PLANETARY SCIENCE INSTITUTE INDIVIDUAL ANNUAL REPORT

Asmin Pathare

Calendar Year 2014

I. Ongoing Research

In 2014, Pathare actively participated in the following areas of research:

(1) Improving Odyssey Neutron Spectroscopy (MONS) mapping of near-surface water-equivalent hydrogen (Pathare et al., 2014). To date, more than 3.5 Mars years of neutron data measured by MONS have been analyzed to develop global maps of hydrogen concentration interpreted using a simple 2D model of the outer ~0.5 meter of Martian crust. A two-layer near-surface regolith model is assumed that expresses hydrogen concentration in terms of: (1) an upper layer of weight fraction Wup having (2) thickness D overlying a (3) semi-infinite lower layer of weight fraction Wdn. Initial MONS-derived global maps of Wdn and D assumed constant Wup. More recently, we self- consistently calculated Wup directly from MONS data using “cross over” of fast vs. epithermal neutrons with unity line for large representative regions of interest (ROIs). However, application of this crossover technique using unweighted sliding 1800-km diameter ROIs resulted in large areas at low latitudes with unphysical negative values of Wup [3]. Therefore we recomputed Wup using distance-weighted ROIs and linear chi- squared minimization regression (Fig. 1), resulting in just one unphysical negative region (in )—thereby creating the best mid-latitude map of Wup to date.

Figure 1: Map of Wup = crossover points from the linear chi-squared minimization regressions between WEHD0(fast) and WEHD0(epi) data in inverse distance-squared weighted regions of interest spaced every two degrees of latitude and longitude. The dynamic range spans 0-6, so the purple (in Valles Marineris) actually corresponds to unphysical negative values, and the red to highly positive values.

(2) Modeling the primary crater production of small (D < 100 m) primary craters on Mars and the Moon using the observed annual flux of terrestrial fireballs (Williams et al., 2014, Icarus). From the size–frequency distribution (SFD) of meteor diameters, with appropriate velocity distributions for Mars and the Moon, we are able to reproduce martian and lunar crater-count chronometry systems (isochrons) in both slope and magnitude. We include an atmospheric model for Mars that accounts for the deceleration, ablation, and fragmentation of meteors. We find that the details of the atmosphere or the fragmentation of the meteors do not strongly influence our results. The downturn in the crater SFD from atmospheric filtering is predicted to occur at D ~ 10–20 cm, well below the downturn observed in the distribution of fresh craters detected by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) or the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX). Crater counts are conducted on the ejecta blanket of crater and the interior of Pangboche crater on Mars and North Ray and Cone craters on the Moon. Our model isochrons produce a similar slope and age estimate for the formation of Zunil crater as the Hartmann production function (~1 Ma). We derive an age of 35.1 Ma for Pangboche when accounting for the higher elevation (>20 km higher than Zunil), a factor ~2 younger than estimated using the Hartmann production function which assumes 6 mbar surface pressure. We estimate ages of 52.3 Ma and 23.9 Ma for North Ray and Cone crater respectively, consistent with cosmic ray exposure ages from Apollo samples. Our results indicate that the average cratering rate has been constant on these bodies over these time periods. Since our Monte Carlo simulations demonstrate that the existing crater chronology systems can be applied to date young surfaces using small craters on the Moon and Mars, we conclude that the signal from secondary craters in the isochrons must be relatively small at these locations, as our Monte Carlo model only generates primary craters.

(3) Characterizing the size-frequency distribution and forward motions of terrestrial dust devils (Pathare and Balme, 2014). We recently presented the results of our quantitative characterization of the size-frequency distribution (SFD) of terrestrial dust devils, which utilizes stereo photography to calculate dust devil diameters via parallax displacement. Initial results show that our methodology is robust and that reliable data for size and position of dust devils can be extracted. Altogether, we were able to successfully stereo photograph approximately 40% of the more than 1000 dust devils that we observed within the study areas during the three field campaigns. Our analysis will not only yield a more comprehensive SFD for terrestrial dust devils, but will also help constrain SFD parameterizations of Martian dust devils based on orbiter and lander observations.

II. Publications

Papers

Williams, J.-P., A. Pathare, O. Aharonson, The Production of Small Primary Craters on Mars and the Moon. Icarus, 235, 23-36, 2014.

Abstracts

Li, J.-Y.; Sykes, M. V.; Pathare, A. V.; Kirby, J. P.; Castillo-Rogez, J. C., Investigating the Habitability of Ceres, Workshop on the Habitability of Icy Worlds, p. 4031, 2014.

Williams, J.-P.; Paige, D. A.; Plescia, J. B.; Pathare, A. V.; Robinson, M. S. (2014), Crater Size-Frequency Distributions on the Ejecta of Giordano Bruno, LPSC 45, p. 2882, 2014.

Williams, J.-P.; Pathare, A. V.; Aharonson, O., The Production of Small Primary Craters on Mars, Mars 8, p.1059, 2014.

Pathare, A. V.; Feldman, W. C.; Prettyman, T. H.; Jensen, E.; Maurice, S., Improved Mars Odyssey Neutron Spectroscopy (MONS) Mapping of Near-Surface Water- Equivalent Hydrogen, Mars 8, p.1150, 2014.

Pathare, A. V.; Balme, M. R., The Devils you Know: Quantifying the Size-Frequency Distribution and Forward Velocities of Terrestrial Dust Devils, with Implications for Mars, Mars 8, p.1197, 2014.

Byrne, S.; , P. S.; Pathare, A. V.; Becerra, P.; Molaro, J. L.; Mattson, S.; Mellon, M. T.; HiRISE Team, Icy Polar Cliffs: Stressed Out and Falling to Pieces, Mars 8, p.1257, 2014.

IV. Service to the Science Community

A. JGR Planets -- Manuscript Peer Review