The Solar System

Astronomers peering at Mars in the 17th and 18th centuries saw signs of life everywhere. Seas! Continents! Canals that carried water to Martian farms!

Water may have flowed on Mars long ago, in ancient seas and riverbeds that early astronomers confused for canals. That was back when the atmosphere was thicker. Now, the air here (mostly carbon dioxide) is too wispy to support water or hold much heat. If you took off your spacesuit and stood on the equator at high noon, your toes would feel toasty but your face would be freezing! The good news is summer here lasts for six months. The bad news: So does winter!

Jupiter is a gas giant, a ball of mostly hydrogen and helium large enough to hold more than 1,300 Earths. You won’t find any solid surface to explore here. Instead Jupiter is made up of ammonia and water vapor. The closer you get to the center of the planet the more liquid it becomes due to the extremely high pressure.

Jupiter is considered by many astronomers to be a failed star. It is so big that it has over 70 moons. Mega-moon Ganymede is larger than Mercury and has its own magnetic field. Volcanoes on Io, the most volcanic body in the solar system, spew clouds of yellow sulfur 500 kilometers high. Frozen Europa might hide a liquid ocean beneath its icy crust. Scientists believe Europa might hide something else inside that ocean: life.

Scientists suspect that a planet-size object knocked Uranus sideways in the early days of its formation. It has spun like a top toppling over ever since, which makes for some oddball seasons and decades devoid of even a glint of the sun’s faint light. The north pole is locked in more than 20 years of darkness in the winter and just as much sunlight in the summer, yet the temperature varies little here on the solar system’s coldest planet.

Like Jupiter and Saturn, Uranus is a gas giant—a ball of gas surrounding an Earth-size core of hot liquids. More specifically, Uranus is considered an “ice giant” because its atmosphere is composed mostly of “icy” water, ammonia, and methane. Researchers have found that Uranus’ crushing atmosphere can compress methane into precious rocks. It is thought that these methane rocks fall as rain, so you could say it rains diamonds!

You better pack for a long trip when you trek to Neptune, the most distant of our solar system’s planets. It’s so far away, in fact, that it’s the only planet you can’t see from Earth with the naked eye. How did astronomers discover a planet they couldn’t even see? Through math! They noticed that Uranus —Neptune’s nearest planetary neighbor—traveled in a way that suggested the gravitational pull of an eighth planet. Crunching the numbers revealed Neptune, first confirmed through a telescope in 1846.

Like Jupiter and Saturn, Neptune is a gas giant—a big ball of gas surrounding an Earth-size core of hot liquids rather than rocks or other solid matter. And like fellow “ice giant” Uranus, Neptune’s atmosphere is composed mostly of water, ammonia, and methane. It’s the methane that gives Neptune its striking blue hue.

Clouds of frozen methane whoosh as fast as a fighter jet through storms the size of Earth.