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Variability of the south polar cap of Mars in Mars years 28 and 29
Icarus 208 (2010) 82–85
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Variability of the south polar cap of Mars in Mars years 28 and 29 P.B. James a,*, P.C. Thomas b, M.C. Malin c a
Space Science Institute, 4750 Walnut St., Boulder, CO 80301, United States Center for Radiophysics and Space Research, Space Sciences Bldg., Cornell University, Ithaca, NY 14853, United States c Malin Space Science Systems, 9115 Brown Deer Rd., Suite 200, San Diego, CA 92121-2239, United States b
a r t i c l e
i n f o
Article history: Received 12 November 2009 Revised 22 January 2010 Accepted 7 February 2010 Available online 25 February 2010 Keywords: Mars, Polar Caps Mars, Polar Geology Mars, Atmosphere Mars, Climate
a b s t r a c t We have used Mars Reconnaissance Orbiter data from 2007 and 2009 to compare summer behaviors of the seasonal and residual south polar caps of Mars in those two years. We find that the planet-encircling dust storm that occurred in the first of the two Mars years enhanced the loss of seasonal CO2 deposits relative to the second year but did not have a large effect on the continuing erosion of the pits and mesas within the residual cap materials. This suggests that the increase of bright frost in some regions of the residual cap observed between Mariner 9 and Viking can be accommodated within observed martian weather variability and does not require unknown processes or climate change. Ó 2010 Elsevier Inc. All rights reserved.
The first data from the Mariner 9 spacecraft orbiting Mars in 1971–1972 showed an immense, planet-encircling dust storm; the south polar cap was barely discernable beneath the dust haze. Over the next several months, as the dust cleared, Mariner 9 documented the summer recession of the seasonal south polar cap to its residual configuration in late summer (Sharp et al., 1971) (Fig. 1-upper left). (For convenience, we adopt the following terminology: a Mars year (MY) is 687 Earth days; MY 1 is defined to start on the vernal equinox in 1955 (Clancy et al., 2000). Mariner 9 observations were of MY 9; Viking observations occurred in MY 12; and Mars Global surveyor observed MY 24–27. Seasons are designated by Ls, where Ls = 0° is northern vernal equinox; southern summer is then Ls 270–360°.) Viking Orbiter 2 observed the entire south polar cap recession and revealed a more extensive and brighter residual cap (Fig. 1-upper right) than three Mars years earlier (James et al., 1979). The CO2 remaining near the pole in late summer in a given year consists of two components: winter snow and frost deposits whose high albedo allows them to survive through the summer and the older, thicker deposits that display a wide variety of erosional forms (Thomas et al., 2000). The observations and calculations reported here are relevant to the former component. South polar cap recessions in MYs 24–27 observed by Mars Orbiter Camera (MOC) on Mars Global Surveyor at synoptic (1/2 km/ pixel) scale were similar to each other and to that observed by Viking when compared at identical Ls values (James et al., 2007; * Corresponding author. E-mail address: [email protected] (P.B. James). 0019-1035/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2010.02.007
Benson and James, 2005; Piqueux and Christensen, 2008). At 1.5– 3 m scales MOC data also revealed mesas and pits in portions of the CO2 surface of the RSPC and their contemporary erosion (Malin et al., 2001; Thomas et al., 2000), raising the questions: did the erosion of features observed within the RSPC suggest the future demise of the cap? Or, taken with the 1970s observations, did they suggest that there was a cycle with periods 10 Mars years of net annual deposition and erosion of CO2 driven by processes unknown? Information on the southern summers of MYs 28 and 29 comes from cameras with pixel scales of about 1 km, 5 m, and 0.25 m on Mars Reconnaissance Orbiter (MRO). Mars Color Imager (MARCI) (Bell et al., 2009) provided daily global images at 1 km/pixel that showed the behavior of the seasonal cap in summer in MY 28 resembled that observed by Mariner 9, while the cap in MY 29 more closely resembled observations by Viking and MGS (Fig. 1-lower left and right respectively). A large outlier of CO2 frost (Brown et al., 2010) at 83°S, 350°–30°W is a particularly notable indicator of variations in seasonal frost behavior. The outlier disappeared before Ls = 320° in MY 9 but in other years persisted as a thin strip until after Ls = 340°, when sublimation gives way to condensation. The behavior of the outlier in MY 28 is qualitatively similar to that observed by Mariner 9 but in MY 29 was again similar to that observed by Viking in MY 12 and by MGS in MYs 24–27. A simple energy balance model may be used to estimate the excess amount of CO2 that sublimed prior to Ls = 320° in MY 9 and MY 28. The CO2 sublimation that balances the insolation absorbed by the outlier frost in the more typical years between Ls = 320° and 360° must be effected by some other source in MY 9 and MY 28
