Lab Experiments (2)

Laboratory Investigations of Mineral Dust (Dunite)

	Fig. 1: Experimental setup. A tube is filled with mineral dust (Dunite with micrometer sized grains) to various heights. With an exciter (surface) and a sensor (bottom) the sound velocity and the frequency dependent attenuation is measured.
	Fig. 2: The sound velocity in Dunite is determined by a linear regression through the data points at different heights. This leads to 55 m/s, which is quite low compared to the sound velocity in air under normal pressure (340 m/s).
	Fig. 3: The attenuation of the acoustic signal depending on the exciting frequency. At low frequency (wavelength larger than the Dunite grain size) the attenuation obeys a square law. Wavelengths smaller than the grain size are attenuated by a factor of the frequency to the power of 4.
This means that at low frequencies, where the wavelength is larger than the grain size, it should be possible to measure signals over the distance of the Rosetta Lander's feet, even if the cometary surface layer consists of micrometer sized dust, like Dunite. Additionally, knowing the frequency where the power law of the attenuation changes, the grain size can be determined.