 SPATIAL AND TEMPORAL DENSITY ANALYSIS OF THE MARS NORTHERN SEASONAL ICE CAP.  C. Mount1,2 and T. N. Titus2, 1Northern Arizona University, Flagstaff, AZ 86001 (chris_mount16@yahoo.com), U.S.G.S., 2255 N. Gemini Dr., Flagstaff, AZ 86001 (ttitus@usgs.gov).  Introduction:  The seasonal polar caps are among the most active regions on Mars.  Twenty-five percent of Mars' total atmosphere cycles annually through the seasonal polar caps [1,2], significantly influencing global climate variations. The two primary CO2 icedeposition mechanisms on Mars are identified as: 1) direct formation of ice onto the surface and 2) CO2 precipitates forming in the atmosphere and falling onto the surface (i.e. CO2 snow) [3,4]. Time-dependent density variations in these ices may indicate which of these processes was responsible for the deposition. We expect densities of CO2 snow-like deposits to increase throughout the sublimation cycle due to fracturing, compaction and settling [5], whereas the more cohesive CO2 slab ice deposits would maintain relatively constant densities before starting to decrease at the peak of the sublimation cycle. Calculation of the volume density requires that column density and the height of the ice cap be known in advance. Column densities are derived in a variety of ways: energy balance calculations [2,6,7], neutron [5,8,9] and gamma ray [1,9] spectroscopy and variations in the gravity field [10,11]. Most density studies of the Martian Polar Caps use the Mars Orbiter Laser Altimeter (MOLA) to measure the height of the cap [5,8,10,11], although the seasonal changes in rock heights on the surface can be used to measure cap heights to a greater accuracy [12]. Here we explore the spatial and temporal density variations of Mars' north seasonal CO2 ice cap, and discuss the processes responsible for seasonal CO2 ice deposition. Methodology and Measurement: The findings presented here focus on a region of the northern polar cap between latitudes 65⁰ N and 75⁰ N. Measurements for the ice depth are determined using the method from Cull et al. [12] in which rock shadow lengths are measured in High Resolution Imaging Science Experiment (HiRISE) images. From a basic geometric relationship between the shadow length and the angle of the incident ray, the rock height is found. The heights of the rocks in spring images are then compared to those in a late summer image, known to have no ice. The average difference in the heights of the rocks yields the seasonal ice depth. This is done until there is no change found in the heights of the rocks. This technique results in crocus dates for specific latitudes and longitudes and also removes extraneous post-crocus-date (ice-free) images from the data sets. Data: Preliminary data were taken for two distinct locations. The first location was the Phoenix landing site at latitude 68.3⁰N and longitude 233⁰E (henceforth referred to as Phoenix1). The second was at latitude 74.6⁰N and longitude 13.4⁰E (Crater1). HiRISE images taken from December 2006 to April 2010 were used for both locations. The images in Phoenix1 ranged in Ls from approximately 11⁰ to 37⁰ with a corresponding summer image at Ls ~ 154⁰. Crater1 has somewhat larger seasonal coverage, ranging from Ls ~ 26⁰ to 59⁰. From Ls ~ 44⁰ to 58⁰ there is a gap in coverage in this region due to poor image resolution. Crater1 has a corresponding summer image at Ls ~ 81⁰. Thermal Emission Spectrometer (TES) observations from June to September 2000 were used to calculate the column density derived from the sublimation and energy balance techniques of Kieffer et al. (2000) [6] and Kieffer and Titus (2001) [7].  Note that HiRISE and TES data are taken from different years, so inter-annual variations could account for regional inconsistencies between them. Results:  An average cap height of 0.175 ± 0.018 m was measured for Ls ~ 11⁰, which is in relatively good agreement with Cull et al. [12], but at Ls ~ 24⁰ and Ls ~ 29⁰ negative heights were measured, indicating an absence of CO2 ice. These negative heights are attributed to changes in shadow length due to non-uniform topography and changes in the illumination azimuth. It is important to note that the only clear rocks in this image are located on a slope which could also account for the negative values. This suggests a crocus date between Ls ~ 11⁰ and Ls ~ 24⁰. The calculated densities range between 361 kg/m3 and 560 kg/m3, and are highly dependent upon crocus date (Table 1). The exact crocus date for this location is unknown due to lack of seasonal coverage, though we do see that it occurs before the crocus date which TES observed.  