Role of Volatiles and Atmospheres on Martian Impact Craters 2005 3012.pdf AGES AND ONSET DIAMETERS OF RAMPART CRATERS IN EQUATORIAL REGIONS ON MARS. D. Reiss1, S. van Gasselt2, E. Hauber1, G. Michael1, R. Jaumann1, G. Neukum2 and the HRSC Co-Investigator Team. 1Institute of Planetary Research, DLR, Rutherfordstr. 2, 12489 Berlin, Germany ([email protected]) 2Institute for Geosciences, Freie Universitaet Berlin, Malteserstr. 74-100, 12249 Berlin, Germany. Introduction: Many large craters on Mars exhibit [13] (Figure 2A). The formation rate of rampart craters ejecta blankets which are not observed on other terres- declines in the Hesperian, whereas the onset diameters trial planets like the Moon [1]. As found by many re- increase. At the Hesperian-Amazonian boundary the searchers [e.g. 2, 3, 4] the morphology is suggested to formation comes to an end. This might indicate a low- be caused by volatile rich target material [2] or atmos- ering of the ground ice table with time which in Xan- pheric effects [5]. However, in a given area a certain the Terra, if present at all, could be several kilometers minimum diameter exists for craters which show fluid- deep in present days. Either all the ground ice was lost ized ejecta blankets [6, 7], called the onset diameter. with time due to diffusion to the atmosphere [e.g. 14] Geographic mapping shows a latitude dependence of or there is still a deep ground ice layer which can only the onset diameters [8, 9]. In equatorial regions the be reached by relatively large (and in recent times rare) onset diameters are typically 4 to 7 km versus 1 to 2 impacts. km in high latitudes (50° latitude), which might indi- cate a ice rich layer at depths of about 300 to 400 m near the equator and ~100 m at 50° latitudes [8]. As pointed out by [1] rampart craters may have formed over a significant time interval and therefore reflect the ground ice depths at a given time. We determined the absolute ages and onset diame- ters of rampart craters in three equatorial regions on Mars by measuring the ejecta blankets’ superposed crater frequencies in Mars Express High Resolution Stereo Camera (HRSC) imagery [10] in three equato- rial regions (Figure 1). Figure 2. (A) Rampart crater in the Xanthe Terra region with an absolute model age of ~3.8 Gyr (HRSC-orbit 927 at 5°N and 310°E). A lateral valley of Nanedi Vallis eroded into the ejecta after the formation of the rampart. (B) Rampart crater in the southern Chryse region on the channel floor of Tiu Vallis with an absolute model age of ~1.5 Gyr (HRSC-orbit 1143 at 15.5°N and 325.3°E). (C and D) Figure 1. Regional context of the study areas. (A) HRSC-image Examples of small rampart craters in the Valles Marineris region. mosaic of orbits 894, 905 and 927 in the Xanthe Terra region; (B) Scale bars are 2 km. C: HRSC-orbit 920 at 15.44°S and 278.42°E; HRSC-image mosaic of orbits 71, 97, 887, 920 and 931 in the Valles D=1.41 km; D: HRSC-orbit 887 at 9.73°S and 280.14°E; D=1.86 Marineris region (C) HRSC-image mosaic of orbits 1143 and 1154 km). in southern Chryse Planitia. Most rampart craters in the Valles Marineris region Methodology: To determine the absolute model show absolute model ages around 3.7 Gyr (Figure 3B), ages of the rampart craters we counted the crater fre- which indicates subsurface ice in the Early Hesperian quencies on the ejecta blankets utilizing the the Mar- most probably shortly after the formation of the Hes- tian impact cratering model of [11] and the polynomial perian aged plateaus. In addition 20 small rampart cra- coefficients of [12]. ters with onset diameters between 1 - 4 km were ob- Results and Discussion: Ages of rampart craters served [16] (Figure 2C and D) and indicate near sur- in the Xanthe Terra region (Figure 3A) are in the range face ice at the time of the impacts in this region. Mor- of ~4 to ~3 Gyr. Most absolute model ages of individ- phologically the ejecta blankets are highly degraded ual ejecta blankets are around 3.8 Gyr. The derived and age determinations by crater counts are not possi- ages imply that their formation is connected with the ble. However, the degraded ejecta blankets as well as Noachian aged fluvial activity (~3.8 Gyr) in this region the lower depth-diameter ratios of the craters in con- Role of Volatiles and Atmospheres on Martian Impact Craters 2005 3012.pdf trast to pristine craters of the same region indicate an 50 45 old, most likely Hesperian age [16]. These unusual A 40 [km] small rampart craters are in agreement with the re- 35 er ported Hesperian-aged fluvial processes in the study t e 30 region reported by [17] and regional variations of 25 equatorial onset diameters of rampart craters by [18]. 