High-temperature laboratory-based measurements of emissivity of eruptive products: a comparison with the existing satellite derived database ASTER GED

Emissivity is rarely measured and usually assumed or inferred to be a constant value between 1.0 and 0.80 for basaltic lavas. Computations based on spaceborne data and models that rely on these temperatures to track cooling with time will produce systematic errors as the literature in optical remote sensing of very-high temperature features lacks in detailed spectral emissivity information for molten materials. Several recent thermal emission studies of silicate glasses and basaltic lavas suggest that the emissivity of molten material is significantly lower than that of the same material in its solid state. To test this on field samples, we measured the spectral emissivity of volcanic samples from Mt Etna (Italy), using a laboratory-based FTIR spectroscopy approach. Mt Etna in Italy was chosen as the study candidate due to its persistent activity, displaying a range of eruptive products and styles, and the availability of spaceborne data with a range of temporal and spatial resolutions. To measure the emissivity of exposed lavas, we collected samples from the three most significant recent events: 1) the major 2001 flank eruption; (2) the long-lived 2002-2003 eruption; and (3) the recent March-April 2017 eruption.

The thermal emission spectra of samples were measured at the German Aerospace Center (DLR, Germany) in vacuum (0.7 mbar) by FTIR at a range of temperatures (400-900 K) and wavelengths (5.0-16.0 μm). The experimental set-up used an external 'simulation chamber' attached to the FTIR spectrometer measuring the emissivity of solid samples (grain size 1000-3000 μm). The emissivity chamber was equipped with temperature sensors to measure the sample/cup temperature, monitor the equipment, and record chamber temperatures. Both the cup and the sample are heated uniformly by induction, and the temperature of the emitting surface is measured using a thermophile sensor in contact with the surface. Data were calibrated using the emissivity spectrum of blackbody material. Our results show that while the mean emissivity data in the static ASTER Global Emissivity Database (GED) fits our data at low/moderate temperature (400 K), emissivity is nonlinearly temperature dependent. We show that measured emissivity decreases with increasing temperature, within the investigated limits of cooling crust lava temperatures.