Extended Data Fig. 3: Iron and nickel carbonyl sublimation properties.

a, Sublimation rates (Z; in molecules cm⁻² s⁻¹) of Fe and Ni carbonyls as a function of temperature, compared to those of the main ices in comets. The carbonyl rates are intermediate between those of H₂O and CO₂. b, The ratio of the sublimation rate of Ni(CO)₄ over that of Fe(CO)₅ shows that the former is considerably higher than the latter. These quantities were computed as follows. As in refs. ^53,54, we estimate the condensation or sublimation temperature T_s of these compounds by solving the equation f_xnkT_s = P_v,x(T_s) where f_x is the relative abundance of species x, n is the number density of the gas, k is the Boltzmann constant, and P_v,x is the vapour pressure, given by the relation log[P_v,x(T)] = −(A/T) + B. The constants A and B for Fe(CO)₅ and Ni(CO)₄ are obtained from refs. ^55,56: A = 2,097 K and B = 11.62 for Fe(CO)₅, A = 1,534 K and B = 10.87 for Ni(CO)₄, with P_v,x in dyn cm⁻². We consider relative abundances f_x of 10⁻³–10⁻⁵ × f_x(H₂O) for both Fe(CO)₅ and Ni(CO)₄, and we adopt n = 10¹³ cm⁻³ as in ref. ⁵⁴. The resulting sublimation temperatures of the iron and nickel carbonyls (97–108 K and 74–82 K, respectively, depending on f_x) are between the sublimation temperatures of H₂O and CO₂ (152 K and 72 K), whereas CO sublimates at 25 K (ref. ⁵⁴). The sublimation rate (in molecules cm⁻² s⁻¹) from the surface of a pure ice into vacuum can be expressed as⁵⁷: Z_x(T) = P_v,x(T)(2πm_xkT)^−1/2, where T is the ice temperature and m_x the mass of species x.