NASA study finds unlikely place to search for water: Uranus' icy moons

The large moons of Uranus

Out of 27 moons, four may contain oceans several miles deep (in a layer between their cores and icy crusts). The team investigated the interior structure and composition of all large moons — Ariel, Umbriel, Titania, Oberon, and Miranda — in this study. 

Titania, which is nearly 982 miles(1,580 kilometers )across, has primarily captured the attention of astronomers in recent years. Because of its enormous size, it is expected that the moon will be able to retain internal heat, implying that the liquid water will not freeze.

“When it comes to small bodies – dwarf planets and moons – planetary scientists previously have found evidence of oceans in several unlikely places, including the dwarf planets Ceres and Pluto, and Saturn’s moon Mimas,” said Julie Castillo-Rogez of NASA’s Jet Propulsion Laboratory in Southern California, and study’s lead author, in an official release. 

She further added: “So there are mechanisms at play that we don’t fully understand. This paper investigates what those could be and how they are relevant to the many bodies in the solar system that could be rich in water but have limited internal heat.”

What lies beneath?

To investigate this, the team built computer models using data from NASA's Galileo, Cassini, Dawn, and New Horizons spacecraft (each of which is credited with discovering ocean worlds). The team specifically infused information on the chemistry and geology of Enceladus (Saturn’s moon), and Charon and Ceres (Pluto’s moons). These icy moons were chosen because they are the same size as the Uranian moons.

The model revealed several parameters that could indicate the presence of liquid water beneath the icy crust. One of them was the porosity of the moon's surface, which could mean the moon's insulation property, revealing the internal heat required for the existence of liquid water. 

They also investigated additional potential heat sources in the rocky mantles of the moons. According to the authors, only internal heat would be insufficient to sustain the moon's subsurface ocean.

The modeling revealed a high concentration of chlorides and ammonia beneath the surface. Ammonia has anti-freezing properties. While it also suggested the presence of salt, which could prevent the water on the icy moons from freezing. 

However, the team claims that there are many unanswered questions about the planet's large moons. “We need to develop new models for different assumptions on the origin of the moons in order to guide planning for future observations,” said Castillo-Rogez.

The new research could aid in the development of specific scientific instruments to better understand the chemistry occurring beneath the surface of Uranus's moons for future missions.

The study has been reported in the Journal of Geophysical Research. 

Study Abstract:

The five large moons of Uranus are important targets for future spacecraft missions. To motivate and inform the exploration of these moons, we model their internal evolution, present-day physical structures, and geochemical and geophysical signatures that may be measured by spacecraft. We predict that if the moons preserved liquid until present, it is likely in the form of residual oceans less than 30 km thick in Ariel, Umbriel, and less than 50 km in Titania, and Oberon. The preservation of liquid strongly depends on material properties and, potentially, on dynamical circumstances that are presently unknown. Miranda is unlikely to host liquid at present unless it experienced tidal heating a few tens of million years ago. We find that since the thin residual layers may be hypersaline, their induced magnetic fields could be detectable by future spacecraft-based magnetometers. However, if the ocean is maintained primarily by ammonia, and thus well below the water freezing point, then its electrical conductivity may be too small to be detectable by spacecraft. Lastly, our calculated tidal Love number (k2) and dissipation factor (Q) are consistent with the Q/k2 values previously inferred from dynamical evolution models. In particular, we find that the low Q/k2 estimated for Titania supports the hypothesis that Titania currently holds an ocean.