PAHs (not LGMs) on Iapetus!

Dr. Eric Wegryn

Science is fun. Not just for the exciting process of discovery, but also because you get to throw around some big words when you want to.  For the past few months I’ve had the amusing pleasure of telling family, friends, and students, “We’ve discovered polycyclic aromatic hydrocarbons(1) on Iapetus,” then watching their faces as they try to deconstruct this polysyllabic jargon.

Those who get the furthest usually respond with, “Really?  What do they smell like?”

For the ones not familiar with the site of my research, those not following the fascinating mission of the Cassini spacecraft—which has been orbiting and exploring Saturn for the past year—I first have to explain that Iapetus, the third largest moon of Saturn, is an enigmatic yin-yang world, with a bright white hemisphere opposite a surprisingly dark hemisphere.  No other world in our solar system has such a large contrast in albedo as Iapetus.

Once we’ve gotten that clear, it’s time to explain the polysomething automatic hydrocarbons. (Aromatic? Oh yes, excuse me.)  Hydrocarbons—now that’s straightforward— molecules containing carbon and hydrogen.  Aromatic does not mean ‘smelly’ though. Aromatic refers to the ring structure of the molecules, usually hexagonal as in benzene C6H6), the simplest kind (as opposed to aliphatic, which describes molecules built on linear or branched chains of carbon atoms).  Finally, polycyclic means that the rings are conjoined, with an extended structure much like the afghans that my mother crochets for holiday gifts.

Of course, once we’ve achieved a scientific understanding of the lilting trochaic hexameter that is polycyclic aromatic hydrocarbons, laziness of speech takes over again, and it usually gets shortened to PAHs.

So how did we find them, and why is this discovery important?

The discovery was not especially complicated.  We conducted spectral analysis of measurements taken by the Visual/Infrared Mapping Spectrometer on Cassini.  Like a regular digital camera, VIMS captures a scene of pixels of Saturn or one of its moons, but instead of just three colors (RGB), VIMS returns a full spectrum for each spatial pixel, 352 wavelengths from 0.35 micrometers to 5.1 m.  This is useful because light reflected from an object varies in intensity with wavelength, as certain wavelengths are absorbed by the chemicals on the reflecting surface. Scientists graph the intensity of reflected light as a function of wavelength, and look for absorption features (dips) corresponding to known chemicals.

We’ve known for a while that the bright side of Iapetus is mostly water ice.  It’s bright to our eyes (in the visible wavelength range), but it has a large absorption band and is very dark between 2.6 and 3.7 m, absorption caused by water ice. But the dark stuff on the opposite side has defied definitive spectral analysis so far.  Part of the reason is because the dark stuff is likely made up of a complex mix of different organic compounds, which have spectra that are less well understood individually, and downright messy to identify in complex mixtures.  But the main reason it hasn’t been figured out yet is simply because we didn’t have the measurements required to do the detailed spectral analysis. Until Cassini.

Doing spectroscopy from Earth is limited by our atmosphere, which only lets through certain wavelengths and absorbs much of the infrared light that contains the telltale features of many interesting molecules.  Even telescopes launched into space, like the new Spitzer infrared telescope, are still only able to see moons of the outer planets as single unresolved pixels from Earth orbit.  By sending a spacecraft out to Saturn, we have been able to acquire measurements over a large wavelength range, but also with the excellent signal to noise ratio and spatial resolution of having the camera right there on the scene.  And since Cassini was designed to go into orbit around Saturn, instead of just flying past like the Voyager probes, we can get multiple passes at each satellite.

From Iapetus, we got our closest look ever just at the turn of the new year.  On 31 December 2004, Cassini’s elongated orbit of Saturn took it out 3.5 million km, where it slowed nearly to a stop just as Iapetus came coasting by in its orbit.  This close encounter was more leisurely than most flybys, and Cassini took four closeup mosaics-with both its main ISS imager and the VIMS spectrometer—of the leading hemisphere, before beginning its fall back in toward Saturn.  In the current Cassini mission plan, we won't get another look this good at the dark side of Iapetus.

And so I’ve had the privilege to work with unprecedented measurements, data full of significant discoveries just waiting to be made.  My particular part of the VIMS team, including Dr. Cristina Dalle Ore and led by Dr. Dale Cruikshank at NASA’s Ames Research Center, is interested in the chemicals and chemical processes that take place on small, cold bodies of the outer solar system—especially chemicals associated with life.

We looked in the spectrum of Iapetus for an absorption band caused by PAHs, between 3.23 and 3.35 m, and we found it. Not only that, we could pinpoint it to specific regions on the surface, map the areas where it is most concentrated. VIMS doesn’t have the great spatial resolution that Cassini’s main camera has (our scene is only 64 x 64 pixels at best), but it’s clear that this stuff is concentrated in and around two large impact basins on the leading hemisphere of Iapetus; and, most importantly, it was detected in the same places in measurements spaced several hours apart.

So what is the significance of this? PAHs are complex organic molecules.  Not exactly amino acids or nucleic acids, but certainly more interesting than water ice or dry ice.  To find them on a satellite in the outer solar system that is frigid and isolated is an indication that at least some level of organic chemistry is going on out there.  This discovery will not only help us understand the composition and perhaps the origin of the dark material covering one side of this 8-ball satellite, but may also helps us understand the chemical processes that lead from the raw materials of life to cells and scientists and students.  Not LGMs, but certainly an important step in the Search.

And as if that’s weren’t exciting enough, we’ve also found this stuff on Phoebe, a more distant, irregular satellite in a retrograde orbit (indicating that it was likely formed somewhere else and captured by Saturn long ago).  So now I get to tell everybody, “We’ve found polycyclic aromatic hydrocarbons on Phoebe!”

(1)  Pronunciation is: POL i - SIK lik AR o - MAT ik HID ro - KAR bons