Abstract. The Mars Odyssey Gamma-Ray Spectrometer is a suite of three different instruments, a gamma subsystem (GS), a neutron spectrometer, and a high-energy neutron detector, working together to collect data that will permit the mapping of elemental concentrations on the surface of Mars. The instruments are complimentary in that the neutron instruments have greater sensitivity to low amounts of hydrogen, but their signals saturate as the hydrogen content gets high. The hydrogen signal in the GS, on the other hand, does not saturate at high hydrogen contents and is sensitive to small differences in hydrogen content even when hydrogen is very abundant. The hydrogen signal in the neutron instruments and the GS have a different dependence on depth, and thus by combining both data sets we can infer not only the amount of hydrogen, but constrain its distribution with depth. In addition to hydrogen, the GS determines the abundances of several other elements. The instruments, the basis of the technique, and the data processing requirements are described as are some expected applications of the data to scientific problems.

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Observations of seasonal variations of neutron flux from the high-energy neutron detector (HEND) on Mars Odyssey combined with direct measurements of the thickness of condensed carbon dioxide by the Mars Orbiter Laser Altimeter (MOLA) on Mars Global Surveyor show a latitudinal dependence of northern winter deposition of carbon dioxide. The observations are also consistent with a shallow substrate consisting of a layer with water ice overlain by a layer of drier soil. The lower ice-rich layer contains between 50 and 75 weight % water, indicating that the shallow subsurface at northern polar latitudes on Mars is even more water rich than that in the south.

Critical comments from Lawrence et al. are considered on the capability of the collimated neutron telescope Lunar Exploration Neutron Detector (LEND) on NASA's Lunar Reconnaissance Orbiter (LRO) for mapping lunar epithermal neutrons, as presented in our paper. We present two different analyses to show that our previous estimated count rates are valid and support the conclusions of that paper.T he major advantage of the collimated neutron telescope Lunar Exploration Neutron Detector (LEND) on board NASA's Lunar Reconnaissance Orbiter (LRO) compared with the previous neutron spectrometer (NS) on Lunar Prospector is the ability of LEND to measure spatial variations of lunar neutrons within a narrow field of view (FOV) (1, 2). The count rate of neutrons within the FOV is our "signal" for such lunar mapping, and all other counts in the collimated sensors are "background" (1, 2). According to our paper (3), the count rate of the collimated sensors is about 5 counts per second (cps), with a signal about 1.5 to 1.9 cps and a background about 3.1 to 3.5 cps. The main criticism of our paper by Lawrence et al. (4) is based on the estimation of a much larger background that was nearly equal to the total count rate of the collimated sensors. If that were the case, LEND could not detect a significant number of neutrons within its FOV, and it would not be able to map neutrons with the required spatial resolution.In our analysis of the LEND mapping capabilities (1, 2), we took into account neutrons at all energies corresponding to the collimated signal and all the components of background on lunar orbit. Lawrence et al. (4) have focused on one particular component of background, which is associated with the partial transparency of the collimator for high-energy epithermal (HEE) neutrons from the Moon. This component was presented in (4) as a new background that was missed in our paper (3), but that is not correct. We estimated a count rate about 0.3 cps for neutrons of all energies, including the HEE.In (4), the count rate for freely propagating HEE neutrons was estimated as 2.25 cps, which is about 7 times higher than our estimation. To get this value, Lawrence et al. (4) performed a numerical simulation of LEND-type sensors for HEE neutrons from soils with different atomic mass. They found that the count rate of HEE neutrons should be about 10% higher for maria than for highlands. Then, using NASA Planetary Data System (PDS) data, they found a difference of about 0.25 cps between the maximum count rate of maria and the mean count rate for highlands. Using these values, they found 2.25 cps for the background from propagating HEE neutrons.We disagree with two statements in (4). The first one is related to the count-rate values they used. The statistics of the counts are low, and one must average data over large areas to properly estimate the difference between maria and highlands. However, Lawrence et al. (4) compared the maximum count rate in one area with the mean count rate in another. We found the difference between mean c...

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