North-south components of slope in the global topography of Mars: Evidence for an ice-rich shallow crust? Jørn Atle Jernsletten Jørn Atle Jernsletten ( Abstract #P31C-0219 ) ( Abstract #P31C-0219 ) Engineering Geophysics Laboratory Engineering Geophysics Laboratory Dept. of Civil & Environmental Engineering Dept. of Civil & Environmental Engineering University of Nevada, Las Vegas University of Nevada, Las Vegas 2005 AGU Fall Meeting 2005 AGU Fall Meeting San Francisco, CA - Wednesday, December 7, 2005 San Francisco, CA - Wednesday, December 7, 2005 Table 1 & Figure 1. Profiles of Trough in Northern Polar Layered Deposits. Northern Polar Layered Deposits. [Adapted from Fenton and Herkenhoff, 2000]

Figure 2. North-South Components of Slope on Mars (Valles Marineris Region) Key: Equatorward slope components are shown in dark red, poleward slope components are shown in dark blue. The way in which these slope components emphasizes reworked materials such as those in impact craters (Figure 2) make them appropriate quantities from which to study the effects of sublimation of ground ice expected to be contained within the shallow crust of Mars at certain latitudes (>30º–40º).

Figure 3. Global North-South Components of Slope on Mars To note in Figure 3 is the fact that given the large number of datapoints (11520 each for equatorward and poleward slope components), equatorward slope components exhibit a higher number of steeper slopes (the bulk of equatorward datapoints are hidden behind the poleward datapoints).

Figure 4. Differences in Global North-South Components of Slope on Mars Figure 4 shows the differences between equatorward and poleward slope components (equatorward slope component minus poleward slope component at each increment in latitude). The steeper equatorward slope components are especially evident in the mid latitudes (30º–60º, Figure 4).

Table 2. Paired-Samples T-Test Differences in North-South Components of Slope (> 5º) The ± ranges are the statistical 95% confidence intervals of the differences. Negative differences are shaded. Equatorward north-south slope components steeper than poleward north-south slope components in all cases, except Southern Hemisphere vs. High Latitudes (*, ‑ 0.072º ± 0.048º). components in all cases, except Southern Hemisphere vs. High Latitudes (*, ‑ 0.072º ± 0.048º). All differences statistically significant at the 99% (0.01) level or above, except Global (Both Hemispheres) vs. High Latitudes at 60°- 80° Latitudes (**), which is significant at the 95% (0.05) level or above. High Latitudes at 60°- 80° Latitudes (**), which is significant at the 95% (0.05) level or above. N (Global, 0°- 90° Latitudes) = 11520, N (N or S Hemisphere, 0°- 90° Latitudes) = 5760, N (Global, 30°- 60° Latitudes) = 3840, N (N or S Hemisphere, 30°- 60° Latitudes) = 1920, N (Global, 60°- 80° Latitudes) = 2560, N (N or S Hemisphere, 60°- 80° Latitudes) = 1280.

Table 3. Spearman Rank Correlations of North- South Slope Components and Differences Positive correlations are shaded. All correlations 2-tailed significant at the 99% (0.01) level or above, except Northern Hemisphere Equatorward vs. Latitude at 60°- 80° Latitudes (*), which is significant at the 95% (0.05) Equatorward vs. Latitude at 60°- 80° Latitudes (*), which is significant at the 95% (0.05) level or above; and Southern Hemisphere Difference vs. Incidence at 0°- 90° Latitudes and level or above; and Southern Hemisphere Difference vs. Incidence at 0°- 90° Latitudes and vs. Latitude at 60°- 80° Latitudes (**), which are not statistically significant. vs. Latitude at 60°- 80° Latitudes (**), which are not statistically significant. N (Global, 0°- 90° Latitudes) = 11520, N (N or S Hemisphere, 0°- 90° Latitudes) = 5760, N (Global, 30°- 60° Latitudes) = 3840, N (N or S Hemisphere, 30°- 60° Latitudes) = 1920, N (Global, 60°- 80° Latitudes) = 2560, N (N or S Hemisphere, 60°- 80° Latitudes) = 1280.

Figure 5. Counts of Global North-South Components of Slope on Mars (> 5º) Figure 5 shows counts of slope components >5º, equatorward and poleward respectively. Note that sample sizes (counts) are statistically significant at all latitude bands, although just barely so at the 60º to 80º latitudes in the northern hemisphere, yielding high uncertainties in the differences at these latitudes (ref. Table 2).

Figure 6. Paired-Samples T-Tests Over 19 Latitude Ranges 10º latitude bands centered at every 10º of latitude from 85º north to 85º, plus equator. latitude from 85º north to 85º, plus equator. All differences statistically significant at the 95% (0.05) level or above, except those labeled (0.05) level or above, except those labeled (65º and 55º north, and 65 south), which (65º and 55º north, and 65 south), which are not significant. are not significant. Note in Figure 6 that equatorward slope components are significantly steeper in the mid latitudes, particularly in the southern hemisphere.

Summary From an analysis of north- south components of slope angle in the global topography of Mars, this study asks the question: Are equatorward facing slopes in the mid to high latitudes are steeper than poleward facing slopes?, based in the hypothesis of sublimation exhumation of very ice-rich materials. Past studies [Fenton and Herkenhoff, 2000 – Icarus, 147, 433 – 443 ] have shown that equatorward (warmer) slopes are steeper than the poleward slopes as measured in individual troughs in the polar layered deposits. Empirical results from this study show that equatorward facing slopes range from 0.1º to 0.3º steeper than poleward facing slopes, thus lending topographical evidence for an ice-rich shallow crust on Mars in the mid to high latitudes. This study is based on MOLA 1/64º gridded elevation data, from which slope angles, slope aspects, north-south components of slope angles, and other derivative data are calculated.