Venus is very similar in size and probable composition to the Earth. During a near - crescent phase the planet is ringed by a faint glow indicating the presence of an atmosphere. This has been confirmed by several spacecraft that have visited the planet since the first visit by Mariner 2 in 1962. The atmosphere consists mainly of carbon dioxide and is very dense; the surface atmospheric pressure is 92 times that on Earth. Thick cloud cover results in a strong greenhouse effect that produces stable temperatures up to 740 K, slightly higher than the maximum day-time values on Mercury, making Venus the hottest of the planets. The thick clouds obscure any view of the surface, which has however been surveyed with radar. The Magellan spacecraft, which was placed in a nearly polar orbit around the planet in 1990, carried a radar-imaging system with an optimum resolution of 100 meters, and a radar altimeter system to measure the topography and some properties of the planet's surface.