39We model the primary crater production of small (D < 100 m) primary craters on Mars and the 40 Moon using the observed annual flux of terrestrial fireballs. From the size-frequency distribution 41 (SFD) of meteor diameters, with appropriate velocity distributions for Mars and the Moon, we 42 are able to reproduce martian and lunar crater-count chronometry systems (isochrons) in both 43 slope and magnitude. We include an atmospheric model for Mars that accounts for the 44 deceleration, ablation, and fragmentation of meteors. We find that the details of the atmosphere 45 or the fragmentation of the meteors do not strongly influence our results. The downturn in the 46 crater SFD from atmospheric filtering is predicted to occur at D ~ 10-20 cm, well below the 47 downturn observed in the distribution of fresh craters detected by the Mars Global Surveyor 48 (MGS) Mars Orbiter Camera (MOC) or the Mars Reconnaissance Orbiter (MRO) Context 49 Camera (CTX). Crater counts conducted on the ejecta blanket of Zunil crater on Mars and North 50 Ray crater on the Moon yielded crater SFDs with similar slopes and ages (~1 Ma, and ~58 Ma, 51 respectively) to our model, indicating that the average cratering rate has been constant on these 52 bodies over these time periods. Since our Monte Carlo simulations demonstrate that the existing 53 crater chronology systems can be applied to date young surfaces using small craters on the Moon 54 and Mars, we conclude that the signal from secondary craters in the isochrons must be relatively 55 small, as our Monte Carlo model only generates primary craters. 56 57 58 59 60 61

[1] For more than 7 years, the Los Alamos built Mars Odyssey Neutron Spectrometer (MONS) has measured the neutron albedo from Mars in three consecutive energy bands: thermal, epithermal, and fast neutron ranges. This paper synthesizes the teamwork on the optimization of the signal extraction, the corrections for observational biases and instrument specific characteristics. Results are presented for neutron time series with an emphasis on seasonal variations at the poles. Frost-free data are mapped on to the surface, and the apparent random nature of the counting-rate distribution per pixel is analyzed: for epithermal neutrons, the relative standard deviation is less than 0.5% equatorward of 45°and up to 2.5% above this latitude limit; for thermal neutrons it is 1% and 2.5% respectively; and for fast neutrons it is 3% and 5.5%, respectively. New science results are obtained with regards to the distribution of water-equivalent hydrogen (WEH) on Mars. Under the assumption of a single uniform distribution of hydrogen with depth, WEH abundances range from 2% near the equator to 80% at the poles, with ±2% to 4.5% relative error bars. A best approximation to a two-layered global distribution of a lower-level hydrogen-rich substrate beneath an upper layer of varying thicknesses is generated using an average hydration level of an upper layer of 2 wt %, derived in the paper by Feldman et al. (2011). Such results are discussed and compared with regard to previous publications on the MONS instrument.

[1] Global maps of hydrogen abundance near the surface of Mars, interpreted as the mass percent of water-equivalent hydrogen (WEH) have been generated from measurements of neutron leakage fluxes from Mars. Although these data provide an unambiguous indicator of the presence of WEH, quantitative details of its magnitude and burial depth depend on the model of the host regolith that is used to interpret the data. Previous models assumed a spatially uniform surface cover layer having a one-to-two mass percent of WEH and thickness D covering a semi-infinite ice-rich deposit. These assumptions allowed the derivation of the relative proportions of ice and regolith in the lower layer, which had been used to create global maps of WEH in the near surface. In this paper we develop a new method that determines, from the Mars Odyssey Neutron Spectrometer (MONS) data, a self-consistent model of the WEH content of both the upper and lower layers as well as the thickness (D) of the upper layer. The results of our model suggest that large areas at nonpolar latitudes may contain water ice deposits that have abundances that are larger than can be held by normal pore volumes. These deposits are buried less than about 1 m below the surface and may represent buried water ice or high concentrations of hydrous minerals. Intriguingly, the most definitive MONS evidence at intermediate latitudes for excess ground ice corresponds to a region of Arcadia Planitia within which High Resolution Imaging Science Experiment (HiRISE) has observed fresh icy craters.

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