Interannual variations of CO2 deposit in Martian caps from the HEND/Odyssey data.

<p>It is known that growing and sublimation of Martian snow caps is seen as seasonal variations of neutron flux above Martian polar regions. The surficial CO<sub>2</sub> deposit attenuates (depends how thick the deposit is) the neutron signal originated at deeper subsurface layers enriched with water ice. It makes neutron observations a valuable technique to monitor behavior of seasonal caps and study their evolution within seasonal cycle or search for inter annual variations. The measured counting rate in neutron detectors and its seasonal variations could be interpreted in terms of column depth (g/cm<sup>2</sup>) and mass (kg) of the seasonal CO<sub>2</sub> deposit based on the numerical models of neutron production and scattering in Martian atmosphere and surface (including upper layer of temporal CO<sub>2</sub> deposit).<br>HEND/Odyssey is continuously operating on Mars orbit and mapping Martian neutron albedo since February 2002 covering period of almost 20 years. Up to now it corresponds to about 10 Martian years. The availability of so long-term and continuous data set could be used to search interannual variations of seasonal cycle. <br>It was found that observations made during different martian years do not match each other in winter demonstating periodical variations. </p>