The Rocky Planets I
VENUS, named after the Roman goddess of love and beauty, is only a little smaller than Earth. It is the second closest planet to the sun, orbiting it every 224.7 Earth days at a distance of about 108 million km (67 million miles). It has no natural satellite. After the Earth's Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of -4.6, bright enough to cast shadows. Because Venus's orbit lies within Earth's orbit, it never appears to venture far from the Sun. Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, for which reason it has been referred to by ancient cultures as the Morning Star or Evening Star.One day on Venus lasts as long as 243 Earth days. Venus makes a complete orbit around the sun (a year in Venusian time) in 225 Earth days. Venus is sometimes called Earth's "sister planet" because of their similar size, gravity, and bulk composition. However, it has also been shown to be very different from Earth in other respects. Venus is shrouded by an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. The planet is covered by a thick, rapidly spinning atmosphere consisting of more than 96% carbon dioxide, which creates a scorched world with temperatures of almost 480 degrees Celsius (900 degrees Fahrenheit), hot enough to melt lead. With a mean surface temperature of 735 K (462 °C; 863 °F), Venus is by far the hottest planet in the Solar System. It has no carbon cycle to lock carbon back into rocks or other surface features, nor does it seem to have any organic life to absorb it in biomass. Its surface pressure is 92 times that of Earth. Venus may have possessed oceans in the past, but these would have vaporized as the temperature rose due to a runaway greenhouse effect.Venus has no moons and no rings. Venus spins backwards (retrograde rotation) when compared to the other planets. This means that the sun rises in the west and sets in the east on Venus.
Mercury, Venus, Earth and Mars are called "rocky" or "terrestrial" planets. They are similar to Earth in composition. Heat from the Sun evaporated lightweight elements like hydrogen and helium into interplanetary space. Mostly rock and metal was left in this zone and clumped together to form the inner rocky planets.
MERCURY is the smallest and closest to the Sun of the eight planets in the Solar System, at a distance of about 58 million km (36 million miles). The planet is only slightly larger than the Earth's moon and makes a complete orbit around the sun (a year in Mercury time) in just 88 Earth days. Seen from the Earth, it appears to move around its orbit in about 116 days, which is much faster than any other planet. This rapid motion may have led to it being named after the Roman deity Mercury, the fast-flying messenger to the gods. One day on Mercury takes 59 Earth days. Because of the very thin atmosphere, or exosphere, which is composed mostly of oxygen (O2), sodium (Na), hydrogen (H2), helium (He), and potassium (K). , Mercury's surface experiences the greatest temperature variation of all the planets, daytime temperatures reaching 430 degrees Celsius (800 degrees Fahrenheit), during the day at some equatorial regions and dropping to -180 degrees Celsius (-290 degrees Fahrenheit) at night. The surface of Mercury is solid and cratered, much like Earth's moon, indicating that it has been geologically inactive for billions of years. Atoms that are blasted off the surface by the solar wind and micrometeoroid impacts create Mercury's exosphere. Mercury has no moons and no rings. Mercury does not experience seasons in the same way as most other planets, such as the Earth. It is locked so it rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates exactly three times for every two revolutions it makes around its orbit. As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years. Because Mercury's orbit lies within Earth's orbit (as does Venus's), it can appear in Earth's sky in the morning or the evening, but not in the middle of the night. Also, like Venus and the Moon, it displays a complete range of phases as it moves around its orbit relative to the Earth. Although Mercury can appear as a very bright object when viewed from Earth, its proximity to the Sun makes it more difficult to see than Venus.
