Mapping Venus

Venus cannot be mapped in visible light. A permanent, complete cloud cover hides its surface. Only radar can produce images of its landforms.

The first radar signal was sent towards Venus in 1959. The data showed that the planet rotates very slowly about its axis, and in the opposite direction to Earth. Before this, it was thought that its rotation period equalled its orbital period. In that case the same side would always face the Sun. In 1962 the Soviet Union beamed the letters СССР (USSR) at Venus in Morse code. Newspapers proudly printed a drawing of the returned signal.

The first surface features were identified in the mid-1960s. They were named Alpha and Beta, and they still carry these names. Both are very bright radar spots. The radar image (see the figure) and the map based on it (see the plate) (Rogers and Ingalls 1969) show the 320° meridian running through the centre of the disc. This is (usually) the hemisphere we see when Venus is closest to Earth.

Observers on Earth usually study Venus when it looks largest, because this gives the best resolution. This position is called inferior conjunction. Venus is then between Earth and the Sun, and we see its night side. The orbits of Venus and Earth are closely linked: exactly 13 Venus years pass in 8 Earth years. This is why Venus turns the same side towards us at its close approaches, time after time.

The next bright feature was named Maxwell by the radio telescope team, after the physicist who worked out the theory of electromagnetic waves in the 19th century. We now know that it is also the highest mountain range on Venus. All later features were named after women, following the name of the planet.

Ground-based radar was soon joined by spacecraft, which mapped Venus from orbit. Pioneer Venus made the first topographic map. From 1983 the Soviet Venera 15 and 16 probes mapped the northern hemisphere at 1–2 km resolution (Barsukov et al. 1986). Soviet researchers led by V. A. Kotelnikov published these data in a thick atlas in 1989. A geological map was also made from them. Unusually, it was published jointly with researchers of the US Geological Survey (USGS).