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Mercury, the smallest and innermost planet, has no moons, or at least none that can be detected to a diameter of 1.6 km (1.0 mi). [2] For a very short time in 1974, Mercury was thought to have a moon. Venus also has no moons, [3] though reports of a moon around Venus have circulated since the 17th century.
Mercury has no natural satellites. As of the early 2020s, many broad details of Mercury's geological history are still under investigation or pending data from space probes. Like other planets in the Solar System, Mercury was formed approximately 4.5 billion years ago.
Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos. The giant planets have extensive systems of natural satellites, including half a dozen comparable in size to Earth's Moon: the four Galilean moons , Saturn's ...
The moons of the trans-Neptunian objects (other than Charon) have not been included, because they appear to follow the normal situation for TNOs rather than the moons of Saturn and Uranus, and become solid at a larger size (900–1000 km diameter, rather than 400 km as for the moons of Saturn and Uranus).
A "moon" was detected moving away from Mercury in 1974, and was eventually identified as a background star, 31 Crateris. 31 Crateris is a spectroscopic binary with a period of 2.9 days, and this may have been the source of the ultraviolet radiation detected in 1974. [6]
There are 293 confirmed moons in our cosmic neighborhood. By studying these worlds, astronomers hope to learn about ancient asteroid collisions, space volcanoes, and the origins of life itself.
It is likely that Mercury has a silicate crust and a large iron core. [94] [95] Mercury has a very tenuous atmosphere, consisting of solar-wind particles and ejected atoms. [96] Mercury has no natural satellites. [97] Venus (0.72–0.73 AU) [D 6] has a reflective, whitish atmosphere that is mainly composed of carbon dioxide.
Many TNOs are often just assumed to have Pluto's density of 2.0 g/cm 3, but it is just as likely that they have a comet-like density of only 0.5 g/cm 3. [ 4 ] For example, if a TNO is incorrectly assumed to have a mass of 3.59 × 10 20 kg based on a radius of 350 km with a density of 2 g/cm 3 but is later discovered to have a radius of only 175 ...