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Fig. 1. Images of martian south polar cap in mid-summer acquired by: upper left Mariner 9 at Ls = 315° in MY 9 (081a21); upper right Viking Orbiter 2 at Ls = 315° in MY 12 (358B07, 48, 57, 58); lower left MARCI at Ls = 313° in MY 28 (P11_005305); and lower right MARCI at Ls = 313° in MY 29 (B11-014113). The grid on the MY 29 image will help locate features mentioned in the text; latitude circles are spaced at 3°, and 0° longitude is at the top. ‘‘X” marks the approximate location of the region shown in Fig. 2.
(assuming that the winter deposition was similar in the two years). We calculated the insolation absorbed by a CO2 surface between the disappearance of the outlier in a dust storm year less than the amount of energy radiated during that time and found that the excess CO2 sublimation was 100 kg/m2 (10 cm thick depending on the CO2 density, which is not well known) depending mainly on the exact Bond albedo (James et al., 1979). MYs 9 and 28 are unique in that they are the only two years observed from orbiting spacecraft that had major perihelic dust storms occurring in late spring when Mars is closest to the Sun near Ls = 250° (Cantor et al., 2008). Large dust storms that result in elevated atmospheric opacity over extended periods accelerate the sublimation of bright CO2 frost deposits by diverting insolation from the visible, where the CO2 reflects strongly, to the infrared, where the CO2 surface is more absorptive (Bonev et al., 2002, 2008). This changing radiative regime explained the more rapid disappearance of the frost feature termed ‘‘Mountains of Mitchel” in MY 25, following a large dust storm near equinox, Ls = 180°, than in MY 24 which lacked a large dust event (Bonev et al., 2002). It was later suggested that the radiative effects of a large perihelic storm, occurring between perihelion and solstice when insolation is maximum and most seasonal frost has sublimed, would be significant for both the outlier and the residual cap (Bonev et al., 2008). A simulated storm that produced a maximum optical depth of 2.0 over the RSPC at Ls = 260° and then decayed to background
opacity in 70 days was modeled and found to cause an additional 40–110 kg/m2 of CO2 sublimation (Bonev et al., 2008), where the range of uncertainty is due to the range of the surface models considered (various dust and water contaminations as well as grain sizes). These large perihelic dust storms are therefore viable candidates for the variations seen in the behavior of the outlier in different years. The maximum covering of seasonal frost is 1 m thick (Kelly et al., 2006; Kieffer et al., 2000; Prettyman et al., 2009; Smith et al., 2001). In MY 28, the Context Imager (CTX 5 m/pixel) on MRO showed little contrast variation within the cap until Ls 300° (Thomas et al., 2009) and further suggested that albedos later in summer reflected the character of the underlying variety of units of the residual cap (Fig. 2 (left)). CTX data in MY 29 showed that at similar seasons (Ls) the cap was brighter than in MY 28 (Fig. 2 (right)). Regions in which the frost cover had been fully or partially removed in MY 28 were still frost covered in MY 29. Although spacecraft problems in the fall of terrestrial year 2009 prevented comparison of the MY 29 cap to the MY 28 cap after Ls = 325°, the increased longevity of frost deposits in the regions of the RSPC in Fig. 2 is established. The ongoing erosion of the residual cap pits and mesas can be assessed for MYs 28 and 29 with HiRISE (McEwen et al., 2007) images at 0.25–0.50 m/pixel. It was previously established for MYs 24–27 (Thomas et al., 2005) and MYs 24–28 (Thomas et al.,
