What are tholins? The mysterious substance that has turned red Ultima Thule

On New Year's Day, NASA's New Horizons probe is interwoven with a tiny world called MU69, or Ultima Thule, the farthest object that mankind has studied closely. While most data on the spacecraft is still waiting to be transmitted, scientists are still learning to know this distant body. We know that it is composed of two pieces of rock stuck together. We know there are no moons or rings in which New Horizons could have approached during his close pass. And we know that Ultima Thule is red.

Carly Howett, a member of the New Horizons team, said that if you stood on New Horizons as time passed, Ultima Thule would appear red to the human eye and very dark. But with the help of improved images, it's also clear that some areas are more red than others, like the edge of the big crater called Maryland.

This redness is probably caused by a mysterious class of compounds called tholins, said Monday the New Horizons team during a mission to the 50th Lunar and Global Planetary Science Conference in Houston.

So what are the tholins?

In general, tholines are complex carbon chains created when ultraviolet light strikes carbon-rich molecules such as methane or ethane. The result is a reddish and tarry substance. This may seem exciting, but it's Carl Sagan, astronomer and star of science communication, who named the material after creating it in his lab (along with his colleague Bishun Khare). They made variations of the famous Miller-Urey experiment, trying to recreate the chemical conditions on the early Earth to see how life could have started.

The idea is that nature can, in the complete absence of biology, produce more and more complex carbon chains, until the transition to a biological protein, and presumably, to life, is in reality only a step. Tholines are complex, organic (ie, carbon-containing) molecules that could be a key step in this process. Scientists are therefore very interested in the presence of this phenomenon in the universe.