Thermal Infrared Emissivity Spectra of Mercury Analogues at PEL in Support for MERTIS on BepiColombo

Abstract

At the Planetary Emissivity Laboratory (PEL, Institute of Planetary Research of the German Aerospace Center, Berlin) we routinely measure emissivity spectra of planetary analogues, in the spectral range from 1 to 100 µm. Complementary measurements include reflectance and transmittance, from the visible to the far-infrared. The MERTIS instrument on the ESA-JAXA BepiColombo mission will collect for the first time thermal infrared (TIR) spectra (7-14 µm) from the hot surface of the innermost planet, which at noon can locally reach a temperature of 450 °C. To prepare for the interpretation of this new dataset, we built a facility to measure the emissivity of very hot surfaces under vacuum (< 0.8 mbar). A Bruker Vertex 80V evacuable spectrometer is connected to an external chamber where the samples are heated under vacuum to temperatures from 50° to above 800° C. The use of an induction heating system to heat-up the stainless steel cups minimizes heat loss to the chamber. As a standard, we measure 4 grain size ranges, from very fines (<25 µm) to larger grain size (125-250 µm), and when possible even larger grain sizes or slabs. A sample carousel mounted on a high-precision rotating motor allows measuring several samples a day without breaking the vacuum. A monitoring webcam and a large number of temperature sensors (on samples, cups or other hardware parts in the chamber) complete the set-up. Ongoing laboratory work shows that the high surface temperature of Mercury analogues can significantly affect the spectral characteristics of surface materials. This can lead to misinterpretations when compared to room temperature measurements. The effects have to be taken into account when interpreting spacecraft data, but even more so when designing new instrumentation.