It’s hard to believe it’s been a full decade, but it really has. Cassini entered orbit around Saturn on July 1, 2004, and almost six months later, it was in position to release its passenger. The Huygens probe was released on Christmas Day, 2004, with batteries fully charged off of Cassini’s plutonium RTG, and on January 14, 2005, it reached Titan.
Entry interface is always an exciting time for any planetary spacecraft, because it’s one of the most dangerous moments in a spacecraft’s mission and, perversely, also the one where mission control has the least opportunity to react to events in time to save the mission. For Mars landers, when you hear the signal that the probe has begun its descent, you know that win or lose, it’s actually already over. With Huygens, this time-delay issue was even worse — over an hour one-way. Given the dense, thick atmosphere (much deeper and denser than our own) and the much lighter gravity, descent was expected to take up to 2.5 hours. All controllers would have would be the carrier signal from Huygens, playing back events transmitted an hour earlier, too faint for Earths’ telescopes to make out the details — Huygens’ real audience was the Cassini orbiter, which would relay the data later. (But not as well as hoped; more on that later.)
It was a fall into the unknown. Over the next decade, Cassini would develop a good picture of Titan via infrared and radar imaging, but none of that had happened yet; they had data from a single pass just two months before release, and that certainly wasn’t enough to even help them target the probe. Knowing nothing about the surface of Titan, they designed the probe to float if it landed in water, and stay mostly upright if it landed on a solid surface, exploiting the thick atmosphere to ensure a gentle touchdown in any case. The descent went better than anyone could have expected, and Huygens not only touched down successfully on the surface of Titan (verdict: solid, but very definitely wet, the first clear evidence of surface liquid other than lava on another planet) but continued to transmit for at least 90 minutes longer. Unfortunately, human error caused much of its data, stored aboard Cassini, to be lost. But what it did return was spectacular.
The rocks in the foreground are small pebbles, smooth and rounded like riverbed rocks, and with telltale depressions from past liquid flowing around them. It looks for all the world like the bed of a river that has just stopped flowing, but is still damp. Huygens cooled rapidly after touchdown, as if it were touching liquid, and the telemetry suggested it shifted as if sitting in sand.
Put this together with everything Cassini’s gleaned over a decade of close orbital passes, gravimetric studies, radar observations, and IR photography, and it’s clear that Titan is the only other place we know with a hydrological cycle — except that it’s different there, because it’s not water that flows. It’s hydrocarbons, and very likely methane in particular. The rock is water ice. And imagine what any inhabitants of Titan would think of our world! A place with seas of molten ice!
Yet despite all that, in many ways it’s the most Earthlike place we know. It has seas, with complex shorelines and archipelagos. It’s got moisture that evaporates into the atmosphere, rises, forms clouds, drifts, and then rains down again. It has erosion by both wind and rain.
I hope we send another spacecraft there someday. It seems like an amazing place.