The construction of Chasma Boreale on Mars

Journal name:
Nature
Volume:
465,
Pages:
446–449
Date published:
DOI:
doi:10.1038/nature09050
Received
Accepted

The polar layered deposits of Mars contain the planet’s largest known reservoir of water ice1, 2 and the prospect of revealing a detailed Martian palaeoclimate record3, 4, but the mechanisms responsible for the formation of the dominant features of the north polar layered deposits (NPLD) are unclear, despite decades of debate. Stratigraphic analyses of the exposed portions of Chasma Boreale—a large canyon 500km long, up to 100km wide, and nearly 2km deep—have led most researchers to favour an erosional process for its formation following initial NPLD accumulation. Candidate mechanisms include the catastrophic outburst of water5, protracted basal melting6, erosional undercutting7, aeolian downcutting7, 8, 9 and a combination of these processes10. Here we use new data from the Mars Reconnaissance Orbiter to show that Chasma Boreale is instead a long-lived, complex feature resulting primarily from non-uniform accumulation of the NPLD. The initial valley that later became Chasma Boreale was matched by a second, equally large valley that was completely filled in by subsequent deposition, leaving no evidence on the surface to indicate its former presence. We further demonstrate that topography existing before the NPLD began accumulating influenced successive episodes of deposition and erosion, resulting in most of the present-day topography. Long-term and large-scale patterns of mass balance achieved through sedimentary processes, rather than catastrophic events, ice flow or highly focused erosion, have produced the largest geomorphic anomaly in the north polar ice of Mars.

At a glance

Figures

  1. Maps of Planum Boreum surface and depositional/erosional history.
    Figure 1: Maps of Planum Boreum surface and depositional/erosional history.

    a, Present surface, based on MOLA data27 with Chasma Boreale and Gemina Lingula indicated. b, Topography of NPLD base as mapped from SHARAD, merged with MOLA surface outside of NPLD. The southern edge of the basal unit beneath NPLD is shown as a dashed line, based on our SHARAD mapping and that of others11. The palaeo-high point of the basal surface is indicated. c, Topography of unconformity surface at base of PLD2 (Fig. 2b), as mapped using SHARAD data. d, Thickness of unit PLD2 (Fig. 2b). For all panels, shading is derived from the present surface, displayed as a semi-transparent overlay in bd for reference. Elevation values are relative to MOLA areoid27. See Supplementary Fig. 3 for data coverage used to create gridded surfaces in bd. The gap in coverage from both MOLA and SHARAD north of 87.4° latitude is indicated by the solid grey circle in each panel.

  2. SHARAD data and interpretation.
    Figure 2: SHARAD data and interpretation.

    a, Depth-corrected radargram crossing Chasma Boreale (portion of SHARAD observation 522402. See Fig. 1a for location). Vertical exaggeration is about 45:1. b, Interpretation of stratigraphy and geologic units, showing representative radar reflectors and the unconformity surface. Unit designations PLD1 and PLD2 are based on radar data and do not necessarily correspond to previously mapped geologic units. Boundaries are dashed where inferred. The Vastitas Borealis interior unit13 is assumed to underlie the basal unit or PLD1 where the basal unit is not present within Gemina Lingula. The base of the basal unit is generally not detectable with SHARAD, but the basal unit itself is distinctive owing to strong volume scattering causing an increased amount of dispersed energy. Red lines indicate off-nadir surface echoes (clutter), as determined by simulations.

  3. HiRISE image PSP_009914_2750 showing depositional relationship between NPLD and basal unit.
    Figure 3: HiRISE image PSP_009914_2750 showing depositional relationship between NPLD and basal unit.

    Illumination from upper right. a, Unconformable contact between lowest NPLD unit (Planum Boreum 1 unit13) and the uppermost basal unit (cavi unit13) on the north margin of Chasma Boreale. Contact, indicated by the upper dotted line, is obscured on the right side by recent deposition of reworked material, the uppermost extent of which is indicated by the lower dotted line. See Fig. 1a for location. b, Close-up view of NPLD–basal unit contact, location indicated by box in a.

  4. Cross-sectional illustration of hypothesized sequence of events leading to the development of Chasma Boreale and Gemina Lingula.
    Figure 4: Cross-sectional illustration of hypothesized sequence of events leading to the development of Chasma Boreale and Gemina Lingula.

    The figure is based on data presented in Figs 1, 2 and 3 and Supplementary Figs 1–5. The location is indicated by solid line in map view (g); it is approximately the same location as Fig. 2. a, Basal unit before modification. b, Erosion of the basal unit. c, Deposition of radar unit PLD1. Presumed deposition across location of present-day Chasma Boreale shown as dashed lines. d, Erosion of PLD1. e, Deposition of radar unit PLD2. f, Recent erosion of NPLD increasing steepness of most slopes. Dashed near-vertical line shows erosion of PLD2 farther west (dashed line on map in g; see Supplementary Fig. 5 for examples of radar data). ABrt, ABbc and ABb1 are, respectively, the Rupes Tenuis, Planum Boreum cavi, and Planum Boreum 1 units, as defined in ref. 13.

References

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Author information

Affiliations

  1. Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin 78758, Texas, USA

    • J. W. Holt &
    • S. Christian
  2. Smithsonian National Air and Space Museum, Washington 20560, District of Columbia, USA

    • K. E. Fishbaugh
  3. Lunar and Planetary Laboratory, University of Arizona, Tucson 85721, Arizona, USA

    • S. Byrne
  4. Bryn Mawr College, Bryn Mawr 19010, Pennsylvania, USA

    • S. Christian
  5. Astrogeology Science Center, US Geological Survey, Flagstaff 86001, Arizona, USA

    • K. Tanaka &
    • K. E. Herkenhoff
  6. Planetary Science Institute, Tucson 85719, Arizona, USA

    • P. S. Russell
  7. Jet Propulsion Laboratory, Pasadena 91109, California, USA

    • A. Safaeinili
  8. Southwest Research Institute, Boulder 80302, Colorado, USA

    • N. E. Putzig &
    • R. J. Phillips
  9. Deceased.

    • A. Safaeinili

Contributions

J.W.H. initiated and led the SHARAD analysis effort, synthesized results and wrote the manuscript. K.E.F. and S.B. led HiRISE analysis, contributed material for early manuscript drafts and assisted in the final manuscript. S.C. assimilated SHARAD data and performed most of the radar mapping. A.S. provided focused and depth-corrected SHARAD data. K.T., K.E.H. and P.S.R. contributed to the HiRISE analysis and the manuscript. N.E.P. and R.J.P. provided radar validation and contributed to the manuscript.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

Data from MRO, including SHARAD and HiRISE, are available at NASA’s Planetary Data System (http://pds.jpl.nasa.gov/).

Author details

Supplementary information

PDF files

  1. Supplementary Information (1.3M)

    This file contains Supplementary Notes A-E comprising: Initial NPLD deposition; PLD1/PLD2 contact mapping; Data coverage for grids; PLD1 thickness; Lower GL/CB radar stratigraphy and Supplementary Figures S1-S5 with legends.

Additional data