The Rocky Planets II
MARS is the fourth planet from the Sun and the second smallest planet in the Solar System. Named after the Roman god of war, it is often described as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance.
Mars is a terrestrial planet with a thin atmosphere, made up mostly of carbon dioxide (CO2), nitrogen (N2) and argon (Ar) It has surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of Olympus Mons, the second highest known mountain within the Solar System (the tallest on a planet), and of Valles Marineris, one of the largest canyons. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature. Mars has two known moons, Phobos and Deimos, which are small and irregularly shaped. These are most likely to be captured asteroids.
Mars had long been thought to have liquid, free flowing water and it wasn't until the first survey of Mars by the Mariner 4 spacecraft in 1965 that this was disproved. The possibility of water was because observers had seen light and dark patches during different times of the Martian year looking like seas and continents. Long straight features were seen by some observers and were interpreted as channeling systems for liquid water. The most famous of these observers was Percival Lowell, who went so far as to propose the idea that the canals were irrigation canals built by a supposed intelligent civilization on Mars. In 1903, Joseph Edward Evans and Edward Maunder conducted visual experiments using schoolboy volunteers that demonstrated how the canals could arise as an optical illusion. William Kenneth Hartmann, a Mars imaging scientist, explains the 'canals' as streaks of dust caused by wind on the leeward side of mountains and craters.
Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude reaches -3.0, which is surpassed only by Jupiter, Venus, the Moon, and the Sun. Optical ground-based telescopes are typically limited to resolving features about 300 km (186 miles) across when Earth and Mars are closest because of Earth's atmosphere.
EARTH is the third planet from the Sun. It is the densest and fifth largest of the eight planets in the Solar System. It is also the largest of the Solar System's four rocky planets. It is sometimes referred to as 'the world' or the Blue Planet. During one orbit around the Sun, the Earth rotates about its own axis 366.26 times, creating 365.26 solar days. The Earth's axis of rotation is tilted 23.4° away from the vertical plane; this is what causes seasonal variations on the planet's surface.
Earth formed approximately 4.54 billion years ago, and life appeared on its surface within a billion years. The rapid spread and increase of life then significantly altered the atmospheric and other basic physical conditions. Chemical interactions on the surface changed the composition of the atmosphere, adding the ozone layer in the upper section. Along with the magnetic field created by Earth's molten core, this provided protection from harmful solar radiation, allowing formerly ocean-confined life to move safely to land. Earth's crust is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. Over 70% percent of Earth's surface is covered with water, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice, the solid ice of the Antarctic ice sheet and the sea ice that is the Arctic ice packs.
The planet's interior remains active, with a solid iron inner core, a liquid outer core that generates the magnetic field and a thick layer of relatively solid mantle on which the continents float.The Moon is Earth's only natural satellite. It was formed about 4.53 billion years ago. The Moon's gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt, and is gradually slowing the planet's rotation.The planet is home to millions of species of life, including humans.
Phases of the Moon