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Fig. 2. (a) (left) is a MY 28 CTX image acquired at Ls = 319° that has been radiatively and geometrically corrected and converted to Lambert albedo by dividing by the cosine of the incidence angle (P11_005438_0936_XN_86S001W_070924). It shows a region of the south polar cap that brightened between Mariner 9 and Viking (the feature in the top right hand corner is the ‘‘tooth” region (Thomas et al., 2009) at 86.3°S, 0.5°W). (b) The ratio of a MY 29 image acquired at Ls = 323° and processed identically to the preceding image, B12_014338_0937_XI_86S000W_090817. Note that the brighter, B12 image is later in the season than the MY 28 image, enhancing the disparity in albedos.
Fig. 3. Erosion of residual cap pits. (a) Changes in pit walls and septa between pits in thicker unit of residual south polar cap, unit A1 (Thomas et al., 2009) measured from HiRISE and MOC images. Changes are relative to initial MOC observation in MY 24. The upper inset shows one of the depressions, 200 m in diameter at Ls 323 in Mars year 28, top, and MY 29, bottom. The smaller inset expands single-wall change; the data are from 86.9S, 6.5W. (b) Changes in thinner B unit (Thomas et al., 2005). The MOC data are from only MY 25. The upper inset shows typical depression at Ls 315.7 MY 28 (top) and Ls 309.4 MY 29 (bottom). The lower inset expands wall view. Images listed in Supplemental data. Located at 86.2S, 9.8W.
2009) that walls of pits in the thicker, older unit retreated at 3.3– 3.8 m/MY, and in the thinner unit at 2 m/MY. Measurement of wall retreat in HiRISE and MOC images shows that MY 28 erosion did not materially deviate from those averages (Fig. 3). The lack of correlation with the simple surface-sky radiative balance is not
surprising, because the backwasting of these pit walls evidently involves fracture and collapse (Thomas et al., 2009; Byrne et al., 2008), not just sublimation loss. However, the upper surface of the residual cap has polygonal troughs, polygonal ridges, and some exposures of 0.1 m layers (Thomas et al., 2009); the visibility of these features might be expected to depend on sublimation (or lack thereof) of materials that are sensitive to radiative balance. Most of these features were unaltered between MYs 28 and 29, indicating that any net erosion or deposition on the bright, horizontal surface of the RSPC is no more than the 0.1 m estimated from the modeled thermal imbalance. Thus, the recent data show perihelic dust storms affect the amount of seasonal frost but do not have a major influence on the erosion of the residual cap. If a residual cap is a relatively long-lived feature of Mars, net condensation of CO2 in some years is required to counteract the physical erosion and the radiative effects of dust that lead to its removal. The changes (Fig. 2 (right)) in the frost distribution at around Ls = 320° in MY 29 suggest that there was net condensation of CO2 in MY 29. However, a simple energy balance model indicates that the insolation absorbed by the bright frost does not fall to the break even point between sublimation and condensation until Ls 330–340°, depending on the Bond albedo of the ice, after MRO data were lost. Therefore, the data do not completely rule out the possibility that the additional frost in MY 29 sublimes before the end of summer. The pattern of large dust storms over the last 40 years is quite variable (Zurek and Martin, 1993) and the cap may be also affected to a lesser degree by smaller dust events which also occur irregularly (Smith, 2009). Effects of dust are a plausible explanation for short term (10 Mars years) variations within the residual cap as in MYs 9 and 28. However, this does not answer questions concerning the longer term evolution, or even the existence of, the RSPC. Parts of the residual cap appear to have been eroding for >100 Mars years (Byrne and Ingersoll, 2003; Thomas et al., 2005), and the thick deposits (>10 m) in the mesas that are currently eroding would require more than 100 Mars years to accumulate at the calculated scale of radiative imbalance (Thomas et al., 2009). Continued erosion at present rates would not erase the residual cap for at least many Mars decades (Malin et al., 2001) even with no surviving winter deposition and assuming no greater occurrence of perihelic storms. The observations reported here indicate that modest variations in weather patterns can switch between net condensation and sublimation of seasonal CO2 while leaving the physical erosion of features within the RSPC unaffected.
