英语时差：土星上真的有生命存在？

Acetylene and hydrogen molecules are disappearing from the atmosphere of Saturn’s moon, Titan. It’s a mystery befitting Sherlock Holmes’ investigative skills, a mystery NASA scientists hope to crack. About fifty percent larger than Earth’s moon, Titan is Saturn’s largest satellite and the only moon with an atmosphere. It has long been known for its methane- and ethane-rich atmosphere, and has intrigued scientists when they discovered lakes of liquid methane on its surface. Titan is a frigid place, with temperatures hovering around minus three hundred degrees Fahrenheit, but despite the cold, scientists wonder if Titan may harbor life. Life as we know it on Earth is carbon based, and Titan has the same hydrocarbon building blocks for living organisms. But, can life exist at such low temperatures, and if it does, can we detect its presence? Missing molecules may be our first clue. Hydrogen molecules should be produced when ultraviolet light breaks down methane and acetylene molecules in Titan’s atmosphere.