Solid CH4 toward low-mass protostars: How much is there to build complex organics?

We use Spitzer IRS spectra to determine the solid CH₄ abundance toward a large sample (52 sources) of low mass protostars. 50% of the sources have an absorption feature at 7.7 μm, attributed to solid CH₄. The solid CH₄/H₂O abundances are 2-13%, but toward sources with H₂O column densities above 2 × 10¹⁸ cm⁻², the CH₄ abundances (20 out of 25) are nearly constant at 4.7 ± 1.6%. Correlations with CO₂ and H₂O together with the inferred abundances are consistent with CH₄ formation through sequential hydrogenation of C on grain surfaces, but not with formation from CH₃OH and formation in gas phase with subsequent freeze-out.