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Therefore it is not necessary to invoke any secular trend in Mars climate to explain variations in the seasonal cap and RSPC observed by spacecraft over the last 20 Mars years. However, the data do not address the possibility of longer-term secular changes. Acknowledgments L. Posiolova, B. Carcich, K. Consroe, N. Button provided technical assistance. B. Cantor, K. Edgett, B. Haberle provided valuable discussions. The early availability of HiRISE image data is gratefully acknowledged. We also appreciate the helpful comments of two reviewers. Supported in part by the Mars Reconnaissance Orbiter Project. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.icarus.2010.02.007. References Bell, J.F. and 21 colleagues, 2009. Mars Reconnaissance Orbiter Mars Color Imager (MARCI): Instrument description, calibration, and performance. J. Geophys. Res. 114, E8 (CiteID E08S92). Benson, J.L., James, P.B., 2005. Yearly comparisons of the martian polar caps: 1999– 2003 Mars Orbiter Camera observations. Icarus 174, 513–523. Bonev, B.P., James, P.B., Bjorkman, J.E., Wolff, M.J., 2002. Regression of the Mountains of Mitchel polar ice after the onset of a global dust storm on Mars. Geophys. Res. Lett. 29, 13-1 (CiteID 2017). Bonev, B.P., Hansen, G.B., Glenar, D.A., James, P.B., Bjorkman, J.E., 2008. Albedo models for the residual south polar cap on Mars: Implications for the stability of the cap under near-perihelion global dust storm conditions. Planet Space Sci. 56, 181–193. Brown, A.J., Calvin, W.M., McGuire, P.C., Murchie, S.L., 2010. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) south polar mapping: First Mars year of observations. J. Geophys. Res., 115 (CiteID E00D13). Byrne, S., Ingersoll, A.P., 2003. Martian climatic events on timescales of centuries: Evidence from feature morphology in the residual south polar ice cap. Geophys. Res. Lett. 30 (13), 29-1. Byrne, S., Russell, P.S., Fishbaugh, K.E., Hansen, C.J., Herkenhoff, K.E., McEwen, A.S., and the Hirise Team. 2008. Explaining the persistence of the southern residual cap of Mars: HiRISE data and landscape evolution models. Lunar Planet. Sci. 39. Abstract 2252.
