In this paper, we have analyzed neutron spectroscopy data gathered by the High Energy Neutron Detector (HEND) instrument on board Mars Odyssey for two Martian years of orbital mapping. We focused on the observations of the Martian southern seasonal cap for this period of time. Nuclear methods based on numerical modeling allow us to derive an estimation of the distribution of CO2 frost column density and mass from the temporal variations of neutron flux between summer and winter seasons. We have estimated the seasonal behavior of both the column density and mass of CO2 frost. Comparison between two Martian years revealed minor (<10%) interannual variations of CO2 thickness at the south pole during the middle of southern winter. It was found that maximal mass of the southern seasonal cap is achieved at Ls = 1600 and is equal to 6.6 1 0.6 W 1015 kg. We found that this result corresponds well with previous HEND results, NASA Ames GCM predictions, Gamma Ray Spectrometer estimates, and estimations from gravity models based on MGS and Mars Odyssey tracking data. In our analysis, we also compared to processed MOLA data (linear thicknesses of snow depths) to measure the value of the volume density of the Southern seasonal cap, and we found that it may vary in the range 400--1250 kg/m3.