49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 2455.pdf

Mars North Polar Cap Recession and Summer Variation: Five Mars Years of MARCI Observations. W. M. Calvin1, B. C. Cantor2, and P. B. James3, 1Dept. Geological Sci., University of Nevada, MS 172, Reno, NV 89557 ([email protected]), 2Malin Space Science Systems, San Diego, CA, 3Planetary Science Institute, Prescott, AZ.

Introduction: The annual condensation and subli- Figure 2 shows the residual ice deposits near ~Ls mation of roughly 25% of the atmosphere onto the pole 130 in each MY from 28-33. Of note is that the albedo that is in winter drives the Martian climate. This sea- pattern on Olympia Planum is highly variable in each sonal advance and retreat has been monitored from a va- MY, and much of the region remains obscured by a dust riety of spacecraft and instruments, including Viking, lag in both MY 32 and 33, in contrast to prior years. This MGS, Mars Express, and MRO [1-7]. The retreat of the region does again brighten after Ls 155, however it is north seasonal cap is generally symmetric about the ge- difficult to discern in MARCI imagery if this is due to ographic pole, with a very repeatable pattern of the lati- deposition of new seasonal frost or late stage removal of tude of the cap edge with Ls [3,6,7]. However, dust dust lags. A large section of the Gemini Scopuli region storms have some impact on recession rate, with Mars remains dust covered in MY 32, unlike most other years. Year (MY) 29 being the fastest northern recession ob- Abalos Mensa was shown to reach a minimum ex- served [3,6,7] following the planet-encircling storm of tent of the bright deposits in MY 30, followed by large 2007. The MY convention is defined by [8]. The local exposures in MY31 [7]. In both MY 32 and 33, this edge of the retreating seasonal cap has a large degree of small mound at the margin of the polar dome continues interannual variability, as does the summer appearance to have large exposures of ice, most similar to MY 24 in of residual ice deposits. extent. Nearby, the Rupes Tenius is also dark in MY 30 Major Features from Past Observations: We and 31, again brightening in MY 32 and 33. have previously reported on the north cap retreat as ob- Movies of daily images show that the Gemini served by MARCI in MY 29-31 [6,7]. That work iden- Scopuli region continues to brighten in localized areas tified several common and repeatable albedo trends. after Ls 117, and the patterns vary from year to year First the albedo pattern of the residual ice and surround- (Figure 2). Large interannual variability suggest the in- ing dark dune field are apparent through the seasonal terplay between surface dust deposits and local weather frost cover, even at the earliest stages of retreat. The sea- and storms, and dust removal is often seen to link to dust sonal frost retreats with latitude and then with elevation, clouds over small areas. leaving the areas of Gemini Scopuli, the lower eleva- Late (up to Ls 85) seasonal frost covering the Olym- tions of Gemina Lingula, Rupes Tenius, and Abalos pia Undae area where gypsum has been identified was Mensa dark, and reaching a minimum of bright deposits noted in MY29 [6]. Frost patterns over Olympia Undae by Ls 95. Subsequently, these areas again brighten, near the summer solstice vary a great deal between Mars reaching a maximum exposure of ice (under clear skies) Years, but no year has the large extent seen in MY29. near Ls 135, just prior to the onset of polar hood for- Future Work: Long observation of the seasonal cap mation. Cantor et al. [6] suggested the brightening retreat by MARCI will allow us to construct an average could be caused either by deposition of new frost or re- or climate model for the optical boundary of the retreat- moval of a dust lag, and Calvin et al. [7] favored the dust ing seasonal cap. We plan to explore separation of wa- lag removal mechanism due to the wide range of eleva- ter and carbon dioxide ices using properties in all of tions and orientations where bright deposits reappear. MARCI’s color filters. Brown et al. [9] used CRISM data to argue for new frost References: [1] James P.B. et al. Ch. 27 in Mars, U deposition on the residual ice dome after ~ Ls 140. Arizona Press, pp. 934-68, 1992. [2] Titus, T.N. 36th New Observations: We now have an additional LPSC Abstract # 1993, 2005. [3] Piqueux, S. et al. Ica- two Mars Years of MARCI observations. Calculation rus, 251, p. 164-180, 2015. [4] Appéré, T. et al., JGR – of the optical edge of the retreating seasonal cap is un- Planets 116, http://dx.doi.org/10.1029/2010je003762, derway and will be synthesized with previous measure- 2011. [5] Brown, A.J. et al., JGR – Planets 117, 2012. ments. Of note in MY33, a late stage ice deposit near http://dx.doi.org/10.1029/2012je004113. [6] Cantor, lat 78N, long 40E retreats slowly until ~ Ls 85, and B.A. et al., Icarus 208, 61–81, 2010. leaves behind a bright dust deposit on the otherwise dark http://dx.doi.org/10.1016/j.icarus.2010.01.032,2010. dune material in a region where extensive bright depos- [7] Calvin, W.M. et al., Icarus 251, 181–190, 2015. its are not observed in any other MY (Figure 1). This https://doi.org/10.1016/j.icarus.2014.08.026. [8] Pi- dust deposit is eventually eroded, by Ls 160, lending ob- queux, S. et al. Icarus, 251, p 332-338, 2015. [9] Brown, servational support in a nearby region to the hypothesis A.J. et al., Icarus, 277, 401-415, 2016. doi:10.1016/j.ic- that large scale brightening on the residual ice dome af- arus.2016.05.007. ter Ls 95 is caused by removal of a dust lag. 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 2455.pdf

Figure 1 (left): In MY 33 late frost leaves a bright dust lag on a dark area of the circumpolar dunes (arrows, Ls 78 and 116) that is eventually re- moved. MY 29 (lowest image) shows the typical appearance of this region in other MY. This observation supports the hypothesis that brightening of the residual ice dome after Ls 95 is related to dust removal rather than new frost deposition.

Figure 2 (above): Appearance of the north residual ice dome in each of the six years observed by MARCI. Ls ranges from 129 to 133 as noted on the image. This Ls was chosen as the most clear image showing the maximum extent of ice deposits prior to the onset of significant cloud (both dust and ice) activity. Also, MRO did not begin systematic mapping until Novem- ber of 2006. Arrows call out the maximum ice exposures on Olympia Planum (MY30), and at the end of Gemini Lingula (MY33), and the mini- mum exposure on Gemini Scopuli (MY32).