ROBEX Part 3: Hammering for science
Mount Etna is very close to being a substitute Moon for the planetary researchers of the ROBEX project – it is here that they find volcanic basalt in quantities similar to those of the constituents of the lunar soil. In addition, the Sicilian volcano is the only place in Europe that experiences earthquakes at a depth of up to 600 kilometres. Mount Etna is like the Moon in this respect as well, as Earth's lunar companion experiences most earthquakes 700 to 1100 kilometres below the surface. "Etna is the only place in Europe where we can measure earthquakes at a similar depth," says planetary researcher Martin Knapmeyer.
Two experiments are to be conducted on the mountain; in the active measurements, the scientists use a hammer to strike the surface and seismometers to measure the transmission of the sound waves through the ground. The volcanic ash of 2001 rests on top of a more solid layer in Piano del Lago, so waves that take different routes through the surface material arrive at the seismometers at different times, providing information on the structures below the ground. The passive experiment uses four measurement stations to listen to the processes inside the volcano.##markend##
The sensor units used to do this are small but effective – stable and yet light at the same time – their structure resembles that of a half-timbered house. The DLR Institute of Composite Structures and Adaptive Systems used the same principle in the MASCOT lander that has been dispatched towards the asteroid Ryugu. These units are equipped with sensors with sufficient sensitivity to register earthquakes several thousand kilometres away. The sensors even detect the wind that – despite the bright sunshine – continues to buffet Mount Etna in stormy gusts.
Blow by blow
The Rover LRU 2 is designed to collect the sensor units from the lander and then position them autonomously at their designated positions. Today, though, this task will be completed by human hand for the first measurements, while the rover is being prepared for its mission. Sabrina Schwinger grasps the five-kilogram hammer. The team members Martin Knapmeyer, Frank Sohl, Alexandra Heffels and Sabrina Schwinger spent time in Berlin, practising how to strike the ground as powerfully and evenly as possible. A 50-metre route of measurement points separates them from the lander. The mobile measurement station and its sensors are initially defined as point zero. Sabrina pauses briefly, the hammer aloft. But then the yellow head crashes down with a muffled bang onto the aluminium plate at her feet. The sound passes through the dust and is picked up by the sensors. The dull thuds of 20 blows resound across the plain, while a black ash cloud rises upward from Etna's crater, as if the volcano were trying to respond.
Caroline Lange receives confirmation via radio; the team from the DLR Institute of Space Systems – that also built the sensor units – has received the data from the mobile measurement stations, transmitted by the lander to the control room in Catania, 23 kilometres away. Receiving a 'thumbs up', the planetary researchers bring the sensor unit to the next measurement point and put it in place for the next cycle. Then everything starts afresh; Frank Sohl takes over – the hammer swings through the air and hits the plate with a muffled thud another 20 times.
Technology for the Moon
The important factor later on during evaluation will be to determine when the sensors recorded which vibration. The sound passes through the basaltic rock at speeds of six to eight kilometres per second, but only 'sneaks' through the volcanic ash at a few 100 metres per second. "Our hammer blows on Earth mimic the meteorite strikes that we want to record and evaluate on the Moon," says Martin Knapmeyer. Until now, seismic measurements on the Moon's surface have only been performed once before. In 1972, astronauts Eugene Cernan and Harrison Schmitt conducted measurements as part of the Apollo 17 mission. ROBEX is intended to demonstrate that the right technology can be used to obtain additional knowledge about the Moon's crust and its interior.
Again, the hammer blows reverberate across the valley. By the end of the day, the planetary researchers will have sent 120 meteorite impacts through Mount Etna. Next time, the rover will be used to transport the sensor units from measuring point to measuring point.