Thermal emission spectra of 24 asteroids and the Galilean satellites

ICARUS27, 463-471

(1976)

Thermal

Emission Spectra of 24 Asteroids and the Galilean Satellites OLAV L. HANSEN

Cerro To1010 Inter-American Observatory,’ Jet Propulsion Laboratory 4800 Oalc Grove Drive, Pasadena, California 91103 Received June 9, 1975; revised September

183-301,

16, 1975

Thermal emission spectra between 8 and 23pm have been obtained for the Galilean satellites and 24 bright asteroids. No significant emission or absorption feature has been found in any of these spectra.

There is no a priori reason to suspect that the thermal emission from asteroids should differ from grey-body radiation. All asteroids that have been investigated are dark (Matson, 1971; Zellner et al., 1974; Morrison, 1974 ; Hansen, 1975) and presumably have no atmosphere. The only source of nongrey radiation would therefore seem to be Restrahlung and/or Christiansen bands from a bare rock face. There is, however, reason to believe that even relat’ively small asteroids (~100 km diameter) have regoliths. In a polarization study of 43 asteroids, Zellner et al. (1974) found that, all showed a well-developed negative branch indicat,ing unconsolidated surface regoliths. Although it is true that powders may also exhibit spectral lines (Conel, 1969), the particle sizes must be limited bo a narrow range for the powder to do so. Under the assumption that asteroids are grey-bodies an earlier study attempted to use relative spectral measurements of Ceres and three of the Galilean satellites to confirm suspected nongrey emission of the latter. To provide a test case, three other asteroids were included in the analysis. Surprisingly, it was found that Ceres exhibited a large emission feature in two adjacent bandpasses near 12 pm relative

to the other asteroids (Hansen, 1974). The three satellites measured also showed emission features near 12pm, but weaker, and in a single bandpass only. The only follow-up observation possible during the 1973 apparition of Ceres was partially frustrated by weather and t’he need to employ a smaller telescope. The results were therefore inconclusive, but appeared to support the earlier observations. Based on the tentative finding that Ceres has a strong emission feature an extensive spectral survey was initiated, leading to investigation of 24 asteroids. Multiple spectra of Ceres, Vesta, Iris, and Fortuna were obtained. None of the spectra, including five of Ceres and a new set of t’he Galilean satellites, shows any significant discrepancies from that of a grey body. The purpose of this paper is therefore twofold : (a) to withdraw the tentative result of Hansen (1974), and (b) to present the measured spectra. OBSERVATIONS

The data reported here were obtained between 1973 August and 1974 November at the Cerro To1010 Inter-American Observatory. Measurements were made with the CT10 infrared, dual-beam photometer at the Cassegrain focus of the 1.5m telescope. The 9 bandpasses used ranged from 8 pm to 23pm with a wide variety of bandwidths ’ Operated by the Association of Universities (Table I). The aperture was 10arcsec; sky for Research in Astronomy, Inc., under contract with the National Science Foundation. chopping was done at 10Hz. Also shown in Copyright 0 1976 by Academic Press, Inc. 463 All rights of reproduction in any form reserved. Printed

in Great

Britain

/

464

OLAV

L. HANSEN

39 40 45 54 56 79 84 139 241 404 410 444 694

Vesta

Laetitia Harmonia Eugenia Alexandra Melete Eurynome Klio Juewa Gennania Arsince Chloris Gyptis Ekard

Iris Iris Iris Flora Hygiea Egeria Melpomene Fortuna (a) Fortuna (b) Bellona Amphitrite

(a) (b) (c) (d) (a) (b) (c)

(a) (b) (c) (d) (e) (f)

1973 1973 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974

7 7 8 10 8 5 8 5 5 8

8

3 5 6 12 1 2 8 8 10 8 7 8 2 10 8

10 10 8 2

8 8

28.0514 28.0347 28.0187 28.0083 14.0180 29.3819 5.2403 6.3486 27.0403 28.0715 5.2139 15.3264 3.2639 1.1389 31.1180 12.2264 6.1528 15.0903 6.3208 14.2319 27.1639 6.1819 29.3000 6.1590 15.1285 27.0958 14.2625 14.2986 29.2326 29.3229 5.1146 28.1805 5.2590 1.0903 5.1701 1.1875 1.3472 5.1910 29.1736

