Research ArticlePLANETARY SCIENCE

Branching geometry of valley networks on Mars and Earth and its implications for early Martian climate

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Science Advances  27 Jun 2018:
Vol. 4, no. 6, eaar6692
DOI: 10.1126/sciadv.aar6692
  • Fig. 1 Global distribution of mean branching angle and aridity index.

    Global distribution of mean river network branching angles (A) and aridity index (AI) (B) averaged over the level 4 basins defined by the Hydrological data and maps based on SHuttle Elevation Derivatives at multiple Scales (HydroSHEDS) data set. Narrow branching angles are more likely to occur in arid regions (low AI, yellow), while in humid regions, branching angles are usually wider (high AI, blue). Latitudes north of 50°N where no stream data are available are marked in cream. (C and D) Examples of stream networks from arid (C) and humid landscapes (D). The arid network is located in eastern Algeria, and the humid network lies in the Amazon rainforest at the border between French Guiana and Suriname. Mean branching angles for binned ranges of AI are shown in (E), together with the corresponding branching angle histograms for the arid (yellow) and humid (blue) tails, respectively. The points with log10(AI) < −1.5 are shaded because they constitute only a small fraction (<5%) of the whole data set spread over a wide range of aridity values; their inclusion or exclusion has no visually detectable effect on the global branching angle distribution.

  • Fig. 2 Comparison of mean valley branching angles on Mars and arid landscapes on Earth.

    (A) Outlet locations of the valley networks mapped by Hynek et al. (29) (orange) and Luo and Stepinski (30) (rose color). Background shading indicates elevation. The corresponding branching angle distributions are shown in (B). The violet solid line represents the branching angle distribution in the Lower Green River, a basin in the arid southwestern United States. The modes of the three data sets are 36° for the Hynek and Hoke networks, 45° for the Luo and Stepinski networks, and 41° for the Upper Colorado-Dirty Devil basin (HUC 1407). These values are considerably smaller than the theoretical angle of 2π/5 = 72° (45) expected for groundwater-driven network growth (black dashed line). Two sample valley networks on Mars are shown in (C) and (D). Scale bar corresponds to both sites. (E) Map of the Upper Colorado-Dirty Devil basin (HUC 1407), where the MDRS (red circle) is located.

  • Fig. 3 River network branching angles in the State of Alaska.

    River network branching angles in the State of Alaska, separated into regions with continuous permafrost (violet; A), discontinuous permafrost (magenta; B), and absent or only sporadic permafrost (green; C). In all three cases, the bifurcation angle histograms peak at roughly 72° (dashed lines), similar to the branching angles observed in other humid landscapes. The black solid lines in (A) to (C) show the kernel-smoothing density estimates of the branching angle distributions.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/6/eaar6692/DC1

    Global aridity and branching angles

    NHDPlusV2 data analysis

    NHD data analysis for the State of Alaska

    fig. S1. Measurement of the branching angle as defined in (21).

    fig. S2. Branching angles for two basins in the arid southwest of the United States.

    fig. S3. Histograms of valley slope for the two Mars data sets (Hynek and Hoke, orange circles; Luo and Stepinski, rose-colored squares) and the Upper Colorado-Dirty Devil basin (HUC 1407, violet solid line) as mapped by the NHDPlusV2 data set.

    fig. S4. Branching statistics of the raw NHD streams of the State of Alaska (50) categorized in regions with continuous permafrost (violet), discontinuous permafrost (magenta), and no permafrost (green).

    References (4650)

  • Supplementary Materials

    This PDF file includes:

    • Global aridity and branching angles
    • NHDPlusV2 data analysis
    • NHD data analysis for the State of Alaska
    • fig. S1. Measurement of the branching angle as defined in (21).
    • fig. S2. Branching angles for two basins in the arid southwest of the United States.
    • fig. S3. Histograms of valley slope for the two Mars data sets (Hynek and Hoke, orange circles; Luo and Stepinski, rose-colored squares) and the Upper Colorado-Dirty Devil basin (HUC 1407, violet solid line) as mapped by the NHDPlusV2 data set.
    • fig. S4. Branching statistics of the raw NHD streams of the State of Alaska (50) categorized in regions with continuous permafrost (violet), discontinuous permafrost (magenta), and no permafrost (green).
    • References (46–50)

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