Horizon 2020
The EU Framework Programme for Research and Innovation

Saturn’s moon Mimas could be hiding an underground ocean

Does Saturn’s moon Mimas have an underground ocean? EU-funded researchers think this is a distinct possibility. The clues are in the way the moon wobbles, which scientists now understand better. This breakthrough and others made by the ESPaCE project are providing new insights into the origins of our solar system.
The network of control points on the surface of Mimas, from which it was possible to determine the rotational motion of the body.

It was by monitoring the rotation of natural satellites – the moons that orbit planets and other celestial bodies – that ESPaCE’s researchers were able to detect oscillations, or deviations in their spinning motion. Insight into the reasons behind such wobbles pointed to the possible existence of an ocean 30 km below the crust of one of Saturn’s moons – Mimas. The alternative explanation for Mimas’ movements is that it has an elongated, rugby-ball-shaped core.

This finding sparked interest within the space community and has been reported by news agencies the world over. It puts Mimas on the list of potential ‘life-friendly’ environments in our solar system. These include several of Jupiter’s moons, and two other moons of Saturn, Enceladus and Titan.

The key to the discovery is ‘ephemerides’ – a scientific term that refers to a mathematical calculation primarily used to determine the positions and rotations of natural satellites, such as moons and planets, as they orbit another body in space.

It was developing new ephemerides that provided the insights into Saturn’s moon. “If you want to predict the motion of a moon, you need to take physics into account,” says Valéry Lainey of the Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE) – the ESPaCE deputy coordinator. This includes gravity, an object’s orientation in space and tidal effects, for example.

What scientists monitor is the sum of all of these effects. Imagine throwing a ball through the air, for example. If this ball contains water, the way it rotates through the air will be different to how it would spin if the centre was completely solid.

Mapping by numbers

Space scientists can use ephemerides to locate where a planet, comet or satellite will be at defined points in the future and how it should rotate. Ephemerides are essential to the quest to learn more about our solar system – if scientists want to collect data on a moon by tracking it, a manoeuvre known as a ‘fly-by’, they need to know exactly where that moon will be. They then need to program a spacecraft so that it tracks the target from an appropriate distance.

Knowing the ephemerides therefore increases the chances of data collection during a space mission. The calculations also ensure a spacecraft has the right propulsion to actually reach a target moon, and that instruments on board are oriented correctly to collect data across the distance between the spacecraft and the satellite.

The ESPaCE project brought together seven universities and research centres with different specialisations to find new ways to calculate these precious ephemerides and other reference systems for natural satellites and spacecraft.

Going back in time

At the heart of the project is data collected from space missions – some of it never used before. This is combined with ground-based data collected from observatories – some of it archival – such as photo plates. Going back in history provides the long timespan necessary to identify tiny motion changes, such as acceleration or deceleration. It is by using this information that researchers can calculate the locations of distant moons.

Some observatories, particularly in the US, have large numbers of photographic plates that have never been analysed using modern computing techniques. Scanning the plates is very time-consuming, but the effort has been worthwhile, says Lainey.

The team found that the accuracy provided by some old images captured from observatories here on Earth is comparable with that of space missions around Mars in the 1970s, says Lainey. “This is amazing because these images were captured very far away compared to space missions,” he adds.

But the researchers behind ESPaCE also expect the impact of the project to be visible long after the news reports have been forgotten. Europe is behind the US in terms of ephemerides, and it is quite clear why. The US’ Jet Propulsion Laboratory has many people working on various aspects of moons and satellites, and all in the same place. “In Europe, the staff are dispersed, says Lainey. “We wanted to see how these people could work together and how their separate data sets could be used.” In doing this, the ESPaCE has created Europe’s first network of space scientists working on ephemerides.

The different partners have concluded that this approach definitely works, and are planning to continue their collaboration after the end of ESPaCE.

Project information