= Solution
The location between pure-silicate and pure-water <planetary mass-radius curves> does not determine a unique composition. Possibilities include a rocky iron-silicate interior with a deep water layer, a rock-water mixture, or a mostly rocky planet whose small hydrogen-helium envelope inflates its radius. The atmosphere could accordingly be hydrogen-helium, steam-rich, carbon-dioxide-rich, or secondary gas released from the interior. Residence in the <circumstellar habitable zone> constrains stellar flux but does not by itself distinguish these models or guarantee surface liquid water.
For an atmosphere spanning fixed base and photospheric pressures,
$$
\Delta R\simeq H\log(P_0/P_{\rm ph}),
\qquad
H=\frac{k_BT}{\mu m_Hg}.
$$
Thus two compositions have
$$
\boxed{
\frac{\Delta R_1}{\Delta R_2}
\simeq\frac{T_1/\mu_1}{T_2/\mu_2}}.
$$
At equal temperature, a hydrogen-helium atmosphere with $\mu\simeq2.3$ is about
$$
\boxed{\frac{18}{2.3}\simeq7.8}
$$
times thicker than a steam atmosphere with $\mu\simeq18$. This large difference is one reason atmospheric composition contributes strongly to the <exoplanet interior-composition degeneracy>.
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