A novel technology for measuring the eruption temperature of silicate lavas with remote sensing: Application to Io and other planets

摘要

The highly variable and unpredictable magnitude of thermal emission from evolving volcanic eruptions creates saturation problems for remote sensing instruments observing eruptions on Earth and on Io, the highly volcanic moon of Jupiter. For Io, it is desirable to determine the temperature of the erupting lavas as this measurement constrains lava composition. One method of determining lava eruption temperature is by measuring radiant flux at two or more wavelengths and fitting a blackbody thermal emission function. Only certain styles of volcanic activity are suitable, those where detectable thermal emission is from a restricted range of surface temperatures close to the eruption temperature. Volcanic processes where this occurs include large lava fountains; smaller lava fountains common in active lava lakes; and lava tube skylights. Problems that must be overcome to obtain usable data are: (1) the rapid cooling of the lava between data acquisitions at different wavelengths, (2) the unknown magnitude of thermal emission, which has often led to detector saturation, and (3) thermal emission changing on a shorter timescale than the observation integration time. We can overcome these problems by using the HOT BIRD detector and a novel, advanced digital readout circuit (D-ROIC) to achieve a wide dynamic range sufficient to image lava on Io without saturating. We have created an instrument model that allows various instrument parameters (including mirror diameter, number of signal splits, exposure duration, filter band pass, and optics transmissivity) to be tested to determine the detectability of thermal sources on Io's surface. We find that a short-wavelength infrared instrument on an Io flyby mission can achieve simultaneity of observations by splitting the incoming signal for all relevant eruption processes and still obtain data fast enough to remove uncertainties in accurate determination of the highest lava surface temperatures. Observations at 1 and 1.5 mu m are sufficient for this purpose. Even with a ten-way beam split, instrument throughput generates acceptable signal-to-noise values. Accurate constraints on lava eruption temperature are also possible with a visible wavelength detector so long as data at different wavelengths are obtained simultaneously and integration time is very short. Fast integration times are important for examining the thermal emission from lava tube skylights due to rapidly changing viewing geometry during close flybys. The technology described here is applicable to instruments observing terrestrial volcanism and for investigating proposed volcanic activity on Venus, where lava composition is not known. (C) 2017 Elsevier B.V. All rights reserved.