\begin{table}%t1
\caption{\label{f_i_cremonese}Mass fraction of elements in the regolith attended in the surface of Mercury.}
%\centerline
{\small
\begin{tabular}{cccc}
\hline
\hline
Mass Fraction &  Cremonese et~al. & Goettel & Smith and Marconi\\
\hline
 $f_{\rm Na}$ & 0.0380 & 0.0044 & 0.0033\\
 $f_{\rm K}$ & 0.0013 & 0.0003 & -- \\
 $f_{\rm Ca}$ & 0.0797 & 0.0375 & --\\
 $f_{\rm Mg}$ & 0.0123 & 0.211 & --\\
\hline
\end{tabular}}
 \tablebib{(1) \citet{cremonese}; (2) \citet{goettel}; (3)~\citet{smithmarconi}.}
\end{table}