The average cap height measured for Crater1 ranges from 0.358 ± 0.029 m at Ls ~ 26⁰ to 0.028 ± 0.036 m at Ls ~ 59⁰, and the TES-derived column density indicates a crocus date at Ls ~ 58⁰. From this we can conclude that the height measured at Ls ~ 59⁰ is due to changes in shadow length due to non-uniform topography and changes in the illumination azimuth. This constrains the crocus dates to between Ls ~ 43⁰ and 58⁰. Table 1. Effect of crocus date on density for Phoenix1. Season (Ls) 11 Crocus Date (Ls) Average Density (kg/m^3) 24 23 22 21 20 567 517 470 403 361  Densities derived from height measurements and TES column abundances increase until Ls ~ 35⁰ (Figure 1). This trend suggests densification of the ice deposits while sublimation is in effect. At Ls ~ 26⁰, calculated ice densities vary from 544 kg/m3 to 603 kg/m3 at crocus dates Ls 43⁰ and 44⁰, respectively. The variation in density due to the crocus dates becomes more extreme as the sublimation season approaches the crocus date. The largest variation in densities occurs at Ls ~ 43⁰, ranging from 383 kg/m3 for crocus date Ls 43⁰ (consistent with CO2 snow) to 1571 kg/m3 for crocus date Ls 44⁰ (comparable to CO2 slab ice). Figure 1 also indicates that for crocus dates after Ls = 44⁰; the densities are not well behaved, becoming denser than the maximum for slab CO2 ice. This further constrains the crocus date to between Ls ~ 43⁰ and 44⁰. It is interesting to note that the densities remain the same for crocus dates of Ls 43.5⁰ and 43.75⁰. This is due to the way in which TES interpolates column density data. In both regions, we determined crocus dates to be earlier than those derived from TES observations. Summary: The preliminary analysis of seasonal rock-shadow measurements from HiRISE images and TES-derived column abundances presented here suggests that the North Polar Seasonal Cap undergoes densification throughout the sublimation season. Measurements from two sites reveal an early spring density averaging much lower than that expected of slab CO2 ice. This suggests CO2 precipitation as the initial depositional process for these locations, which is in disagreement with spectral observations [6, 7]. Only one location had enough seasonal coverage to measure the density at multiple degrees of Ls and it was found that the ice deposits become much denser as the crocus date approaches. The density becomes unconstrained late in the sublimation process as it is highly dependent on the crocus date. Although crocus dates for both locations are not well constrained due to lack of seasonal coverage, our results indicate that CO2 disappearance occurs before the date observed by TES. Further study is necessary to determine whether this is a general trend or if this behavior is specific to these locations.   Figure 1. Combined height and column density data. Average density vs. season for Crater1. Each line corresponds to a different crocus date at Ls denoted by the numbers on the right side of the graph. At Ls ~ 43⁰ the density varies greatly with differing crocus dates. References: [1] Kelly, N. J. et al. (2006), JGR, 111, E03S07 [printed 112(E3), 2007]. [2] Titus, T. N. et al. (2008), in The Martian Surf.: Comp., Mineralogy, and Phys. Prop., Edited by J. F. Bell III, Cambridge University Press., 578-598. [3] Titus, T. N. et al (2001), JGR, 106, 23181-23196. [4] Haberle R. M. et al. (2004), GRL, 31, L057025. [5] Aharonson, O. et al. (2004), JGR, 109, E05004. [6] Kieffer, H. H. et al. (2000), JGR, 104, 9653-9699. [7] Kieffer, H. H. and Titus, T. N. (2001), Icarus, 154, 162-180. [8] Prettyman et al. (2009), JGR, 114, E08005. [9] Feldman, W. C. et al. (2003), JGR, 108(E9), 5103. [10] Smith, D. E. et al. (2001), Science, 294, 2141-2146. [11] Smith, D. E. et al. (2009), JGR, 114, E05002. [12] Cull, S. et al. (2010), JGR, 115, E00D16. 