20 However, only one younger relatively large (D = 8.6 15 km) Amazonian aged (~2.5 Gyr) rampart crater was crater diam 10 identified, which could indicate a lowering of the 5 ground ice table after a volatile rich Hesperian phase. 0 In southern Chryse Planitia the rampart craters 30 B where the ejecta have been eroded by fluvial events 25 show absolute model ages around 3.8 Gyr and between 20 ~0.5 to ~1.5 Gyr of channel superposed ramparts (Fig- ure 2B and 3C). These ages are in good agreement 15 with the fluvial activity of Tiu Vallis derived from 10 crater counts between ~3.6 to ~1.5 Gyr BP [19]. The crater diameter [km] 5 formation of relatively young Amazonian aged ram- part craters with onset diameters of 3 km indicates that 0 ground ice could still be present in this region at ] 70 C m k [ 60 depths of a few hundred meters. The ground ice might r have been recharged by the last fluvial episode of Tiu 50 ete Vallis and sheltered from diffusion by thick fluvial m 40 ia sediments. The volatile layer in Chryse Planitia in gen- d 30 eral could possibly be as shallow as ~ 60 m as onset r ate 20 r diameters indicate [20, 21]. c 10 Conclusions: The ages and onset diameters of 0 rampart craters in three equatorial study regions indi- 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 cate that the formation is connected to volatile rich ejecta ages BP [Gyr] phases in the Martian history. The present ground ice Figure 3. Absolute model ages of rampart craters versus crater table in present times might strongly vary regionally. diameter of the Xanthe Terra region (A), the Valles Marineris region 1. The correlation of rampart ages with fluvial ac- (B) and southern Chryse Planitia (C). Gray triangles show rampart tivity and the lack of young rampart craters in the Xan- craters which are superposed on fluvial features. Gray circles show the Terra region indicates that the ground ice table is rampart craters which are eroded by fluvial activity. Black squares show no relative age relationships. Error bars are 30% for model possibly at a depth of several kilometers or non- ages younger than 3 Gyr and ±200 Myr for model ages higher than existent at present times. 3 Gyr [15]. 2. Small onset diameters and ages of rampart cra- ters in the Valles Marineris region indicate a volatile References: [1] Squyres S. W. et al. (1992) Mars, Univ. of Arizona Press, 523-554. [2] Carr M. H. et al. (1979), JGR, 82, rich phase in the Hesperian, most probably in the early 4055-4065. [3] Allen C. C. (1978), NASA Tech. Mem, 79729, Hesperian. The present ground ice table in this region 160-161. [4] Mouginis-Mark [1979] JGR, 84, 8011-8022. [5] might be several hundred meters deep as indicated by a Schulz P. H. and Gault D. E. (1979) JGR, 84, 7669-7687. [6] relatively large Amazonian aged rampart crater. Boyce J. M. (1980) NASA Tech. Mem., 82385, 140-143. [7] 3. In southern Chryse Planitia young Amazonian Kuzmin R. O. (1980) Dokl. ANSSSP, 252, 1445-1448. [8] Kuzmin R. O. et al. (1988) Solar Sys. Res., 22, 195-212. [9] aged rampart craters with onset diameters of ≥ 3 km Costard F. (1989) Earth, Moon, and Planets, 45, 265-290. [10] formed after the last fluvial activity in this region. This Neukum, G., et al. (2004) ESA Special Publications, SP-1240. indicates a ground ice table (possibly a few hundred [11] Hartmann W. K. and Neukum G. (2001) Space Sci. Rev., meters deep) in recent geological times which might be 96, 165-194. [12] Ivanov B. A. (2001) Space Sci. Rev., 96, 87- still present today. 104. [13] Masursky H. et al. (1977) JGR, 82, 4016-4037. [14] Carr M. H. (1996) Water on Mars, Oxford Univ. Press, pp. 229. Acknowledgements: This work was supported by a grant [15] Neukum G. et al. (2004) Nature, 432, 971-979. [16] Reiss from the German Research Foundation (DFG) within the scope D. et al. (2005) GRL, in press. [17] Mangold N. et al. (2004) of the priority programme “Mars and the Terrestrial Planets”, Science, 305, 78-81. [18] Barlow N. G. et al. (2001) GRL, 28, SPP 1115. This work was also supported by the Programme 3095-3098. [19] Neukum G. and Hiller (1981) JGR, 86, 3097- National de Planétologie and by the the European Community’s 3121. [20] Costard F.