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Cantor, B.A., Malin, M.C., Wolff, M.J., Haberle, R.M., James, P.B., Clancy, R.T., Lee, S.W., 2008. Observations of the Martian Atmosphere by MRO-MARCI, an Overview of One Mars Year. Mars Atmosphere: Modeling and Observations Workshop, Williamsburg, VA. Abstract 9075. Clancy, R.T., Sandor, B.J., Wolff, M.J., Christensen, P.R., Smith, M.D., Pearl, J.C., Conrath, B.J., Wilson, R.J., 2000. An intercomparison of ground-based millimeter, MGS TES, and Viking atmospheric temperature measurements: Seasonal and interannual variability of temperatures and dust loading in the global Mars atmosphere. J. Geophys. Res. 105, 9553–9572. James, P.B., Briggs, G., Barnes, J., Spruck, A., 1979. Seasonal recession of Mars’ south polar cap as seen by Viking. J. Geophys. Res. 84, 2889–2922. James, P.B., Thomas, P.C., Wolff, M.J., Bonev, B.P., 2007. MOC observations of four Mars year variations in the south polar residual cap of Mars. Icarus 192, 318– 326. Kelly, N.J., Boynton, W.V., Kerry, K., Hamara, D., Janes, D., Reedy, R.C., Kim, K.J., Haberle, R.M., 2006. Seasonal polar carbon dioxide frost on Mars: CO2 mass and columnar thickness distribution. J. Geophys. Res. 111, E3 (CiteID E03S07). Kieffer, H.H., Titus, T.N., Mullins, K.F., Christensen, P., 2000. Mars polar spring and summer behavior observed by TES: Seasonal cap evolution controlled by frost grain size. J. Geophys. Res. 105, 9653–9699. Malin, M.C., Caplinger, M.A., Davis, S.D., 2001. Observational evidence for an active surface reservoir of solid carbon dioxide on Mars. Science 294, 2146–2148. McEwen, A.S., and 14 colleagues 2007. Mars Reconnaissance Orbiter’s High Resolution Imaging Science Experiment (HiRISE). J. Geophys. Res. 112, E5 (CiteID E05S02). Piqueux, S., Christensen, P.R., 2008. Deposition of CO2 and erosion of the martian south perennial cap between 1972 and 2004: Implications for current climate change. J. Geophys. Res. 113, E2 (CiteID E02006). Prettyman, T.H., Titus, T.N., Colaprete, A., 2009. Characterization of Mars’ seasonal caps using neutron spectroscopy. J. Geophys. Res., 114 (CiteID E08005). Sharp, R.P., Murray, B.C., Leighton, R.B., Soderblom, L.A., Cutts, J.A., 1971. The surface of Mars. 4. South polar cap. J. Geophys. Res. 76, 357–368. Smith, M.D., 2009. THEMIS observations of Mars aerosol optical depth from 2002– 2008. Icarus 202, 444–452. Smith, D.E., Zuber, M.T., Neumann, G.A., 2001. Seasonal variations of snow depth on Mars. Science 294, 2141–2146. Thomas, P.C., Malin, M.C., Edgett, K.S., Carr, M.H., Hartmann, W.K., Ingersoll, A.P., James, P.B., Soderblom, L.A., Veverka, J., Sullivan, R., 2000. North–south geological differences between the residual polar caps on Mars. Nature 404, 181–184. Thomas, P.C., Malin, M.C., James, P.B., Cantor, B.A., Williams, R.M.E., Gierasch, P., 2005. South polar residual cap of Mars: Features, stratigraphy, and changes. Icarus 174, 535–559. Thomas, P.C., James, P.B., Calvin, W.M., Haberle, R., Malin, M.C., 2009. Residual south polar cap of Mars: Stratigraphy, history, and implications of recent changes. Icarus 203, 352–375. Zurek, R.W., Martin, L.J., 1993. Interannual variability of planet-encircling dust storms on Mars. J. Geophys. Res. 98, 3247–3259.
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Variability of the south polar cap of Mars in Mars years 28 and 29 P.B. James a,*, P.C. Thomas b, M.C. Malin c a
Space Science Institute, 4750 Walnut St., Boulder, CO 80301, United States Center for Radiophysics and Space Research, Space Sciences Bldg., Cornell University, Ithaca, NY 14853, United States c Malin Space Science Systems, 9115 Brown Deer Rd., Suite 200, San Diego, CA 92121-2239, United States b
a r t i c l e
i n f o
Article history: Received 12 November 2009 Revised 22 January 2010 Accepted 7 February 2010 Available online 25 February 2010 Keywords: Mars, Polar Caps Mars, Polar Geology Mars, Atmosphere Mars, Climate
a b s t r a c t We have used Mars Reconnaissance Orbiter data from 2007 and 2009 to compare summer behaviors of the seasonal and residual south polar caps of Mars in those two years. We find that the planet-encircling dust storm that occurred in the first of the two Mars years enhanced the loss of seasonal CO2 deposits relative to the second year but did not have a large effect on the continuing erosion of the pits and mesas within the residual cap materials. This suggests that the increase of bright frost in some regions of the residual cap observed between Mariner 9 and Viking can be accommodated within observed martian weather variability and does not require unknown processes or climate change. Ó 2010 Elsevier Inc. All rights reserved.
