Sludge of the universe

Many moons, dwarf planets, comets, and trans-Neptunian objects are covered in a kind of complex chemical sludge sometimes called “tholins.” And this sludge may be much more common throughout the universe.

Titan against Saturn
Kevin Gill, CC BY 2.0, via Wikimedia Commons

Last year I wrote about the Miller-Urey experiment, an attempt to synthesize organic matter from inorganic compounds. In the 1970s, Carl Sagan and Bishun Khare conducted a similar experiment – and it produced an impossibly complex sticky tar-like sludge. They dubbed the melange of chemicals in this mixture “tholins.” And we have good reason to believe that this sludge, or something a lot like it, coats a lot of the solar system and beyond.

First, the experiment. Sagan and Khare took a bunch of gases known to exist out in space and blasted them with UV light or electrical sparks. The resulting sludge proved extremely difficult to identify. As it turns out, it wasn’t just one thing, it was a complex brew:

aromatic hydrocarbons, pyrroles, pyridines and other rings, and crosslinked with long-chain alkanes, alkenes and polypeptides

Tholins: organic chemistry of interstellar grains and gas

This strange collection of molecules, this reddish mud, can be created from “cosmically abundant” gases. And it just so happens that a lot of the solar system beyond is coated in a very similar sludge.

Consider the following. The surface of Titan, Saturn’s largest moon:

Surface of Titan
ESA/NASA/JPL/University of Arizona, Public domain, via Wikimedia Commons

Arrokoth, formerly known as Ultima Thule, the weirdly-shaped object floating about in the Kuiper Belt:

Europa, one of Jupiter’s moons:

Europa
NASA/JPL/DLR, Public domain, via Wikimedia Commons

And hey, our old friend Pluto:

Astronomers suspect that the reddish bits of all these objects, and many others, are actually tholins. And Sagan and Khare proposed that this cosmic sludge may also be plentiful in that otherwise empty space between star systems. And what’s more, tholins contain a lot of molecules that could – under the right conditions – even lead to organic life.

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