2.344 2.523

2.204 1.954 2.006 2.098 2.037 3.021 2.581 1.976 2.324 2.310 2.359 2.415 2.499 2.178 2.597 2.187 1.995 1.970 1.809 2.339 2.803 2.059 2.099

4.976 4.976 4.976 4.976 2.843 2.982 2.982 2.982 2.972 2.971 2.426 2.293 2.279 2.227

10 10 10 10 8

1974 1974 1974 1974 1973 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1973 1974 1974 1974 1973 1974 1974 1974 1974 1974 1974

Jl .I2 53 54 1 1 1 1 1 1 3 4 4 4 4 7 7 7 8 10 13 18 19 19 28 29

IO Europa Ganymede Callisto ceres Ceres Ceres Ceres Ceres Ceres Juno Vesta vesta Vesta

r

UT(Year,Month,Date)

Object

9.54 9.54 9.54 9.54 20.14 12.80 10.89 10.57 17.29 17.44 14.27 19.77 14.78 15.99 25 .OO 10.70 5.60 22.51 11.92 1.90 4.57 3.81 4.95 2.07 11.76 12.02 6.40 7.86 8.17 5.45 13.61 10.73 20.80 15.34 5.10 13.86 22.94 7.05 11.17

4.354 4.354 4.354 4.354 2.416 2.137 2.085 2.078 2.383 2.396 1.532 1.548 1.401 1.343 1.590 1.005 1.033 1.363 1.071 2.012 1.598 .968 1.320 1.297 1.447 1.504 1.511 1.191 1.625 1.184 1.039 1.012 .907 1.461 1.808 1.120 1.346 1.352 1.585

a

A

52 9

-2.58 -2.74

-2.59

6

-3.30

-3.67

-5.20 -4.51

-5.13 -5.28 -5.36 -4.88 -4.65 -3.36

6 2

2

4 10

10 1

5

6

1 1 1 2 2 2

-3.86 -3.02

1 1 1 1

-3.86 -3.04 -5.02 -5.40

-2.74 -2.75

-4.94 -5.28 -4.00 -5.21 -5.28 -5.34 -4.78 -4.48 -3.35 -4.40 -5.06 -5.26 -4.55 -4.14 -4.23 -3.00 -3.40 -3.48 -3.03 -3.46 -3.74 -3.35 -1.87 -3.19 -2.86 -2.11 -3.01 -3.42 -3.14 -2.24 -2.49 -2.63 -2.07 -2.39 -2.44

19.63p

%

-3.03 -3.34 -3.08

-2.31 -3.09

II

19.67v

3

3 3 6

%

-3.31 -3.67 -3.41

-3.64 -4.07 -3.80

-3.86

-5.60 -4.82

-5.60 -5.62 -5.51 -5.30 -4.92

-4.29 -3.21 -5.31 -5.77

22.79U

TABLE OBSERVATIONAL DATA

6 3 8 2

20 2

1 2 4 4 6 10 1 1 1 7 1

5 1 3 10

%

-2.47

-2.72

-2.79 -3.08 -2.83

-3.19 -3.40 -3.42

-3.38

-5.07 -4.06

5 2 7

11

8

1

4 5 5 1 1 6

1 6 5 2

-3.57 -2.76 -4.64 -5.03 -4.70 -4.89 -4.89 -4.65 -4.27 -2.89

%

18.07~

-1.56 - .79 - .23 - .75 - .98 -1.01 -1.37

-1.98 - .46 -1.44 -1.47 - .63 -1.31 -1.73 -1.89 - .98

-1.26 - .Ol -2.16 -2 .a9 -3.43 -3.12 -3.36 -3.36 -2.86 -2.67 -1.60 -2.93 -3.63 -3.44 -2.90 -2.36 -2.43 -1.14 -1.90 -1.85 -1.17 -1.90 -1.72 -1.78 .52 -1.14 -1.27 - .71 -1.14 -1.50 -1.68 - .62 - .a4 - .90 - .17 - .64 - .80 - .80 -1.13

