Monitoring the spectral signatures of Earth's climate change from the Moon with the Lunar Earth Temperature Observatory – LETO

In the frame of the Italian National Plan for Recovery and Resilience (PNRR), the Earth-Moon-Mars (EMM) infrastructure project includes an activity for the development of a permanent lunar base. Several instruments will be operated from this base for the observation of both the Earth and the deep space. Among these instruments, the Lunar Earth Temperature Observatory (LETO) will consist of a Fourier transform spectro-radiometer (LETO-FTS) and of an imager (LETO-IMG). LETO-FTS will measure the longwave spectral radiance emitted by the Earth’s full disk, in a broad spectral range, extending from the Far- to the Mid- Infrared (FIR+MIR) regions. The planned long life span of the mission and its high and SI traceable accuracy, will permit to observe the trends in the spectral signatures of the Earth’s climate change. To simulate LETO’s measurements, a comprehensive suite software tools was adapted and re-arranged at CNR-INO. The suite includes an Earth-Moon orbital simulator and a radiative transfer algorithm to simulate the spectral radiance emitted by the portion of the Earth’s disc viewed from the Moon, as a function of time and of the position of the lunar base. The orbital simulator is able to provide at a given time the geophysical coordinates of the Earth’s portion observed by LETO. The observed region is then fractionated into small homogeneous pixels. The σ-IASI fast radiative transfer model is used to compute this spectral radiance. The total radiance reaching the instrument is finally obtained as the summation of the contributions arising from the individual pixels. The algorithm is able to model both Earth’s surface and atmospheric properties. As an example, hourly simulations of the spectral radiance emitted by the visible portion of the Earth’s disk were obtained for a specific day, assuming a lunar site located on the prime meridian at a latitude of -70°. The time variability of the spectral radiance in specific spectral intervals located between 100 and 1600 cm-1 is studied. More specifically, we study the correlations of the spectral radiance changes with the variability of geophysical parameters, such as the global outgoing longwave radiation, the average global temperature, the water vapor column, etc. The overall objective is to build a long-term dataset to monitor Earth’s climate changes.