On New Year’s Day, NASA’s New Horizons probe streaked by a tiny world dubbed MU69, or Ultima Thule , the farthest object humankind has studied up close. With most of the data still on the spacecraft waiting to be transmitted, scientists are still getting to know this distant body. We know that it’s composed of two chunks of rock loosely stuck together. We know that it doesn’t have moons or rings that New Horizons might have careened into on its close pass. And we know Ultima Thule is red.Carly Howett, a member of the New Horizons team, said that if you were standing on New Horizons as it sped past, Ultima Thule would appear red to the human eye and very dark. But with the aid of enhanced imagery, it’s also clear that some patches are redder than others, like the rim of the large crater known as Maryland.That redness is likely caused by a mysterious class of compounds called tholins, the New Horizons team said Monday during a mission update at the 50th Lunar and Planetary Science Conference in Houston.So what are tholins?Broadly speaking, tholins are complex carbon chains made when ultraviolet light strikes carbon-rich molecules like methane or ethane. The result is a reddish, tarry substance. That may not sound exciting, but it was astronomer and science communication all-star Carl Sagan who named the material after creating it in his lab (along with fellow researcher Bishun Khare). They were performing variations on the famous Miller-Urey experiment , trying to recreate the chemical conditions on early Earth to see how life might have started.The idea is that nature can, in the total absence of biology, produce more and more complicated carbon chains, until the leap to a biological protein, and presumably, life, is in fact just a single step. Tholins are complex, organic (meaning it contains carbon) molecules that could be a key step in this process. So scientists are very interested in where it’s occurring in the universe.