Saturn's moon Enceladus is a thrilling target in the search for life in the Universe. Under its surface, which consists of a massive ice sheet, there are large bodies of water which scientists suspect may be populated by living microorganisms. A space mission called Enceladus Explorer (EnEx) is to investigate that question.
Yet the technical challenges involved are enormous: while the movement of an ice probe can be controlled relatively easily here on Earth, systems designed for use on distant heavenly bodies must be capable of operating largely autonomously. Under the EnEx-RANGE project (Robust Autonomous Acoustic Navigation in Glacier Ice), scientists of RWTH Aachen are developing a localisation network of autonomous melt probes, in which all probes that belong to the network can spot each other by acoustic signals.
Each network probe is equipped with a transmitter and receiver. Mutually exchanging signals permits automatically determining the position of every single probe in the ice at high precision. Flexible and efficient data systems register these signals, control the depth to which a probe has penetrated the ice, and evaluate all recorded data. There are two things that are particularly important in the investigation: ensuring optimum localisation quality, and ensuring the best possible coverage of the area under scrutiny. To be able to do this in spite of possible interferences and failures, each melt probe autonomously optimises its own position in the ice.
An ice mole helps in exploration
The chief purpose for which the RANGE system will be employed is localising, positioning, and navigating the EnEx-IceMole - a melt probe that is capable of manoeuvring in ice, collecting water samples from crevasses and transporting them to the surface. The EnEx-IceMole was developed under the DLR Space Administration's Enceladus Explorer project, in which six German universities developed the initial technological groundwork for the future EnEx space mission in the period from February 2012 to March 2015.
During a test drill that took place in an Antarctic glacier in November 2014, the position of the EnEx-IceMole in the ice was determined on the basis of run-time measurements of acoustic signals. Signals sent by sound generators to the surface of the glacier were registered by receivers in the EnEx-IceMole. Scientists determined the position of the probe by converting measured signal run times into distances, followed by trilateration.
Exploring the environment with ultrasound
This acoustic localisation system is now being developed further under the RANGE project. The objective is to enhance its sensitivity and reduce the size of the electronics while at the same time increasing its performance. The intention is for the system to set up a self-optimising localisation network that is distributed around a body of ice in three dimensions. The ultrasound system integrated in the melt head of the EnEx-IceMole will be improved as well. It enables the probe to recognise obstacles like rocks or cavities from a distance, as well as spotting interesting regions like water-filled crevasses in the ice. Such crevasses will be approached selectively and the water they contain thoroughly tested for microorganisms.
In this preliminary study, similar to a medical ultrasonic scan, special ultrasonic arrays use directed sound signals to examine the environment, receiving the echoes reflected by any structural transitions. Building on the results of the previous EnEx project, data quality is being improved by more efficient hardware. Moreover, it is hoped that using optimised algorithms will permit extracting more information from the data.
Tests in an indoor pool and an alpine glacier
Regular test measurements in water and ice enable scientists and engineers to test the RANGE system and improve it quickly step by step. Tests are being carried out in the facilities of RWTH Aachen, the Ulla Klinger indoor pool at Aachen, in Lake Rursee near Simmerath, and on the Hintereisferner glacier in Austria.
Within RWTH Aachen, the 3rd department of Physics collaborates with the IMA/ZLW & IfU Cluster which includes the Instititute of Information Management in Mechanical Engineering (IMA), the Centre for Learning and Knowledge Management (ZLW), and the associated Institute of Business Cybernetics e.V. (IFU). Forming part of the EnEx initiative of the DLR Space Administration, the project is sponsored by DLR with funds from the Federal Ministry for Economic Affairs and Energy (BMWi).