Mechanical Properties of the Surface Material of Comet 67P/Churyumov-Gerasimenko Measured By the Casse Instrument Onboard the Philae Lander

Abstract:

The Comet Acoustic Surface Sounding Experiment (CASSE) is housed in the six soles of Philae’s landing feet, thus being the first part of the spacecraft that has mechanical contact with the comet. It consists of three piezoelectric, triaxial accelerometers and three transducers, each of which consists of a stack of 80 piezoelectric elements connected electrically in parallel. This combination allows for both passive listening to the comet and active sounding of the subsurface.

After a calibration during descent, when the answer of the unfolded landing gear to active soundings will be determined, the touchdown is the first event that will be recorded for scientific purposes: The shock experienced by the accelerometers in the first milliseconds after touchdown is indicative for elastic/plastic deformation parameters of the cometary soil. The time series recorded by all three accelerometers will also support the reconstruction of the landing process itself, by indicating the touchdown sequence of the feet, and also a possible activity of the lander’s ice screws.

During the First Science Sequence, expected to last 4 or 5 cometary days, a number of measurements will be conducted. In a sounding experiment, the piezoelectric transducers will send rectangular signals with frequencies up to 640 Hz between the feet. It is expected that both compressional and shear waves will be recorded. Additionally, CASSE will listen to the MUPUS hammering: this is a strong acoustic source of known location, as MUPUS measures its current depth of penetration. Signal travel times between feet and from MUPUS can be interpreted in terms of elastic constants, possibly as functions of depth and time, while the presence and decay of scattered waves depends on structural parameters like porosity and layering on wavelength scale.

After the First Science Sequence, a Long-Term Science Sequence will allow the repetition of measurements in order to monitor possible changes of the comet during its passage towards the sun, and periods of passive listening to the acoustic emissions of the comet itself.

In this contribution, we will give an overview of the data recorded so far, first evaluations of the touchdown signals in comparison to previous calibration landing tests carried out on sand, and possibly first results of the sounding and listening measurements.