If you’ve followed the mission or any of the recent popular science pieces on it, you know that the probe is being dropped into Saturn’s atmosphere to protect the moons Enceladus and Titan from any potential contamination: these two worlds, in very different ways, proffer two of the solar system’s best candidates for extraterrestrial life and scientists hoping for future missions to explore them wish to minimize the chance that any life found comes from us. So, the solution is to crash the probe on Saturn, which shows every indication of being entirely inhospitable to life as we understand it.
In all of the articles I’ve read about this, the story ends with the loss of signal from Cassini and perhaps descriptions of scientists shutting down computers, turning off monitors, rolling chairs under desks, and walking out of Mission Control to drink a solemn toast to a collective job well done. But Saturn, while unwelcoming to life, is far from dead; it is a rich, complex, and almost unimaginably vast system, a churning, roiling, rapidly rotating ball of gas -- with a (relatively) little bit of rock. What happens to Cassini after transmission ceases? This has been a fascinating question to me and a little reading and Googling has yielded some interesting possible answers; here is what I imagine based on what I found.*
Science journalists describe Cassini’s last waking moments: edging into Saturn’s upper atmosphere at tens of thousands of miles per hour, it begins to be buffeted and tossed about. Its aerodynamics pull it to begin to tumble, but its stabilizers, at first, keep its antenna pointed at Earth, but this does not last long. In short order, the crashing of the wisps of gas at horrendous speeds pummel the craft until its ability to direct itself -- designed for near-vacuum -- is overwhelmed and Cassini tumbles and (some light-minutes further/later) Earth loses its signal.
Almost immediately after this, as the drag on Cassini brings it ever more quickly into thicker and thicker air, the probe begins to break up, heat up, and vaporize. Different parts will presumably respond differently to the atmospheric insults and, as bits break off and apart, some will slow more quickly than others, and what remains of Cassini will, after probably no more than a few dozen seconds, slow and cool enough to become a minor rain of detritus falling into Saturn’s cloudtops. I imagine some of these as relatively light pieces of framework or housing, unmassive enough to decelerate rapidly enough to avoid completely vaporizing, as well as various chunks of dense, durable stuff -- the most formidable of which might be whatever remains of Cassini’s power core: 66 pounds of plutonium.
As these bits variously plummet, tumble, or float their way into Saturn’s depths, they will pass first through its upper layers with clouds of ammonia and water ice and bands of ammonium hydroxide. As pressure in these upper layers increases, water can form droplets and mix with ammonia. Saturn is, however, by far mostly hydrogen and it is through this that Cassini’s remnants will pass for a very long time. The outer layers are gaseous hydrogen, but, as pressure increases, this becomes liquid and, eventually, metallic hydrogen. As the scattered bones of Cassini works their way down through the gas and liquid hydrogen, its densest pieces may come to rest somewhere around the transition from liquid to metal, some 18,000 vertical miles from where it sent its last radio pulse.
Think about this: Earth is about 7,900 miles in diameter; that means that the shards of Cassini could fall through two and a half Earths before stopping. For how long will they fall? Years? Decades? Centuries? Millenia? Just imagine...
One question I didn’t get an answer to (mine was not an exhaustive search) was at what point might different parts float -- particularly the plutonium? Metallic hydrogen can, in theory (at least as I understand the lay descriptions), exist as a liquid under sufficient pressure: would plutonium float in this ocean metal if it made it down that far? Or would it continue falling? Down, down below inconceivable depths of the most abundant constituent of the universe, there is probably a hunk of iron, nickel, and rock: Saturn’s core. might the densest pieces work their way through the metallic hydrogen, all the way down to stone? Also, what (if anything) happens when radioactive material like Cassini’s heart comes in contact with a highly conductive material like metallic hydrogen? And is it possible -- even likely -- that Saturn’s core already has at least a few dozen pounds of radioactive elements? The metallic-rocky core may be larger than the Earth: what would that look like?
In my dreams tonight, I imagine Cassini’s bones wafting through ammonia clouds and currents of liquid hydrogen. May they find a well-deserved and worthy rest there.
*NOTES: This is pure speculation on my part, based on a relatively cursory search of a handful of what I perceive to be relatively reliable sites (e.g., NASA, Wikipedia, etc.). I’d actually be thrilled if a real planetary scientist did this thought experiment! Also, I normally hyperlink my essays extensively, but, tonight, I'm sleepy and actually just proud I got this written and posted. I invite anyone to explore and/or correct any of the facts I list or ideas I have shared.