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During the April 1971 inferior conjunction, the planet Mercury was observed with the Haystack radar operating at a wavelength of 3.8 cm. Sufficient sensitivity was available to measure scattering characteristics and topography over part of the equatorial region. The reflections corresponded typically to mean surface slopes of about 100 and although these showed some measurable variation with longitude, the reflections were far less variable than on Mars or Venus. Topographic variations of the order of a kilometer were observed, but the planet appears to have less variation in this respect than either Venus or Mars.
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Ingalls et al. (1972) studied this question.
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