  The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) suite
of instruments operated throughout the over two years of the Rosetta mission
operations in the vicinity of comet 67P/Churyumov-Gerasimenko. It measured gas
densities and composition throughout the comet's atmosphere, or coma. Here we
present two-years' worth of measurements of the relative densities of the four
major volatile species in the coma of the comet, H2O. CO2, CO and O2, by one of
the ROSINA sub-systems called the Double Focusing Mass Spectrometer (DFMS). The
absolute total gas densities were provided by the Comet Pressure Sensor (COPS),
another ROSINA sub-system. DFMS is a very high mass resolution and high
sensitivity mass spectrometer able to resolve at a tiny fraction of an atomic
mass unit. We have analyzed the combined DFMS and COPS measurements using an
inversion scheme based on spherical harmonics that solves for the distribution
of potential surface activity of each species as the comet rotates, changing
solar illumination, over short intervals and as the comet changes distance from
the sun and orientation of its spin axis over long time intervals. We also use
the surface boundary conditions derived from the inversion scheme to simulate
the whole coma with our fully kinetic Direct Simulation Monte Carlo model and
calculate the production rates of the four major species throughout the
mission. We compare the derived production rates with revised remote sensing
observations by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)
as well as with published observations from the Microwave Instrument for the
Rosetta Orbiter (MIRO). Finally we use the variation of the surface production
of the major species to calculate the total mass loss over the mission and, for
different estimates of the dust/gas ratio, calculate the variation of surface
loss over the nucleus.
