  We present the mass excesses of 52-57Sc, obtained from recent time-of-flight
nuclear mass measurements at the National Superconducting Cyclotron Laboratory
at Michigan State University. The masses of 56Sc and 57Sc were determined for
the first time with atomic mass excesses of -24.85(59)(+0 -54) MeV and
-21.0(1.3) MeV, respectively, where the asymmetric uncertainty for 56Sc was
included due to possible contamination from a long-lived isomer. The 56Sc mass
indicates a small odd-even mass staggering in the A = 56 mass-chain towards the
neutron drip line, significantly deviating from trends predicted by the global
FRDM mass model and favoring trends predicted by the UNEDF0 and UNEDF1 density
functional calculations. Together with new shell-model calculations of the
electron-capture strength function of 56Sc, our results strongly reduce
uncertainties in model calculations of the heating and cooling at the 56Ti
electron-capture layer in the outer crust of accreting neutron stars. We found
that, in contrast to previous studies, neither strong neutrino cooling nor
strong heating occurs in this layer. We conclude that Urca cooling in the outer
crusts of accreting neutron stars that exhibit superbursts or high temperature
steady-state burning, which are predicted to be rich in A=56 nuclei, is
considerably weaker than predicted. Urca cooling must instead be dominated by
electron capture on the small amounts of adjacent odd-A nuclei contained in the
superburst and high temperature steady-state burning ashes. This may explain
the absence of strong crust Urca cooling inferred from the observed cooling
light curve of the transiently accreting x-ray source MAXI J0556-332.