The first data from the Mariner 9 spacecraft orbiting Mars in 1971–1972 showed an immense, planet-encircling dust storm; the south polar cap was barely discernable beneath the dust haze. Over the next several months, as the dust cleared, Mariner 9 documented the summer recession of the seasonal south polar cap to its residual configuration in late summer (Sharp et al., 1971) (Fig. 1-upper left). (For convenience, we adopt the following terminology: a Mars year (MY) is 687 Earth days; MY 1 is defined to start on the vernal equinox in 1955 (Clancy et al., 2000). Mariner 9 observations were of MY 9; Viking observations occurred in MY 12; and Mars Global surveyor observed MY 24–27. Seasons are designated by Ls, where Ls = 0° is northern vernal equinox; southern summer is then Ls 270–360°.) Viking Orbiter 2 observed the entire south polar cap recession and revealed a more extensive and brighter residual cap (Fig. 1-upper right) than three Mars years earlier (James et al., 1979). The CO2 remaining near the pole in late summer in a given year consists of two components: winter snow and frost deposits whose high albedo allows them to survive through the summer and the older, thicker deposits that display a wide variety of erosional forms (Thomas et al., 2000). The observations and calculations reported here are relevant to the former component. South polar cap recessions in MYs 24–27 observed by Mars Orbiter Camera (MOC) on Mars Global Surveyor at synoptic (1/2 km/ pixel) scale were similar to each other and to that observed by Viking when compared at identical Ls values (James et al., 2007; * Corresponding author. E-mail address: [email protected] (P.B. James). 0019-1035/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2010.02.007
Benson and James, 2005; Piqueux and Christensen, 2008). At 1.5– 3 m scales MOC data also revealed mesas and pits in portions of the CO2 surface of the RSPC and their contemporary erosion (Malin et al., 2001; Thomas et al., 2000), raising the questions: did the erosion of features observed within the RSPC suggest the future demise of the cap? Or, taken with the 1970s observations, did they suggest that there was a cycle with periods 10 Mars years of net annual deposition and erosion of CO2 driven by processes unknown? Information on the southern summers of MYs 28 and 29 comes from cameras with pixel scales of about 1 km, 5 m, and 0.25 m on Mars Reconnaissance Orbiter (MRO). Mars Color Imager (MARCI) (Bell et al., 2009) provided daily global images at 1 km/pixel that showed the behavior of the seasonal cap in summer in MY 28 resembled that observed by Mariner 9, while the cap in MY 29 more closely resembled observations by Viking and MGS (Fig. 1-lower left and right respectively). A large outlier of CO2 frost (Brown et al., 2010) at 83°S, 350°–30°W is a particularly notable indicator of variations in seasonal frost behavior. The outlier disappeared before Ls = 320° in MY 9 but in other years persisted as a thin strip until after Ls = 340°, when sublimation gives way to condensation. The behavior of the outlier in MY 28 is qualitatively similar to that observed by Mariner 9 but in MY 29 was again similar to that observed by Viking in MY 12 and by MGS in MYs 24–27. A simple energy balance model may be used to estimate the excess amount of CO2 that sublimed prior to Ls = 320° in MY 9 and MY 28. The CO2 sublimation that balances the insolation absorbed by the outlier frost in the more typical years between Ls = 320° and 360° must be effected by some other source in MY 9 and MY 28
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Fig. 1. Images of martian south polar cap in mid-summer acquired by: upper left Mariner 9 at Ls = 315° in MY 9 (081a21); upper right Viking Orbiter 2 at Ls = 315° in MY 12 (358B07, 48, 57, 58); lower left MARCI at Ls = 313° in MY 28 (P11_005305); and lower right MARCI at Ls = 313° in MY 29 (B11-014113). The grid on the MY 29 image will help locate features mentioned in the text; latitude circles are spaced at 3°, and 0° longitude is at the top. ‘‘X” marks the approximate location of the region shown in Fig. 2.
