The surface of Saturn's largest satellite--Titan--is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion. The Huygens probe entered Titan's atmosphere on 14 January 2005 and descended to the surface using a parachute system. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing.

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A new version of the linear critical-angle refractometer for liquids is described: a 1728 element linear CCD array is used to scan the intensity profile of the output image, within which the position of a cut-off edge is a linear function of refractive index. The precision in locating this cut-off edge is greatly increased, especially for milky liquids, by differentiating the intensity profile numerically. Further information about complex particulate liquids is found to be given by the shape of the differentiated profile, and alternative internal and external illumination modes are provided which may give more information. The present instrument covers the refractive index range 1.3000 to 1.4700, with an accuracy of +or-0.0001 for transparent liquids. The light source is an LED at 635 nm. The intensity profile, the differentiated intensity profile, and the refractive index value are all computed, displayed and printed by a desk-top computer.

Telescopic observations of the polarization of light by asteroids are interpreted on the basis of a systematic polarimetric analysis of terrestrial, meteoritic and lunar samples. Laboratory measurements were made using samples with different surface textures, and scanning electron microscope pictures were used to investigate the influence of microtexture and crystalline structure.It is demonstrated that asteioid surfaces do not accumulate thick regolithic layers of micro-fragments, as do the Moon and Mercury. This is because the majority of debris ejected by impacts are lost, due to the low gravitational escape velocity from these bodies. However, asteroids are not bare rocks, but are coated with a thin layer of adhesive debris. This coating apparently has the composition of the body itself. The fact that there is no indication of significant maturation by space weathering suggests that the dust which coats the surface of asteroids is frequently replaced by further impacts.Asteroids may be classified polarimetrically in several groups: those in group C are made of very dark material and behave like carbonaceous chondrites, or very dark Fe-rich basalts; Those in group S correspond to silicates and stony meteorites. A third group represented by Asteroid 21 Lutetia and 16 Psyche may be metallic.

Summary Some optical polarization effects for very fine powders are shown. The well-known curve of polarization versus phase angle is found to change in shape and scale when the grain size is reduced to less than the wavelength; strong negative polarization is found, even for a smooth powder surface of high albedo. The negative peak changes in shape, and its parameters are found to be related to grain size in simple ways. This distinctive shape provides a possible remote-sensing method for detecting subwavelength grains on planetary surfaces.