Left: χ² values (compared to the best fit) of each temperature retrieval for the range of fixed D/H ratios. Measured values are marked by the black asterisks, the blue dotted line marks the quadratic fit to the D/H ratios less than 1.65 × 10⁻⁵ and the dashed red line marks the quadratic fit of the D/H ratios that are greater than 1.65 × 10⁻⁵. The minimum χ² of 84.487 has been subtracted from all values so that this plot can show Δχ². The horizontal black lines delineate the Δχ² of 1, 4 and 9 which correspond to σ₁, σ₂ and σ₃ respectively. The vertical dotted line marks the inferred value of methane D/H 2.9 × 10⁻⁵ converted from the hydrogen proto-solar D/H of Geiss & Gloecker (1998) using a fractionation factor of 1.34, demonstrating that the proto-solar quantity of deuterated methane provides a significantly worse fit using our data and retrieval. Right: temperature profiles associated with CH₃ D abundances set to 50% (blue dashed line), 60% (blue dotted line), nominal levels (solid black line), 140% (red dotted line) and 150% (red dashed line). We note that the nominal level is in the centre of these adjusted temperature retrievals, this indicates that the black profile is the best temperature fit for these data.