An ice age recorded in the polar deposits of Mars

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Science  27 May 2016:
Vol. 352, Issue 6289, pp. 1075-1078
DOI: 10.1126/science.aad6968
  1. Fig. 1 Color mosaic of the north polar region from Viking Visual Imaging Subsystem data collected between 1976 and 1980 (32), including the north polar layered deposits (NPLD) and distal outliers.

    Ground-track footprints of radargrams in later figures are shown.

  2. Fig. 2 SHARAD radargrams highlighting the widespread, recent accumulation package (WRAP).

    The WRAP unconformity (turquoise lines) is shown (A) within the uppermost 100 m of the NPLD, observation 1998901; (B) within the uppermost 90 m of the NPLD with changing properties of two sets of undulations, observation 2007101; (C) within the uppermost 80 m of the NPLD, observation 761602; (D) within the uppermost 150 m of the NPLD, observation 1247002; (E) with increased trough migration path slope in observation 598301; (F) above buried spiral troughs in Gemina Lingula, observation 725402; and (G) above buried spiral troughs and with reversing undulations, observation 2007101. Magenta lines, erosion and migration; yellow dashed lines, trough migration paths; orange dashed lines, undulation migration paths. Green and red reflectors are reflections 25 and 26 (R25 and R26) from (14), respectively. Arrows point north for each image. Ground track of each image is shown in Fig. 1. Uninterpreted radargrams are in fig. S2. Vertical exaggeration ranges between 50:1 and 150:1.

  3. Fig. 3 Interpolated thickness maps of the NPLD WRAP, two outlier deposits, and the SPLD.

    (A) NPLD WRAP thickness ranges from 0 m at outcrops to 320 m at the NPLD interior. The integrated volume of the WRAP and outlier deposits is ~80,000 km3. Fully interpolated region poleward of 87.4°N is shaded in gray. (B) Corresponding SPLD unit is discontinuous and less widespread, with a volume of ~7000 km3. Thickness reaches 210 m locally. For maps showing uninterpolated thickness along radargram ground tracks, see fig. S1.

  4. Fig. 4 Modeled ice accumulation and stratigraphic unconformities in the NPLD.

    (A) Solar insolation (W m−2) correlated to ice thickness modeled for the north polar region from 4 million years ago (Ma) to the present [from (2)]. The accumulated thickness is shown normalized to the current NPLD value, so units are not displayed. Insolation varies by a factor of 2 through time, creating four climatologically distinct periods (vertical bars separated by colors). Applying the same logic that associates the WRAP (unit D) with the latest increase in accumulation modeled by (2), we assign units to periods of relatively low accumulation rates (units A and C) and relatively high accumulation rates (unit B). (B) SHARAD radargram 529701 annotated with stratigraphic unconformities mapped here and in (13, 14). Yellow dashed line highlights spiral trough migration paths. The red reflector mapped between units B and C is reflector 29 (R29) from (14) and is associated with an unconformity in other locations. It represents the surface nearest which most of the spiral troughs formed. Packets of high radar reflectivity [in (B)] may correspond to darker shaded bands [in (A)] highlighting periods of greater insolation variability and potentially explaining the packet-interpacket reflectivity zones that SHARAD detects (15, 16).

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