1

-1.67 - .32 -2.61 -3.27 -3.37 -3.43 -3.51 -3.77 -3.07 -2.95 -1.96 -3.35 -3.81 -3.67 -3.12 -2.55 -2.45 -1.31 -2.23 -2.05 -1.38 -2.13 -1.91

8

29 3 2 18 21 16 4 20 1

t

1 3 1 42 1 10 17

15 1 11

2 1 1 5 3

1 1

1

11.55~

%

12.28l~

7 4 10 8 10 4 8 6

8

3 19 1 6 14 3

2 10 5 3 1

3 4

1 4 3 1

%

.79 .47 -1.70 -2.42 -2.32 -2.95 -2.87 -3.15 -2.62 -2.42 -1.42 -2.64 -3.35 -3.25 -2.75 -1.98 -2.08 -1.45 -1.67 -1.24 - .87 -1.69 -1.53 -1.60 - .49 - .91 -1.06 - .17 - .88 -1.29 -1.40 - .54 -1.03 - .53 .38 - .48 - .62 - .43 - .96

-

10.77~

6 4

2 9 1 1 4 1 1 2 2 1 2 1 15 1 2 6 5 11 1 6 8 3 1 2 15 4 9 17 7 3 4 2 3 4 1 6 4

%

-

.53 .40

-.18 1.02 -1.05 -1.84 -2.15 -2.38 -2.62 -2.68 -2.10 -1.97 - .91 -2.18 -3.02 -2.76 .-2.39 -1.81 -1.82 - .83 -1.21 -1.07 - .35 -1.36 -1.09 -1.08 .43 - .48 -.37 - .04 -.32 -.92 -1.16 - .03 - .75 - .21 .14 - .02 -.12

10.04~

5 5

6 11 3 1

13 1

3 6 10 9 4

8

2 2 1 1 1 2 1 3 3 5 1 1 3 1

2 2 1 2 1 1

2 6 1

%

.30 .71 .30 .07 .67 .43 - .28 - .37 .56 .03 .43 1.48 .46 .91 .37 .77

-

.13 - .74 -,l.84 -1.50 -1.72 -1.85 -1.26 -1.10 - .20 -1.46 -2141 -2.02 -1.54 -1.13 -1.19 - .20 - .62 - .61 .53 - .59 - .36

.54 2.57

0.78p

2 4 8 10 1 4 11 22 3 13 20 9 3 18 13 5 11 36 7 19 27 33

1 23 5 2 1 1

%

466

OLAV L. HANSEX

Table 1 are zero-magnitude fluxes taken from Schild et aE. (1971), mean airmass coefficients determined during two years of observing at CTIO, and effective wavelength coefficients that will be explained later. Calibration was accomplished in two separate stages : the first for the t,wo broad bandpasses centered at 10.04pm and 19.63@; the second for all the narrow bandpasses. Stage 1 rests on a broad-band calibration of a Ori and tc Sco at 10 and 2Oprn (Becklin et al., 1973) as well as on unpublished work carried out at) CT10 t,o tie y Cru and p Gru into the network. Stage :! rests on the ast’eroid data and analysis reported here, and is based on the assumption t,hat t’he 24 asteroids in the sample taken as (x whole exhibit smooth spectra between x and 23pm. The exact’ route to this part of t’he calibration will be exWAVELENGTH 9

IO

I

I

12 ’

I

(/on) 15

-

l”“l”’ 0 Ori

*

a

I

”

SC0

l

I

500

1000

WAVENUMBER

20

(cm-‘)

FIG. 1. Derived fluxes of the calibrittion stars bawd on a calibration at 10.04pm and 19.63~r11 (Bwklin et a7., 1973). arid tile assumption that tlvz weiglltod mean of tlrc- asteroid sprctra is that of a gray body.

IR SPECTRA

9 ,

I

IO ,

I

I

1000

OF SMALL

WAVELENGTH 12 15 20 . , . I ,..I’,.‘.

I

I

800

1

I

1

600

(

467

BODIES

pm 1 9

IO ,

rO,

II I

I

WAVENUMBER

I 1000

I2 I5 20 Qo, 11 t “‘.I”’

I

I 800

I

I 600

I

L

(cm?

