The thickness of seasonal CO2 frost that covers the north pole of Mars during the winter and spring seasons is studied using gamma ray and neutron data measured using the gamma ray spectrometer suite of instruments aboard Mars Odyssey. The resultant seasonal variation of CO2 ice/frost thickness for latitudes greater than +85¿ is in close agreement (within 9% at maximum) with those predicted using all three major Mars general circulation models presently in use. These observed CO2 ice results are significantly different from those inferred from the Mars Obiter Laser Altimeter experiment aboard Mars Global Surveyor (MGS). A possible explanation is that the density of the CO2 ice/frost cap is considerably lower than that inferred from observed variations of the MGS orbit. Other significant results are that the thermal neutron counting rates provide evidence for variations in the abundance of noncondensable components of the polar atmosphere of Mars and that the basement terrain below the seasonal cover of CO2 frost within about 10¿ of the pole is nearly 100% water ice.