A reanalysis of data from NASA’s Voyager spacecraft, along with new computer modeling, has led NASA scientists to conclude that the four largest moons of Uranus likely contain a layer of ocean between their cores and icy crust.
Their study is the first to detail the evolution of the internal composition and structure of all five large moons: Ariel, Umbriel, Titania, Oberon and Miranda. The work suggests that four of the moons contain oceans that could be tens of miles deep.
In total, at least 27 moons orbit Uranus, with the four largest ranging from Ariel, averaging 720 miles (1,160 kilometers) to Titania, averaging 980 miles (1,580 kilometers). Scientists have long believed that Titania, given its size, would most likely retain internal heat from radioactive decay.
The other moons were previously generally thought to be too small to retain the heat necessary to keep the internal ocean from freezing, especially since the heating produced by Uranus’ gravitational pull is only a minor source of heat.
“When it comes to small bodies dwarf planets and moons planetary scientists have previously found evidence of oceans in some unlikely places, including the dwarf planets Ceres and Pluto and Saturn’s moon Mimas,” she said. “So there are mechanisms at play that we don’t fully understand.” This paper explores what these might be and how they are relevant to the many bodies in the Solar System that may be rich in water but have limited internal heat.
What lies above and below
The researchers used this modeling to measure how porous the surfaces of the Uranian moons are, and found that they are likely insulated enough to retain the internal heat that would be needed to host an ocean. They found what could be a potential source of heat in the rocky mantles of the moons.
By examining the composition of the oceans, scientists can learn about the materials that could also be found on the icy surfaces of moons, depending on whether the substances beneath have been pushed up from below by geological activity. There is evidence from telescopes that at least one of the moons, Ariel, has material that flowed onto its surface, possibly from ice volcanoes, relatively recently.
Miranda, the innermost and fifth-largest moon, also hosts surface features that appear to be of recent origin, suggesting that it may have had enough heat to sustain an ocean at some point. Ammonia has long been known to act as an antifreeze. In addition, modeling suggests that salts likely present in the water would be another source of antifreeze, maintaining the bodies’ internal oceans.
Of course, there are still many questions about the large moons of Uranus, Castillo-Rogez said, adding that there is still a lot of work to be done: “We need to develop new models for different assumptions about the origin of the moons in order to guide the planning of future observations.”
The surface of different moons
Digging into what lies beneath and on the surface of these moons will help scientists and engineers choose the best scientific instruments to explore them. For example, determining that ammonia and chlorides may be present means that spectrometers that detect compounds by their reflected light would have to use a wavelength range that covers both kinds of compounds.
Similarly, they can use this knowledge to design instruments that can probe liquid in the deep interior. Looking for the electrical currents that contribute to the moon’s magnetic field is generally the best way to find the deep ocean, as scientists from the Galileo mission did on Jupiter’s moon Europa.
However, the cold water in the interior oceans of moons like Ariel and Umbriel could make the oceans less able to carry these electrical currents, presenting a new kind of challenge for scientists trying to figure out what lies beneath.
