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The passage around this formula

Atmospheric composition is inferred the same way interiors are — from a signal, not from the substance itself — but the physics is radiative transfer rather than mechanics. The workhorse technique is transmission spectroscopy : as a planet with an atmosphere passes in front of its star (for an exoplanet) or as sunlight grazes a planet’s limb (for a solar-system body observed from orbit), starlight is filtered through the atmosphere’s outer layers before reaching the detector. Each gas absorbs a characteristic set of wavelengths, governed to first approximation by the Beer–Lambert relation, $I(\lambda) = I_0(\lambda)\, e^{-\tau(\lambda)}, \qquad \tau(\lambda) = \int n(z)\,\sigma(\lambda, z)\, dz$. where $I_0(\lambda)$ is the incident spectrum, $\tau(\lambda)$ the…

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