(assuming that the winter deposition was similar in the two years). We calculated the insolation absorbed by a CO2 surface between the disappearance of the outlier in a dust storm year less than the amount of energy radiated during that time and found that the excess CO2 sublimation was 100 kg/m2 (10 cm thick depending on the CO2 density, which is not well known) depending mainly on the exact Bond albedo (James et al., 1979). MYs 9 and 28 are unique in that they are the only two years observed from orbiting spacecraft that had major perihelic dust storms occurring in late spring when Mars is closest to the Sun near Ls = 250° (Cantor et al., 2008). Large dust storms that result in elevated atmospheric opacity over extended periods accelerate the sublimation of bright CO2 frost deposits by diverting insolation from the visible, where the CO2 reflects strongly, to the infrared, where the CO2 surface is more absorptive (Bonev et al., 2002, 2008). This changing radiative regime explained the more rapid disappearance of the frost feature termed ‘‘Mountains of Mitchel” in MY 25, following a large dust storm near equinox, Ls = 180°, than in MY 24 which lacked a large dust event (Bonev et al., 2002). It was later suggested that the radiative effects of a large perihelic storm, occurring between perihelion and solstice when insolation is maximum and most seasonal frost has sublimed, would be significant for both the outlier and the residual cap (Bonev et al., 2008). A simulated storm that produced a maximum optical depth of 2.0 over the RSPC at Ls = 260° and then decayed to background
opacity in 70 days was modeled and found to cause an additional 40–110 kg/m2 of CO2 sublimation (Bonev et al., 2008), where the range of uncertainty is due to the range of the surface models considered (various dust and water contaminations as well as grain sizes). These large perihelic dust storms are therefore viable candidates for the variations seen in the behavior of the outlier in different years. The maximum covering of seasonal frost is 1 m thick (Kelly et al., 2006; Kieffer et al., 2000; Prettyman et al., 2009; Smith et al., 2001). In MY 28, the Context Imager (CTX 5 m/pixel) on MRO showed little contrast variation within the cap until Ls 300° (Thomas et al., 2009) and further suggested that albedos later in summer reflected the character of the underlying variety of units of the residual cap (Fig. 2 (left)). CTX data in MY 29 showed that at similar seasons (Ls) the cap was brighter than in MY 28 (Fig. 2 (right)). Regions in which the frost cover had been fully or partially removed in MY 28 were still frost covered in MY 29. Although spacecraft problems in the fall of terrestrial year 2009 prevented comparison of the MY 29 cap to the MY 28 cap after Ls = 325°, the increased longevity of frost deposits in the regions of the RSPC in Fig. 2 is established. The ongoing erosion of the residual cap pits and mesas can be assessed for MYs 28 and 29 with HiRISE (McEwen et al., 2007) images at 0.25–0.50 m/pixel. It was previously established for MYs 24–27 (Thomas et al., 2005) and MYs 24–28 (Thomas et al.,
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Fig. 2. (a) (left) is a MY 28 CTX image acquired at Ls = 319° that has been radiatively and geometrically corrected and converted to Lambert albedo by dividing by the cosine of the incidence angle (P11_005438_0936_XN_86S001W_070924). It shows a region of the south polar cap that brightened between Mariner 9 and Viking (the feature in the top right hand corner is the ‘‘tooth” region (Thomas et al., 2009) at 86.3°S, 0.5°W). (b) The ratio of a MY 29 image acquired at Ls = 323° and processed identically to the preceding image, B12_014338_0937_XI_86S000W_090817. Note that the brighter, B12 image is later in the season than the MY 28 image, enhancing the disparity in albedos.
