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Rosetta Permittivity Probe SESAME / PP

The Permittivity Probe (PP) is part of the physical properties package "Surface Electric Sounding and Acoustic Monitoring Experiment" (SESAME) on board Rosetta's Lander Philae. Principle Investigator: Dr.Walter Schmidt, Finnish Meteorological Insitute / Earth Observation, Helsinki, Finland.

Primary objectives:

Determine the water ice content in the cometary surface layer and its variation with time as the comet approaches the Sun.

Used technique:

The electrical conductivity and permittivity of the comet surface material is measured with a quadrupole technique, where two electrodes, connected with a generator for variable voltages, induce a variable current into the cometary matter. Two additional electrodes detect the resulting potential variations and their phase difference to the generator current. See also here.

There are three transmitter electrodes on the Philae cometary lander (one in a lander foot and the other two close to the sensors of the experiments MUPUS-Pen and APXS).The two receiving electrodes are mounted into the soles of the remaining two feet (details). Using the variable geometry of different transmitter electrode combinations one can infer the complex permittivity down to a depth of about 2 m.

The measured parameters react quite sensitive to the amount of polar molecules, especially water, and their temperature. Using these data one can calculate the water content of the surface, which can vary during one comet day and with the distance of the comet to the Sun, and possibly also the water vapor production rate. From this it is possible to describe the outgassing behavior and to infer the composition of the comet.

 

Monitoring only the potential variations without generating an additional current, the PP instrument can detect plasma waves up to 10 kHz, which originate from the interaction of the Solar Wind with the emitted cometary particles and gas and are a measure of the cometary activity. 

The picture below shows the MUPUS PEN instrument for thermal conductivity measurements with the PP transmitter electrode mesh attached. After insertion of the black rod into the comet material, the mesh will lie on the surface and couple capacitively the AC-current into the surface material

 

PP electrode at MUPUS-PEN

SESAME in a nutshell

Details about the SESEAME instrument, sensors, operations and the complete Rosetta mission timeline can be found in this PDF-document, put together by Dr. M.Knappmeyer on behalf of the Rosetta/Philae/SESAME team.