The latest research from NASA shows that the internal structure of Saturn's moon Titan, which has long been thought to hide a global liquid ocean under its icy shell, may not be as "oceanic" as previously thought, but more like a "cosmic shaved ice" of unprecedented scale - a semi-solid mud composed of high-pressure ice and rock.
The study is based on a reanalysis of data taken by the Cassini spacecraft during its 2012 flyby of Titan. For many years, the scientific community has generally believed that many icy moons in the outer solar system have a huge underground ocean sandwiched between thick icy shells and rocky cores. Since 2008, Titan, which is shrouded in a methane atmosphere, has also been included in this "ocean world" candidate list. However, the latest modeling results suggest that Titan's interior may not be filled with large areas of free-flowing liquid water, but rather a semi-solid mixture that resembles a frozen dessert.
The research team used NASA's Deep Space Network (DSN) to measure the Doppler frequency shift of the radio signals sent back by Cassini when it orbited Saturn and passed by Titan. By analyzing these frequency shifts, scientists were able to infer Titan's gravitational pull on the detector and further determine the degree of "softness" of the satellite under the influence of Saturn's tides, that is, the extent and speed of its shape changing with the pull of the tides. Earlier analysis believed that Titan can respond to Saturn's tidal changes almost instantly and complete its deformation almost simultaneously during its elliptical orbit. The absence of an obvious time lag is regarded as important evidence for the existence of a large-scale liquid water layer inside it. Coupled with previous estimates of stiffness (rigidity) and the deduction that sufficient heat is retained inside, a global underground ocean once became a mainstream hypothesis.
However, new analysis using more complex models overturns this conclusion. Researchers found that Titan does not "keep up with the beat" in its tidal response, but has a lag of about 15 hours. This phenomenon neither conforms to the characteristics of a typical liquid internal structure, nor can it be explained by a completely solid interior. However, it is highly consistent with a "semi-solid" and "mud-like" internal environment. New calculations point to deviations in earlier understandings of Titan's stiffness, which have been revised to more closely match an internal structure composed of a mixture of ice and rock with an overall low-viscosity "mushy" structure. At the same time, the data also shows that Titan's interior has the ability to effectively transport heat from the core outward, causing local areas to refreeze, thereby forming a thick "rock ice and mud zone" between the hard ice shell and the rock core.
According to the latest models, Titan's so-called "ocean" is closer to an icy slush composed of high-pressure phase ice VI and ice VII, mixed with rock debris and filled with many pockets of warm water. Both Ice VI and Ice VII are water ice that remains solid or semi-solid under great pressure, and the temperature in these water pockets can reach about 20 degrees Celsius (68 degrees Fahrenheit). The research team pointed out that if minerals in rocks can migrate into these small "pockets" that are relatively warm and rich in liquid water, theoretically, conditions may exist to breed primitive microbial life, although this idea is still far away from the actual discovery of life.
Flavio Petricca, a postdoctoral researcher at NASA's Jet Propulsion Laboratory (JPL) who is responsible for the relevant analysis, said that the scientific community did not expect such a strong energy dissipation phenomenon inside Titan. By further reducing the "noise" in the Doppler data, the research team was able to capture subtle signal fluctuations that had been previously obscured. These "small ripples" became key "smoke evidence" proving that Titan's internal structure is completely different from previous understandings. Petricca explained that the low-viscosity rock-ice slurry not only allows Titan to bulge and compress significantly under the influence of Saturn's tides, but is also efficient enough to dissipate internal heat, thereby preventing large-scale melting from forming a truly global liquid ocean and maintaining the moon in a "semi-melted" edge state for a long time.
In addition to its scientific significance, this metaphor of "the largest shaved ice in the universe" also adds a touch of humor to future detection missions. The article joked that if this "mud sea" model is finally confirmed by further observations, then the mission team heading to Titan in the future may need to consider not only how to safely dive between high-pressure ice and rock mud, but also "incidentally discuss" which flavor of syrup should be carried to best match this planet-level "smoothie". Relevant research has been published in the magazine "Nature", and a research briefing issued by NASA announced this latest result.