We present a simple analytic model, that captures the key features of the emission of radiation from material ejected by the merger of neutron stars (NS), and construct the multi-band and bolometric luminosity light curves of the transient associated with GW170817, AT,2017gfo, using all available data. The UV to IR emission is shown to be consistent with a single $approx0.05$,M$_odot$ component ejecta, with a power-law velocity distribution between $approx 0.1,c$ and $>0.3,c$, a low opacity, {$kappa<1$,cm$^2$,g$^{-1}$}, and a radioactive energy release rate consistent with an initial $Y_{rm e}<0.4$. The late time spectra require an opacity of $kappa_ uapprox0.1$,cm$^2$,g$^{-1}$ at 1 to $2mu$m. If this opacity is provided entirely by Lanthanides, their implied mass fraction is $X_{rm Ln}approx10^{-3}$, approximately 30 times below the value required to account for the solar abundance. The inferred value of $X_{rm Ln}$ is uncertain due to uncertainties in the estimates of IR opacities of heavy elements, which also do not allow the exclusion of a significant contribution to the opacity by other elements (the existence of a slower ejecta rich in Lanthanides, that does not contribute significantly to the luminosity, can also not be ruled out). The existence of a relatively massive, $approx 0.05$,M$_odot$, ejecta with high velocity and low opacity is in tension with the results of numerical simulations of NS mergers.
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