General Astronomy/The Terrestrial Planets
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Besides Astronomical Units (AU), astronomers also use light years and parsecs to measure distances in space.
Pluto is 40 AU away from Sol. The sun looks like a very bright star from there. Voyager I is 71 AU away from Earth.
The planets may be found in the sky along the ecliptic. They are constantly changing position. Only Mercury, Venus, Mars, Saturn and Jupiter may be seen with the naked eye.
A planet's albedo is the ratio of reflected light to incident light.
The terrestrial planets lose energy from volcanoes. They are characterized by craters (caused by falling meteorites), volcanoes (caused by magma and radiation) and rift valleys (caused by magma plumes). An example of a rift valley is Valles Marineris on Mars.
The heat inside the core of terrestrial planets is caused by the slow decay of uranium, potassium, and thorium.
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[edit] Mercury
Mercury is the closest planet to the Sun. Its orbit is a mere 88 days, meaning that in the time it takes for the Earth to make one full revolution around the Sun, Mercury makes a little over four revolutions.
Days relative to Mercury are extremely long. It takes two revolutions for Mercury to complete one full orbit around its axis, so most points on the planet experience direct sunlight for a (Mercurial) year at a time.
[edit] Venus
Venus is the second planet from the sun, and the only terrestial planet to rotate in a retrograde manner.
Every 500 million years, the surface of Venus melts. After heat escapes, it solidifies again.
Normal telescopes cannot penetrate the very thick atmosphere, but radar can be used to map out the topography of the land.
[edit] Earth
The planet Earth is the planet on which all known life in the universe lives. It is the third planet from the sun and the largest terrestrial planet.
Life on Earth is made possible by its massive reserves of liquid water. No other body in the solar system is known to have massive quantities of liquid water, though several Jovian moons are suspected to have oceans under their crust.
The Earth's only natural satellite, the Moon, is the largest satellite in the solar system relative to its planet. This means that the Moon and Earth are sometimes considered to be a double planet. The Moon is also the only celestial body aside from the Earth that has been set foot upon by humans.
[edit] Mars
The highest mountain on Mars is Olympus Mons. The northern hemisphere is dominated by volcanic plains; the south by heavily-cratered highlands. The surface is very dusty, and covered with dunes. The sky is a pinkish-orange, from dust storms.
Mars has a chaotic terrain. Astronomers believe it was created when magma melted underground permafrost, causing the ground to collapse. Valley networks are believed to have been caused by rain, and water flowing under ice.
Dried up lake beds indicate the presence of water in Mars' past. Another indicator is calcium carbonate, found in a Martian meteor in 1996. Yet another is the presence of outflow channels (believed to have been caused by underground permafrost melted by magma) and alluvial fans. Evidence of seepage flows have also been found. These occur when water breaks through ice and cascades down a cliff.
Martian craters are different from the craters found on the Moon, Venus, or Earth. They're surrounded by mud flows.
The first to map the surface of Mars was Italian astronomer Giovanni Schiaperelli in 1877. He observed a series of lines criscrossing the Martian landscape, which he called canali (channels). Some astronomers translated canali as canal, implying that they had been built by intelligent beings.[1] This idea was advanced by Percival Lowell in his books Mars and Its Waterways (1903) and Mars as the Abode of Life (1908)[2]. It was later discovered that the Martian canals don't actually exist, and were very likely an optical illusion.
