Dr. Bonnie Buratti Looks Back at Iapetus

Todd J. Barber, Cassini lead propulsion engineer

Continuing the format of my prior column, I decided to interview Bonnie Buratti, a key member of the Cassini science team and an expert on small icy solar system bodies. We had a wonderful phone chat about the latest thinking regarding Iapetus, our shared commitment to outreach, and the allure and appeal of space science. Bonnie has been on Cassini for many years, as I have, but sometimes team members don't get much of chance to interact on this rather large planetary mission. I am the better for my brief conversation with Dr. Buratti, yet another example of the hallmark cooperation between science and engineering that permeates the Cassini mission. I've heard her speak so eloquently on many occasions, but this was a rare opportunity for a more personal exchange.

Hardly able to contain my excitement, I asked Bonnie for her scientific take on Cassini's recent encounter with bizarre Iapetus. She remarked that this world was "more amazing than she would have thought" and how surprising the close-up images were, showing patterns of black and white segregation down to the smallest distance scales. How familiar this feeling is when seeing high-resolution images of a solar system body for the first time! Is there no end to the ability of planetary science to leave us scratching our heads in puzzlement? The key questions now about Iapetus are where did the dark material originate, and how did it form such intricate patterns. Most scientists, Bonnie included, believe it is exogenic ("It Came from Outer Space," to shamelessly parrot a 1953 movie title), but its ultimate source is unknown. The dark material is not red enough to match another vexing moon of Saturn, Phoebe, and other distant satellites in retrograde orbits of Saturn probably don't have enough material to blanket Iapetus (and possibly Hyperion). Bonnie wonders if enigmatic Titan could be the source of the material, a theory I hadn't heard before, off in my engineering corner.

I asked Bonnie about early thoughts regarding the chemistry of the dark material, and without hesitation she confirmed the presence of hydrocarbons, even polycyclic aromatic hydrocarbons (PAHs). These intriguing molecules are nothing less than the suspected building blocks of the molecules of life, so their incidence raises more than a few eyebrows among scientists attempting to understand the origins of life on early Earth. Seemingly impossibly, Iapetus just became a lot more interesting. Bonnie also told me about an early theory for the sharp boundaries between light and dark areas, even in the finest resolution images. She explained the scientific theory of thermal segregation even in a way an engineer could understand, for which I was grateful. Dark material that falls in a crater, say, will experience much more warming due to solar radiation, just by virtue of its color (anyone ever wear a black t-shirt on a sunny day?). This heating may well evaporate water ice, which then may settle as white frost on cooler areas, which are by definition away from the darker areas. She made it sound so simple! Given her experience and panache with outreach, I shouldn't have been surprised. Bonnie also confirmed the Iapetus mountain peaks seen by Voyager were indeed part of the "belly band" of Iapetus, but I was surprised when she told me this ancient equatorial ridge may show signs of landslides. Could this moon be any more peculiar? Bonnie's final comment brought a smile to my face, as one of many engineers helping to keep Cassini scientifically productive. She closed by saying that we were "now very data rich," with enough raw data to keep scientists and modelers busy for many years. With safing and Iapetus behind us, I bid Bonnie adieu and returned to my "day job" of propulsion engineering for this wondrous spacecraft.