This morning I was looking at a nice website that lets you zoom in or out and see objects of different sizes, ranging from the Planck length to the entire observable Universe:
• Cary Huang, The Scale of the Universe.
When I zoomed out to a size somewhat larger than Rhode Island, I saw something strange:
Huh? I recently saw a movie called Melancholia, about a pair of sisters struggling with fear and depression, and a blue planet of that name that’s headed for Earth. And indeed, dysnomia is another mental disorder: a difficulty in remembering words, names or numbers. Are astronomical bodies named for mental disorders catching on?
I don’t know—but unlike Melancholia, Dysnomia is real! It’s the speck at left here:
Why the funny name? In Greek mythology, Dysnomia (Δυσνομία), meaning ‘lawlessness’, was the daughter of Eris, the goddess of strife and discord. You may remember the dwarf planet Eris under its tentative early name ‘Xena’, from the TV character. That was deemed too silly to stand.
Eris is 30% more massive than Pluto, and thus it helped lead to a redefinition of ‘planet’: both are now called dwarf planets, because they aren’t big enough to clear their neighborhood of debris.
Eris has a highly eccentric orbit, and it takes 580 years to go around the Sun. Right now it’s at its farthest from the Sun: three times as far as Pluto. So it’s very cold, about 30 kelvin (-243 °C), and its surface is covered with methane ice. But in about 290 years its temperature will nearly double, soaring to a balmy 56 kelvin (-217 °C). Its methane ice will melt and then evaporate away, giving this world a new atmosphere! That’s pretty amazing: an annual atmosphere.
The surface of Eris is light gray. So, it’s quite different than Pluto and Neptune’s moon Triton, which are reddish due to deposits of tholins—complex compounds formed by bombarding hydrocarbons and nitrogen with sunlight.
Remember the smoggy orange surface of Titan?
That’s due to tholins! It’s possible that on Eris the tholins are currently covered by methane ice.
I wish I knew more about what tholins actually are—their actual chemical structures. But they’re a complicated mess of stuff, and they vary from place to place: people talk about Titan tholins, Triton tholins and ice tholins.
Indeed, the term “tholin” comes from the Greek word tholós (θολός), meaning “not clear”. It was coined by Carl Sagan to describe the mysterious mix of substances he created in experiments on the gas mixtures found in Titan’s atmosphere… experiments a bit like the old Urey-Miller experiment which created amino acids from stuff that was present on the early Earth—water, methane, ammonia, hydrogen—together with lots of electrical discharges.
Might tholins become complicated enough to lead to life in the outer Solar System, at least on relatively peppy worlds like Titan? There’s a complex cycle going on there:
Here ‘Da’ means daltons: a dalton is a unit of mass equal to about one hydrogen atom, so a molecule that’s 2000 Da could be made of 2000 hydrogens or, more reasonably, 2000/12 ≈ 166 carbons, or various other things. The point is: that’s a pretty big complicated molecule—big enough to be very interesting!
On Earth, many soil bacteria are able to use tholins as their sole source of carbon. Some think that tholins may have been the first food for microbes! In fact, some scientists have speculated that Earth may have been seeded by tholins from comets early in its development, providing the raw material necessary for life. But ever since the Great Oxygenation Event, the Earth’s surface has been too oxidizing for tholins to exist here.
Tholins have also been found outside our Solar System:
• Red dust in disk may harbor precursors to life, Carnegie Institute news release, 5 January 2008.
I bet tholins often go hand-in-hand with PAHs, or polycyclic aromatic hydrocarbons. PAHs are also common in outer space. In Earth you can find them in soot, or the tarry stuff that forms in a barbecue grill. Wherever carbon-containing materials suffer incomplete combustion, you’ll find PAHs.
PAHs are made of hexagonal rings of carbon atoms, with some hydrogens along the edges:
Benzene has a single hexagonal ring, with 6 carbons and 6 hydrogens. You’ve probably heard about naphthalene, which is used for mothballs: this consists of two hexagonal rings stuck together. True PAHs have more. With three rings you can make anthracene:
With four, you can make napthacene:
And so on! The game just gets more complicated as you get to use more puzzle pieces.
In 2004, a team of scientists discovered anthracene and pyrene in an amazing structure called the Red Rectangle!
Here two stars 2300 light years from us are spinning around each other while pumping out a huge torus of icy dust grains and hydrocarbon molecules. It’s not really shaped like a rectangle or X—it just looks that way from here. The whole scene is about 1/3 of a light year across.
This was first time such complex molecules had been found in space:
• Uma P. Vijh, Adolf N. Witt, and Karl D. Gordon, Small polycyclic aromatic hydrocarbons in the Red Rectangle, The Astrophysical Journal 619 (2005), 368-378.
By now, lots of organic molecules have been found in interstellar or circumstellar space. There’s a whole "ecology" of organic chemicals out there, engaged in complex reactions. Life on planets might someday be seen as just an aspect of this larger ecology. PAHs and tholins are probably among the dominant players in this ecology, at least at this stage.
Indeed, I’ve read that about 10% of the interstellar carbon is in the form of PAHs—big ones, with about 50 carbons per molecule. They’re common because they’re incredibly stable. They’ve even been found riding the shock wave of a supernova explosion!
PAHs are also found in meteorites called "carbonaceous chondrites". These space rocks contain just a little carbon: about 3% by weight. But 80% of this carbon is in the form of PAHs.
Here’s an interview with a scientist who thinks PAHs were important precursors of life on Earth:
• Aromatic world, interview with Pascale Ehrenfreund, Astrobiology Magazine.
Also try this:
• PAH world hypothesis, Wikipedia.
This speculative hypothesis says that PAHs were abundant in the primordial soup of the early Earth and played a major role in the origin of life by mediating the synthesis of RNA molecules, leading to the (also speculative) RNA world.
Another radical theory has been proposed by Prof. Sun Kwok, author of Organic Matter in the Universe. He claims that instead of PAHs, complex molecules like this would do better at explaining the spectra of interstellar clouds:
Would this molecule count as a tholin? Maybe so: I don’t know. He says:
Our work has shown that stars have no problem making complex organic compounds under near-vacuum conditions. Theoretically, this is impossible, but observationally we can see it happening.
For more see:
• Sun Kwok and Yong Zhang, Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features, Nature 479 (2011), 80–83.
This paper isn’t free, but here’s a summary that is:
• Astronomers discover complex organic matter exists throughout the Universe, ScienceDaily, 26 October 2011.
However, I’d take this with a grain of salt until more confirmation comes along! They’re matching very complicated spectra to hypothetical chemicals, without yet any understanding of how these chemicals could be formed in space. It would be very cool if true.
Regardless of how the details play out, I think we’ll eventually see that organic life across the universe is a natural outgrowth of the organic chemistry of PAHs, tholins and related chemicals. It will be great to see the whole story: how much in common life has in different locations, and how much variation there is. It may be rare, but the universe is very large, so there must be statistical patterns in how life works.
It goes to show how everything is connected. Starting from a chance encounter with Dysnomia, we’ve been led to ponder another planet whose atmosphere liquifies and then freezes every year… and then wonder about why so many objects in the outer solar system are red… and why the same chemicals you find in the tarry buildup on a barbecue grill are also seen in outer space… and whether life on Earth could have been started by complex compounds from comets… and whether life on planets is just part of a larger interstellar chemical ‘ecology’. Not bad for a Saturday morning!