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and the negative branch of its polarization curve. (The pictures were obtained by J. E. Geake at Manchester.)
the negative branch of its polarization curve. (The pictures were obtained by J. E. Geake at Manchester.)
INTERPRETATION OF THE POLARIZATION CURVES OF ASTEROIDS
From the previous polarimetric results, combined with other optical or physical data available, models of the nature of the asteroidal surfaces can be derived, or at least indicated. For Vesta the diameter is known to be of the order of 410 km (see A. Dollfus”) and the corresponding albedo is 0.40. This high value is compatible with frost deposits but excludes aggregated cosmic-dust coatings. The presence of polarization, in turn, excludes frost deposits, but agrees with solid surfaces. The spectral reflectivity curves between 0.35 and 1.1 pm observed by McCord et al. (1970)* show a pronounced dip at 0.9 pm found on Mg-rich orthopyroxene and recognized on samples of basaltic achondrites. On account of its deeper negative branch, the shape of the polarization curve excludes a glassy surface of the type displayed in figure 19, and also the average lunar rock structures shown in figure 18; more multiple scattering is needed and involves more rugged surfaces. The negative branch of polarization is not incompatible with a regolith layer resulting from fragments, generated by impacts on a light basaltic achondrite mineral. For an assumed density of 3.3 g/cm3, the escape velocity is 140 m/s and a large fraction of the ejecta produced by impacts should be lost in space; however, the cohesion of grains in vacuum may help to retain sticky grains. For Ceres, the diameter of about 770 km is definitely larger than that for Vesta, and the darker albedo of 0.13 is similar to the case for the Moon and Mercury. For Pallas, some inconsistencies remain between diameter measurements (see A. Dollfus?), but the size is intermediate between that of Ceres and Vesta, with an albedo not higher than that of Ceres. The diameter of Iris has not yet been measured but belongs to the Ceres' or Pallas' range of sizes. Although polarimetric measurements should be continued, all three of these asteroids apparently display the same type of polarization curves, with an inversion angle near 18° and a negative minimum as high as 1.7 percent. The low albedo and high negative branch of polarization exclude frost deposits on these three objects but characterize surface structures and composition definitely different from that for Vesta. The reflection spectrum from Pallas, and probably Ceres, obtained by McCord et al. (1970) does not show the 0.9 pm band seen on Vesta. With escape velocities of the order of 250 m/s, these bodies should retain more easily ejected fragments from impacts, as Vesta does, but nevertheless the polarization curves depart from the characteristics of the lunar fines and, despite the similarity in albedo, exclude a pure lunar type regolithic powdered layer. The polarimetric properties may indicate cohesive grains but do not rule out, with the present data, a loose aggregate of gently deposited cosmic dust; more observational and laboratory work is to be performed.
For Icarus, the measurements of figure 7 refer to a body belonging to a completely different range of sizes. The diameter of nearly 1 km (Gehrels et al., 1970) gives a mass on the order of 108 times smaller than for the other asteroids polarimetrically analyzed. The escape velocity of 0.35 m/s rules out the retention of any kind of ejecta resulting from impacts. The maximum of polarization of 7 percent almost excludes bare rock with an albedo of 0.26 assumed by Gehrels et al. (1970), but authorizes a fluffy, loosely aggregated deposit of (cosmic) dust. However, a cometary model with stones embedded on ice is perhaps not ruled out on the basis of the current polarimetric data available.
POSSIBILITIES FOR FUTURE WORK
In addition to the refinement and extension of the ground-based telescopic techniques currently used, major results could be expected from space missions near the asteroidal belt.
For the minor planets of the main belt, the phase angle observable from Earth is limited to the range between 0° and about 30°. Space missions will reveal the shape of the curves near their maxima, occurring between 90° and 120°, and determine the highest value of the polarization Pimax. Together with the albedo A, deduced from the direct measurements of the diameters, these values of Pmax are basic for the telescopic determinations of the composition of these celestial bodies.
For instance, in the case of the Moon, the determinations of Pmax for areas of different albedo plotted on a logarithmic scale as a function of A display a linear relation between log A and log Pmax, as seen in figure 21 (Dollfus and
Figure 21.-Albedo of 146 areas of the lunar surface plotted as a function of their maximum of polarization Pmax in logarithmic scale. Measurements in orange light.
Bowell, 1971). Comparisons with mineralogic samples, summarized in figure 22, demonstrate that several compositions have to be ruled out as candidates for simulation of the optical properties of the Moon, namely, sands, clays, chalks, crushed rocks, pulverized meteorites, ignimbrites, vitric basalts, and most of the volcanic ashes. On the contrary, the pulverized basalts from lava flows fit the optical properties of the lunar regolith very well (Dollfus et al., 1971a; Dollfus and Titulaer, 1971). The subsequent confirmation of this result by the direct exploration of the Moon again accredits the significance of the polarimetric technique for remote analysis of the composition of planetary surfaces.
Figure 22.-Domains occupied by different kinds of terrestrial and meteoritic samples in a log A versus log Pmax plot similar to figure 21. The domain for lunar measurements is derived from figure 21 after normalization of the albedo scale with the Apollo lunar samples returned to Earth.
Figure 23 further refines the determination by taking into account the spectral variation of Pinax and A. Measurements at five wavelengths for each sample gave five dots in the log A -log Pmax plot, alined along a segment represented in figure 23; location, length, and slope of these segments characterize still more precisely the surface and again strengthen the optical similarities between the lunar regolith and terrestrial pulverized basalts.
Spaceborne measurements on minor planets will characterize the asteroidal surfaces by dots in figure 22 and segments in figure 23 and will provide specific data for compositional comparison and simulation.