Volcanology of Venus
The surface of Venus is dominated by volcanic features and has more volcanoes than any other planet in the Solar System. It has a surface that is 90% basalt, and about 65% of the planet consists of a mosaic of volcanic lava plains, indicating that volcanism played a major role in shaping its surface. There are more than 1,000 volcanic structures and possible periodic resurfacing of Venus by floods of lava. The planet may have had a major global resurfacing event about 500 million years ago, from what scientists can tell from the density of impact craters on the surface. Venus has an atmosphere rich in carbon dioxide, with a density that is 90 times greater than Earth's atmosphere.
Atmosphere of Mercury
The Mercurian exosphere consists of a variety of species originating either from the Solar wind or from the planetary crust. The first constituents discovered were atomic hydrogen (H), helium (He) and atomic oxygen (O), which were observed by the ultraviolet radiation photometer of the "Mariner 10" spaceprobe in 1974. The near-surface concentrations of these elements were estimated to vary from 230 cm for hydrogen to 44,000 cm for oxygen, with an intermediate concentration of helium. In 2008 the MESSENGER probe confirmed the presence of atomic hydrogen, although its concentration appeared higher than the 1974 estimate. Mercury's exospheric hydrogen and helium are believed to come from the Solar wind, while the oxygen is likely to be of crustal origin.