FIG. 2. Normalized emission spectra of the Galilean satellites and Ceres. On this, and the following figures, statistical errors not shown are smaller than the symbol. The spectrum of Ceres marked (a) is probably erroneous (see text).

plained together with the reduction of the asteroid data. The data are listed in Table II together with the relevant configuration parameters : Sun-asteroid distance, r ; asteroidEarth distance, A ; and Sun-asteroidEarth phase-angle, cc. All measurements have been adjusted to equal airmass using the adopted mean coefficients from Table I. To minimize errors introduced by applying mean airmass corrections as opposed to nightly determined corrections, the measurements of both asteroids and calibration stars were confined to be within z airmasses. In addition, the satellite and asteroid measurements have been corrected for effective wavelength variation, a source-

dependent correction (Hansen, 1972), making them correspond to infinitely narrowband measurements at the stated wavelengths. The correction, applicable for 1OOK -c 7’ -c 300K, is given by m = m, + A + B(T/lOOK) +C (T/100K)2, where m, is the measured magnitude at 1 airmass. The coefficients A, B, and C are listed in Table I. The temperature used need not be known to a high degree of accuracy. For this work, an effective equilibrium temperature was calculated for each bandpass by using an assumed Bond albedo of 0.05 and integrating over the asteroid disc. For stars the correction is negligible. The error following each magnitude in the table is the internal, percentage, rms error in the

OLAV L. HANSEN

468

WAVELENGTH

9

12

15

1.1.

1000

20 “‘I”

I...

800

( pm 9

,

600

WAVENUMBER

IO

I

1000 ( cd

12

,

I5

20

I,

I,,

800

600

)

FIG. 3. Kormalized emission spectra of asteroids.

measured flux-expressed that way for the sake of data compression. For the purpose of studying normalized spectra, systematic errors should have no effect’ other than of changing the slope of the spectra. RESULTS In order to look for features in an asteroid spectrum it is convenient to normalize it to the spectrum of a hypot’hetical, cool, black body and plot the normalized spectrum, expressed in magnitudes, as a function of wavenumber. The reason is that, for grey body temperatures below %‘FiK, such a plot will approximate a straight line. The method is therefore a sensitive way to look for spectral features.

Because the magnitudes of the calibrat,ion stars were known only for the broad IO and 2Opm bandpasses, an iterative procedure has been followed to derive magnitudes for hhe seven narrow bands. The procedure consisted of minimizing differences from a straight line drawn t,hrough the magnitudes at 10.04 and 19.63pm of the normalized asteroid spect’ra. Weight was assigned t’o individua,l differences according to the inverse square of t’he recorded stat’istical uncertainty. In other words, it was assumed that’ t,he weighted a,verage of the asteroid spect’ra would approximate the spectrum of a grey body. The resulting stellar calibration, which is thus int’imately related Do tfhe work rc-

IR SPECTRA

9

CJ

IO I -0% -

OF SMALL

WAVELENGTH 12 15 20 ’ I ’ ’ I ““I”” I3 EGERIA

( pm) 9 I

IO I -

’

12 I5 20 ’ ’ ” ““I’.’ 29 AMPHITRITE

‘00

39 LAETITIA

I8 MELPOMENE

40

I9 FORTUNA

Es!

469

BODIES

HARMONIA

45

EUGENIA

;\eu,,

P

54

28 BELLONA

9

ALEXANDRA

--$---+‘---1,

I

1000

I

I

800

I

I 600

III

IIII 1000

WAVENUMBER FIG. 4. Normalized

*

1 cm-’

800

11 600

a

)

emission spectra of asteroids.

