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DISCUSSION

GEHRELS (added after the conference): I think it dangerous in principle to calibrate with meteoritic material; one is then likely to derive meteoritic characteristics. The calibration should, instead, be done with asteroids and perhaps satellites. We must obtain good diameter measurements, possibly from space missions.

PHYSICAL STUDIES OF ASTEROIDS BY POLARIZATION OF THE LIGHT

AUDOU//V DOLLFUS
Observatoire de Paris

Curves of polarization are available at present for asteroids Vesta, Ceres, Pallas, Iris, Flora, and Icarus. These curves are compared with those of the satellites of Jupiter and Mercury, the Moon, and Mars. Laboratory simulations had already proved that the Moon's surface behaves like a powder of pulverized basalts; the recent confirmation by direct exploration is proving the significance of the method for remote determination of the surface properties of celestial bodies. The simulation of the Martian surface is found on small grained powders oxidized by ferreous limonite or goethite. New laboratory measurements were conducted to prepare the simulation of the asteroidal surfaces. Samples of the lunar surface returned to Earth provide impact-generated regolith and bare rocks superficially pitted and etched by impacts of the types suggested to be found on asteroidal surfaces; they were analyzed polarimetrically.

Preliminary interpretations show that Vesta departs significantly from the other asteroids and cannot be covered by frost deposits or by aggregate cosmic dusts; a regolith-type surface generated by impacts or a coating of cohesive grains is indicated.

Ceres, Pallas, and Iris are darker, and their polarizations do not suggest a pure regolithic surface, but cohesive grains or aggregates of dust are indicated.

Icarus is 10° times smaller in mass; its polarization authorizes a fluffy, loosely aggregated dust deposit; however, a cometary model with stones embedded in ice is perhaps not ruled out on the basis of the present data.

The way in which deep-space missions near the asteroidal belt can improve these results is discussed.

ANALYSIS OF THE REMOTE SURFACE CHARACTERISTICS
OF THE ASTEROIDS BY POLARIZATION

Telescopic observations permit determination of the amount of polarization P of the light received from asteroids. The plot of these measurements as a function of the phase angle a defines the “curve of polarization” of an asteroid. This curve characterizes the mineralogic properties and structural texture of the asteroidal surface. These curves can be compared from asteroid to asteroid and with other celestial bodies, and simulated by laboratory measurements on different kinds of mineralogic samples.

POLARIZATION MEASUREMENTS AVAILABLE ON ASTEROIDS

The first curves of polarization on asteroids were derived in France by B. Lyot (1934). He used a photographic polarimeter attached to the 100 cm reflector of Meudon Observatory." A curve of polarization was obtained for Vesta in 1934 and Ceres in 1935. These curves were published later by A. Dollfus (1961) and are reproduced again in figures 1 and 2. The curve for Vesta (fig. 1) starts with a negative branch (electric vector maximum in the plane through the Sun, asteroid, and Earth) having a minimum of about Pmin = -1.0 percent near a = 12” and then rises to cross the P=0 value at o, = 26°. Ceres (fig. 2) displays a more pronounced negative branch with Pm in near - 1.3 percent and oo = 17°. Then, the polarization is positive (major electric vector perpendicular to the plane of vision) and rises steeply.

Later, S. Provin (1955), with the assistance of J. S. Hall and A. A. Hoag at the U.S. Naval Observatory, Washington, D.C., used a photoelectric polarimeter on Ceres, Pallas, and Iris. His curves were republished with additional information by A. Dollfus (1961). Figure 3 reproduces the curves for Ceres; the agreement with Lyot's curves (fig. 2) is not perfect, the negative branch being more pronounced and the slope, near the inversion point, being steeper. Pallas (fig. 4) is similar to Ceres. For Iris (fig. 5), Provin followed the variation of polarization as a function of time during more than a complete photometric light period (fig. 6); the phase angle was 27° and provided an average polarization of +1.2 percent; no significant variations were detected, proving

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Figure 1.-Curve of polarization of Vesta. Figure 2. —Curve of polarization of Ceres. (1934 observations of B. Lyot; 100 cm (1935 observations of B. Lyot; 100 cm refractor of Meudon Observatory; pho- reflector of Meudon Observatory; photographic polarimeter.) tographic polarimeter.)

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Figure 3.-Curve of polarization of Ceres. (1954 observations of D. C. Provin, U.S. Naval

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Figure 4.—Curve of polarization of Pallas. Figure 5.-Curve of polarization of Iris. (Observations of D. C. Provin, U.S. (Observations of D. C. Provin, U.S. Naval Observatory; photoelectric polar- Naval Observatory; photoelectric polar

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Figure 6.-Sixty-seven measurements of the amount of polarization of Iris, near a = 27°, plotted as a function of the phase of the lightcurve. Open circles give normal points. (Observations of D. C. Provin.)

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Figure 7.-Curves of polarization of Icarus obtained on the occasion of close encounter with Earth in 1968 by T. Gehrels. The values for 0.64 and 0.52 um are grouped in the same curve; the other curve and open circles are for 0.43 um.

that the physical properties of the surface were the same on the different parts of the asteroid successively seen from Earth as a result of its rotation.” The next polarization curve for an asteroid was obtained by T. Gehrels et al. (1970) on Icarus. Making use of its flyby in the vicinity of Earth in 1968, the authors collected photoelectric polarization measurements in seven colors at the Catalina Station, Arizona. Figure 7 shows the polarization curves in red and blue light derived from the published measurements. Figure 8, taken from the Gehrels publication, compares the polarization (normalized) of Icarus, as a

*See, however, p. 72 of Applied Optics 2, 1963; Provin may have been observing Iris near pole-on aspect.

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