 1990.
Jones, T. D., L A. Lebofsky, J. S. Lewis, and M. S. Marley. The composition and odgin of the C,
P, and D asteroids: Water as a tracer of thermal evolution in the outer belt, Icarus, 88,172-192,
1990.
Lebofsky, L A., W. B. Hubbard, E. Asphaug, D. M. Hunten, R. L. Millis, O. G. Franz, L H.
Wasserman, M. A'Heam, R. Schnurr, A. R. K]emda, W. Osbom, F. VUas, A. E. Potter, P. D.
MaJey, and P. L Manly. Bull. Amer. Astron. Soc., 22, 1113, 1990.
Lebofsky, L A., T. D. Jones, P. D. Owensby, M. A. Feierberg, and G. J. Consolmagno. The nature
of low aibedo asteroids from 3_um spectrophotometry. Icarus, 83, 12-26, 1990.
Nelson, M. L Sensitivity of Hapke modeling of mixtures to grain elze. Lunar and Ranetary Science
XXI, 869-860, 1990. (Abstract of talk presented at the LPSC XX! Conference 3/90).
Nelson, M. L and R. N. Clark. Comparison of different measures of the grain size In Hapke
modeling. Bull. Amer. _. Soc., 22, 1033, 1990. (Abstract of talk presented at the Division
of Planetary Science Conference 10/90).
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wavelength, /=m
Rg. 1. Observations of the spectral reflectance of Ceres between 1 and 4 # m. The sd.id
lines are high-resolution Fourier spectra and the circles are medlurn-resolutlon
spectrophotometry. The upper curves are the observed spectra of Ceres, including
the thermal emission from the asteroid at longer wavelengths. The bottom curves
are the relative reflectance with the thermal emission removed using the "standard
thermal model."
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CG5121 - SEM Graizt Size Compared to Fit Grain Size
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Rgure 2. Calculated spectra of coming glass 5121. The solid line is calculated using the
measured grain size and the dashed using the model grain size.
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CG5121 - Measured Spec_ compared to Spectra Calculated
From the SE_ Grain Size and From a Grain Size Mixture
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Figure 3. The measured spectrum of coming glass 5121 is plotted as a solid line. Spectra
calculated using the measured grain size (dashed) and the grain size mixtures
(dash-dot) are plotted as dashed lines. The calculated spectra are virtually
identical.
IV. SYSTEMAUTOMATION AND OPTIMIZATION


Compositions of near-Earth asteroids

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