The hint comes in the form of a ratio. All elements have a certain number of known isotopes — variants of that element with the same number of protons that differ in their number of neutrons. The ratio of one isotope to another isotope is a crucial diagnostic tool.
In planetary atmospheres and surface materials, the amount of one isotope relative to another isotope is closely tied to the conditions under which materials form. Any change in the ratio will allow scientists to deduce an age for that material...
"When we looked closely at how this ratio could evolve with time, we found that it was impossible for it to change significantly," Mandt said in a press release. "Titan's atmosphere contains so much nitrogen that no process can significantly modify this tracer even given more than four billion years of Solar System history."
Pluto, a distant icy dwarf planet, orbits the Sun 29 times farther out than the Earth and has estimated surface temperatures of -380 degrees Fahrenheit (-229 degrees Celsius). These frigid temperatures are far too cold to allow liquid water on Pluto's surface. Its location and small size make it very difficult to observe; however, with NASA's New Horizons mission slated to reach the distant world next year, scientists hope to map Pluto and its moons in great detail.
Current models predict one moon in particular, Charon, is of great interest to study. The models indicate Charon has surface fractures, indicative of a possible subsurface ocean. Further analysis is needed to determine in the moon's interior is warm enough to support liquid water.