Enceladus, only about 500 km in diameter and once believed too small to be active, has been found one of the most geologically dynamic objects in the Solar System. An icy moon of Saturn, it is considered along with Jupiter’s Europa one of the prime targets for the search for life in the solar system. In 2006, the Cassini spacecraft discovered water plumes including simple organic compounds emanating from geysers, or “cryovolcanoes”, from within canyons on the south pole of that moon.
A 10km-deep ocean of water, larger than Lake Superior under the south pole of Enceladus was recently confirmed by Cassini measurements. This means that there is a very strong possibility that these plumes are in direct contact with, and fed by, the ocean.
Figure 1: An artist’s impression of the interior of Saturn’s moon Enceladus, based on recent data from the Cassini space probe suggesting the moon contains a water ocean beneath its south pole. Credit: Nasa/JPL-Caltech
For this recently discovered ocean to be a potential habitat for life as we know it three conditions must be satisfied: the existence of liquid water, an energy source, and the chemicals necessary for life. The interaction between the recently confirmed ocean and the moons rocky core, means that very possibly also the latter two conditions are met for Enceladus.
Water from the ocean is assumed to upwell through cracks in the ice and then eject into space through the plumes. This water would remain liquid up to shallow depths in the order of even of tens of meters. Such reservoirs of liquid water under the ice near one of the geyser sources would thus make a prime target in the search for extraterrestrial life, and would be much more easily accessible than the ocean itself. Biomarkers and even existing microorganisms would not have been destroyed by exposure to vacuum.
To access these potential habitat of extraterrestrial life, the Enceladus Explorer (EnEx) project sponsored by the German DLR aims to design a mission to Enceladus, as well as to develop an operable drilling technique to penetrate the icy surface of the moon using the IceMole, a maneuverable subsurface ice melting probe for clean sampling and in-situ analysis of ice and subglacial liquids. Its design is based on the novel concept of combined melting and drilling (or – more precisely – screwing) with a hollow ice screw – as it is used in mountaineering – and a melting head at the tip of the probe. The screw presses the melting head firmly against the ice, which optimizes conductive heat transfer into ice and aids steering in the desired direction. The IceMole can change melting direction by differential heating of the melting head, forcing the probe into a curve.
Figure 2: The IceMole in field testing on the Morteratsch glacier in Switzerland by the FH Aachen development team. Credit: FH Aachen
The general mission concept is to land EnEx at a safe distance from an active plume. The IceMole would then be deployed, melting its way through the ice crust towards the target water pocket at a depth of up to a couple of hundreds of meters for an in situ examination for the presence of microorganisms and biomarkers.
The EnEx mission is comprised of a Lander carrying the IceMole, and an Orbiter with the main function to act as a communications relay between the Lander and Earth. Due to the very low ice temperatures (100 – 150 K) electrical power in the order of 5 kW is needed to power the IceMole melting head. This large amount can only be provided at this far distance from the Sun by a small nuclear reactor.
Figure 3: Impression of the combined lander and orbiter in orbit around Saturn near Enceladus. The reactor on the lander (front part) powers electric thrusters (rear part) during this phase. The middle elongated structure is there to put distance between radiation coming off the reactor, and sensitive electronics on the orbiter. Credit: Bundeswehr University Munich
After launch, the Lander and Orbiter perform the interplanetary transfer to Saturn together, using the on-board nuclear reactor to power electric thrusters. Once captured in Saturn orbit, the combined spacecraft perform a gravity assisted moon tour in order to reach the orbit of Enceladus, while continuing using nuclear electric propulsion. After orbit insertion at Enceladus the orbiter will perform a detailed reconnaissance of the south polar terrain of the moon. At the end of the reconnaissance phase, the lander will separate from the orbiter and an autonomously guided landing sequence will commence, that will place the lander near one of the vapor plumes within one of the valleys. Once landed, the IceMole will be deployed and start melting through the ice, while navigating around hazards and towards a target subglacial water pocket.
Figure 4: Operations concept for the IceMole on Enceladus. The IceMole melts through the ice while maneuvering around hazards and towards the target water pocket close to a plume. Credit: FH Aachen
Critical areas of technology development for this mission to happen include the development of a small nuclear reactor (several have operated in space since the 70s, but none for landed applications) and the capability to use this reactor to power electric thrusters (so far, only solar power has been used for that purpose). Another critical field is the development of autonomous landing and hazard avoidance systems, to ensure a safe and accurate landing. Navigation techniques for the IceMole are already under active development in the project. For the eventual application of the IceMole on Enceladus, sufficiently miniaturized in-situ instrumentation should be developed.
In summary, the IceMole technology is a viable approach for clean sampling and analysis of deep ice and extraterrestrial subglacial environments. With the recent discovery of plumes on Jupiter’s moon Europa, a shift towards icy moon exploration can be expected. It is clear that with minimum modification, the EnEx mission concept could be applied also there, as also to other areas of interest, such as the north pole of Mars. The IceMole will thus serve as a tool for astrobiological exploration throughout the solar system.
The Enceladus Explorer project is carried out by a research consortium of seven German universities with the Institute for Space Technology and Space Applications (ISTA) of the Bundeswehr University Munich responsible for the overall mission and system design of the EnEx spacecraft. The project is based on an idea and initiative of, and is sponsored by, the DLR German Space Administration.
Links: Presentation at the Habitability of Icy Worlds Workshop in Pasadena including info on EnEx, February 2014 (video). [http://connect.arc.nasa.gov/p7q9c3hujgf/]
EnEx news article on the DLR website [http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10212/332_read-2751/year-all/#gallery/4874]
Links to papers and presentations: Konstantinidis et al., „Enceladus Explorer (EnEx): A lander mission to probe subglacial water pockets on Saturn‘s moon Enceladus for life“, Acta Astronautica, under review
Dachwald et al., “A Maneuverable Probe for Clean In-Situ Analysis and Sampling of Subsurface Ice and Subglacial Aquatic Ecosystems”, Annals of Glaciology, under review
Dachwald et al., “Development and Testing of a Maneuverable Subsurface Probe That Can Navigate Autonomously Through Deep Ice”, Presentation at the 9th International Planetary Probe Workshop, Toulouse, France, 18th – 22nd June 2012 [https://solarsystem.nasa.gov/docs/03_Development%20and%20Testing%20of%20a%20Maneuverable%20Subsurface%20Probe...B.%20Dachwald1.pdf]