ported here, is given in Table III and Fig. 1. As an aid to judging the stellar spectra, the scaled spectrum of a 4000K black body is also shown. The excess radiation near 10pm in the stellar spectra is due to circumstellar grains ; see, for example, Gillett et cd. (1968) and Woolf and Ney (1969). Because 20ym is an even multiple of 1Oprn it is reasonable to expect excesses in that region also. Using this calibration and normalization black-body temperature of 220K, spectra of the Galilean satellites and the 24 asteroids have been derived, and are shown in Figs. 2-5. The assumption that the weighted mean of the asteroid spectra is that of a grey body is supported by the overall good agreement with straight-line spectra (drawn through the broad-band

magnitudes at 10.04 and 19.63pm). The one obvious exception is the spectrum of Ceres marked “a” in Fig. 1. That is the spectrum which led to the tentative conclusion discussed earlier and reported in Hansen (1974). Inviewofthenewresultsfor Ceres, it now appears almost certain that spectrum “a” is erroneous, although the source of the error is somewhat mysterious. The fact that its slope is steeper than the others is consistent with Ceres being closer to the sun at the time of that observation (see Table II), but the slope difference is too great to be explained by a distance effect alone. A more likely explanation is that atmospheric absorption was abnormal, removing 20pm flux disproportionately while affecting the other bandpasses to a lesser and variable extent.

470

OLAV L. HANSEX

9

IO

WAVELENGTH 15 20

12

( pm) 9 IO

12

I5

694

20

EKARD

241 GERMANIA

1 I

1

I

I

1000

I

I

800

I

I

I

-I

600 WAVENUMBER

I

I

1000

I

I

800

I

I

I

I

600

(cm-‘)

Fru. 5. Normalized emission spectra of asteroids.

CoNCLUsIoNs

Neither the Galilean satellites nor the 24 asteroids in the present sample appear to have significant spectral features in their thermal emission. They are therefore suitable objects for interpolation between existing stellar calibrations at IO and 20pm. Indeed, if radii and thermal models could be derived independently to satisfactory accuracy, asteroids could be used for absolute calibration at infrared wavelengths. This work is part of a larger study to derive radiometric albedos and radii of 84 asteroids for which the spectral slopes are interpreted to yield effective temperatures (Hansen, 1976).

ACKNOWLEDGMENTS It is a pleasure to thank Dr. Paul Herget of tlm Cincinnati Observatory for supplying excellent ephemerides for all but the brightest asteroids, Mr. Klaus Czuia for urging me on to this study. Mr. Victor P6rez for preparing the figures, and Mrs. Elisa Baucr for typing the manuscript. Financial support was derived from the National Science Foundationthrough its operating grant to CTIO. REFERENCES BECKLD, E. E., HANSEN, 0. L., KIEFFER, H., AND NEUGEBAUER, G. (1973). Stellar flux calibration at 10 and 20pm using Mariner 6, 7, and 9 results. As&on. J. 78, 1063-1066. CONEL, J. E. (1969). Infrared emissivities of silicates: Exuerimental results and a cloudy

IR SPECTRA

OF SMALL

atmosphere model of spectral emission from condensed particulate mediums. J. Geophys. Res. 74, 16141634. GILLETT, F. C., Low, F. J., AND STEIN, W. A. (1968). Stellar spectra from 2.8 to 14 microns.

Astrophys.

J. 154, 677-687.

HANSEN, 0. L. (1972). Thermal radiation from the Galilean satellites measured at 10 and 20 microns. Ph.D. Thesis, California Institute of Technology, Pasadena, California. HANSEN, 0. L. (1974). 12-micron emission features of the Galilean satellites and Ceres.

Astrophys.

J. 188, L31-L33.

HANSEN, 0. L. (1976). Radii and albedos of 84 Asteroids from simultaneous visual and infrared photometry. A&on. J., 81, 74-84.

BODIES

471

MATSON, D. L. (1971). Infrared observations of asteroids. In Physical Studies of Minor Planets (T. Gehrels, Ed.), pp. 45-50. NASA SP-267. MORRISON, D. (1974). Radiometric diameters and albedos of 40 asteroids. Astrophys. J. 194,

203-212. SCHILD, R., PETERSON, D. M., AND OKE, J. 13. (1971). Effective temperatures of B- and Atype stars. Astrophya, J. 166, 95-108. WOOLF, N. J., AND NEY, E. D. (1969). Circumstellar infrared emission from cool stars.

Astrophys.

J. 155, LlSl-L184.

ZELLNER, B., GEHRELS, T., AND GRADIE, J. (1974). Minor planets and related objects XVI. Polarimetric diameters. d.stronj. J. 79, 1100-1110.