Complex sentence: The story is set on Mars, imagined as a dying planet whose harsh desert environment is based on theories of the astronomer Percival Lowell that were current the late 19th and early 20th centuries.
Split into simpler sentences: The story is set on Mars, imagined as a dying planet with a harsh desert environment. This vision of Mars was based on the work of the astronomer Percival Lowell, whose ideas were widely popularized in the late 19th and early 20th centuries.