Exploration of Mars seasonal vegetation and apparent linear features were ascribed to intelligent design. Further telescopic observations found two moons, Phobos and Deimos, polar ice caps and the feature now known as Olympus Mons, the solar system's tallest mountain. The discoveries piqued further interest in the study and exploration of the red planet. Mars is a rocky planet, like Earth, that formed around the same time, yet with only half the diameter of Earth, and a far thinner atmosphere; it has a cold and desert-like surface. The minimum-energy launch windows for a Martian expedition occur at intervals of approximately two years and two
Shepherd Moons and elements of Celtic and New age music, though Enya believes her music does not belong in the latter genre. "Shepherd Moons" received generally positive reviews from critics and became a greater commercial success than "Watermark". It went to number one on the UK Albums Chart and peaked at number seventeen on the "Billboard" 200 in the United States. The album was certified multi-platinum by the British Phonographic Industry and Recording Industry Association of America for shipments of 1.2 million and five million copies, respectively. Between 1991 and 1994, Enya released four singles from "Shepherd Moons": "Caribbean Blue", "How Can