In this composite image, the sun has reached its northernmost point in Earth's sky, marking a season change and the first solstice of the year 2004. NASA/ESA hide caption

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Earth has been many different planets in its long history — and could be more. Some imaginary views are shown here. Top (from left): a water world before large-scale continents had formed; a "snowball" Earth phase with extreme glaciation; and a world shrouded in smoke after a large asteroid impact. Bottom: Earth today; Earth losing its oceans as the sun becomes hotter; and a final scorched Earth. Courtesy of Don Brownlee, University of Washington hide caption

toggle caption Courtesy of Don Brownlee, University of Washington

In 2013, NASA's Cassini spacecraft captured this image of Earth from Saturn. Seen here, our planet is 898 million miles away (1.44 billion kilometers) and appears as a blue dot at center right. NASA/JPL-Caltech/Space Science Institute hide caption

toggle caption NASA/JPL-Caltech/Space Science Institute

The Cathedral Spires in the Black Hills of South Dakota are just one of innumerable formations across the planet that speak to the Earth's ancient history. K. Scott Jackson/Ohio Water Science Center/USGS hide caption

toggle caption K. Scott Jackson/Ohio Water Science Center/USGS

That little blue dot is how Earth will likely appear in a photo shot from a spacecraft that is studying Saturn. NASA/JPL-Caltech simulation hide caption

toggle caption NASA/JPL-Caltech simulation

Will we need to tread lightly, in deference to the locals, when finally do make it to Mars? Pat Rawlings/SAIC/NASA hide caption

toggle caption Pat Rawlings/SAIC/NASA