The four inner planets are as follows from closest to the sun to the farthest from the sun: Mercury, Venus, Earth, and Mars. Mercury, the closest planet to the sun, is similar to our moon, in the aspect that it is desolate and has almost no atmosphere. Mercury's surface also contains craters left by impacts from objects in outer space. However, mercury's surface contains radioactive isotopes and "creases" in the surface thought to be created by the shrinkage of mercury's core. Water has been found at the north pole of mercury in the form of ice, but life can not be sustained here for the above reasons.
Many would infer that because Mercury is the closest to the sun, it would be the hottest planet. This is Incorrect, Venus is the hottest planet in our solar system. This is because of the high concentration of Venus' greenhouse gasses in it's atmosphere. This makes Mercury the second hottest planet in our solar system after Venus during the day. Due to mercury having no atmosphere, temperatures vary amazingly between the days and nights of the planet. They range from -170C and less during the night and 420C+ during the day.
The third planet would be Earth. Earth is a special case because it is the only planet we have observed to harbor life in our solar system (and everywhere!). This is because of a combination of earth's distance from the sun, the contents of our atmosphere, and the contents of the planet itself. These three factors are all adjusted in a very specific way that supports life.
The final planet would be Mars. Mars is a very cold planet that could most likely not support life in it's current state. The red surface of mars is attributed to the presence of Iron Oxide (Rust) which supports a theory that there is (or was) water on mars. Temperatures can range from highs of around 20C to lows of less than -150C. If mars had a suitable atmosphere it may have been suitable for life.
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