Modelled temperature profiles for the observed area for MERTIS during the 5th flyby of BepiColombo.

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. Introduction:Mercury is a fascinating planet for thermal modelers. With extreme temperatures ranging from 100 K at night to over 700 K during the day [1, 2], and its 3:2 spin-orbit resonance with the sun [3], modelling Mercury’s thermal environment presents a unique challenge. Because of its proximity to the Sun and the orbit-spin resonance, Mercury also experiences differences in regional temperature identified as “hot” and “cold” poles. Various factors influence the temperature on Mercury’s surface like the albedo variation of impact craters, topography, surface morphology, distance to the sun, density and conduction of the material etc.Over the years, many studies have attempted to model the thermal properties of Mercury, often by using the Moon as a reference due to both being an airless body [4, 5]. From Mariner 10 to BepiColombo, the accuracy of these thermal models for Mercury has significantly improved, thanks to mission data. However, due to limited knowledge about the surface composition and other constraints, thermal models still require further refinement.In this study, we present a thermal model aimed at improving our understanding of Mercury’s local temperature behavior, ahead of MERTIS’s arrival at Mercury.2. MERTIS and the observation area:MERTIS (Mercury Radiometer and Thermal infrared Imaging Spectrometer), onboard the ESA-JAXA mission BepiColombo, is a thermal infrared spectrometer (TIS) and radiometer (TIR) that is part of the Mercury Planetary Orbiter payload [6]. TIS has a spectral range of 7-14 μm with a resolution of 90 nm, and TIR, a radiometric range of 7-40 μm, divided into two bands - 8-14 μm and 7-40 μm [6].One of the primary objectives of MERTIS is to study surface temperature distribution on both day and night side of Mercury. It first observed Mercury during BepiColombo’s 5th flyby on 1st December, 2024. The spacecraft achieved its closest approach at 37,000 km and MERTIS was able to observe the planet, through its space baffle, for 37 minutes, with a pixel resolution of 27 km for TIS and 100 km for TIR.The observed area extended from 140°E to 230°E longitude on Mercury and covered major craters such as Bashō crater in the south and parts of Caloris Basin in the north including the hot pole at 180°. MERTIS was able to observe the planet with an incidence angle of as little as 0.2° to as high as 95.00°, where the Sun is below the horizon, covering therefore a large range of local time.3. Methodology:The thermal model in our study takes the energy balance thermal equation for airless bodies as a reference to calculate the temperature distribution within the observed area, where absorbed solar energy equals emitted thermal radiation.Equation 1: Energy balance equation for airless bodies.Where: T = Temperature A = Bond Albedo - 0.058 S = Solar Flux 𝜎 = Stefan-Boltzmann Constant 𝜀 = Surface emissivityThe incoming radiation, or solar flux is calculated using a constant bond albedo [2], the distance from the sun and the incidence angle. The incidence angles of more than 90° are ignored as this indicates that the sun is below the horizon.Equation 2: Incident solar energy or solar flux.Where:F0 = Solar constant for Mercury at distance from the sun.𝜃 = Incidence angle (

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