Gassy Moon

Over the holidays in late 1943 and early ’44, Gerard Kuiper was on leave from military duty in Europe. Instead of taking in the sights of home, though, he took in the sights of the solar system.

Kuiper was an astronomer at the Yerkes and McDonald observatories. During his time off, he used McDonald’s 82-inch telescope to study several worlds. His list included Titan, the largest moon of the planet Saturn. It’s about 3200 miles in diameter — larger than Mercury, the smallest of the Sun’s major planets.

Before Kuiper began observing Titan, there had been hints that it had an atmosphere. But no one had been able to prove it. Kuiper did. His spectra of the little world showed that it was surrounded by methane gas. It was the first detection of an atmosphere for any moon.

Kuiper recognized why Titan could have an atmosphere, despite its weak gravity. It’s because Titan is so cold — hundreds of degrees below zero. As a result, the methane molecules in the atmosphere move slowly enough that Titan can hold onto them.

In 1980, though, scientists learned that methane is only a minor part of Titan’s air. Instead, as the Voyager 1 spacecraft flew past Saturn, it discovered that the atmosphere is made mostly of nitrogen — the same gas that makes up most of Earth’s atmosphere. It’s the main ingredient in an atmosphere that is much thicker than our own — an atmosphere discovered 75 years ago.

Script by Ken Croswell