Fig. 3. Erosion of residual cap pits. (a) Changes in pit walls and septa between pits in thicker unit of residual south polar cap, unit A1 (Thomas et al., 2009) measured from HiRISE and MOC images. Changes are relative to initial MOC observation in MY 24. The upper inset shows one of the depressions, 200 m in diameter at Ls 323 in Mars year 28, top, and MY 29, bottom. The smaller inset expands single-wall change; the data are from 86.9S, 6.5W. (b) Changes in thinner B unit (Thomas et al., 2005). The MOC data are from only MY 25. The upper inset shows typical depression at Ls 315.7 MY 28 (top) and Ls 309.4 MY 29 (bottom). The lower inset expands wall view. Images listed in Supplemental data. Located at 86.2S, 9.8W.
2009) that walls of pits in the thicker, older unit retreated at 3.3– 3.8 m/MY, and in the thinner unit at 2 m/MY. Measurement of wall retreat in HiRISE and MOC images shows that MY 28 erosion did not materially deviate from those averages (Fig. 3). The lack of correlation with the simple surface-sky radiative balance is not
surprising, because the backwasting of these pit walls evidently involves fracture and collapse (Thomas et al., 2009; Byrne et al., 2008), not just sublimation loss. However, the upper surface of the residual cap has polygonal troughs, polygonal ridges, and some exposures of 0.1 m layers (Thomas et al., 2009); the visibility of these features might be expected to depend on sublimation (or lack thereof) of materials that are sensitive to radiative balance. Most of these features were unaltered between MYs 28 and 29, indicating that any net erosion or deposition on the bright, horizontal surface of the RSPC is no more than the 0.1 m estimated from the modeled thermal imbalance. Thus, the recent data show perihelic dust storms affect the amount of seasonal frost but do not have a major influence on the erosion of the residual cap. If a residual cap is a relatively long-lived feature of Mars, net condensation of CO2 in some years is required to counteract the physical erosion and the radiative effects of dust that lead to its removal. The changes (Fig. 2 (right)) in the frost distribution at around Ls = 320° in MY 29 suggest that there was net condensation of CO2 in MY 29. However, a simple energy balance model indicates that the insolation absorbed by the bright frost does not fall to the break even point between sublimation and condensation until Ls 330–340°, depending on the Bond albedo of the ice, after MRO data were lost. Therefore, the data do not completely rule out the possibility that the additional frost in MY 29 sublimes before the end of summer. The pattern of large dust storms over the last 40 years is quite variable (Zurek and Martin, 1993) and the cap may be also affected to a lesser degree by smaller dust events which also occur irregularly (Smith, 2009). Effects of dust are a plausible explanation for short term (10 Mars years) variations within the residual cap as in MYs 9 and 28. However, this does not answer questions concerning the longer term evolution, or even the existence of, the RSPC. Parts of the residual cap appear to have been eroding for >100 Mars years (Byrne and Ingersoll, 2003; Thomas et al., 2005), and the thick deposits (>10 m) in the mesas that are currently eroding would require more than 100 Mars years to accumulate at the calculated scale of radiative imbalance (Thomas et al., 2009). Continued erosion at present rates would not erase the residual cap for at least many Mars decades (Malin et al., 2001) even with no surviving winter deposition and assuming no greater occurrence of perihelic storms. The observations reported here indicate that modest variations in weather patterns can switch between net condensation and sublimation of seasonal CO2 while leaving the physical erosion of features within the RSPC unaffected.
P.B. James et al. / Icarus 208 (2010) 82–85
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