Onset and migration of spiral troughs on Mars revealed by orbital radar

Journal name:
Nature
Volume:
465,
Pages:
450–453
Date published:
DOI:
doi:10.1038/nature09049
Received
Accepted

The landscape of the north polar layered deposits of Mars (NPLD) is dominated by a pinwheel array of enigmatic spiral troughs1. The troughs have intrigued planetary scientists since the Mariner 9 spacecraft returned the first close-up image in 1972, but conclusive evidence of their origin has remained elusive. Debate continues regarding all aspects of the troughs, including the possibility that they have migrated2, 3, 4, 5, their age in relation to the current NPLD surface6, and whether they are fundamentally erosional6, 7 or constructional2, 4 features. The troughs are probably related to climatic processes2, 8, yet the nature of this relationship has remained a mystery. Previous data characterizing only the exposed NLPD surface were insufficient to test these hypotheses. Here we show that the central spiral troughs initiated after deposition of three-quarters of the NPLD, quickly reached a stable morphology and migrated approximately 65 kilometres poleward and 600 metres in altitude over the past two million years or so. Our radar stratigraphy rules out hypotheses of erosional incision post-dating deposition6, 7, 9, 10, and instead largely validates an early hypothesis for constructional trough migration2, 3, 4, 5 with wind transport and atmospheric deposition as dominant processes. These results provide hard constraints for palaeo-climate models and a new context for evaluating imagery, spectral data, and now radar sounding data, the better to understand the link between orbital parameters and climate, the role of climate in shaping the polar ice of Mars, and eventually, the age of the polar deposits themselves8, 11, 12, 13.

At a glance

Figures

  1. NPLD with locations of data.
    Figure 1: NPLD with locations of data.

    a, Shaded-relief NPLD surface from MOLA elevation data29 showing morphology of troughs, overall wind patterns from wind streak mapping5 (black arrows) and locations of other figures. b, Upward projection of mapped discontinuities. Colours indicate trough migration path depth below surface. Location of orbit segments indicated in black. GL, Gemina Lingula; SR, saddle region.

  2. SHARAD data over troughs.
    Figure 2: SHARAD data over troughs.

    a, Observation 6247_02 (location indicated with yellow line in Fig. 1a), with vertical axis converted from time delay to depth assuming the dielectric constant of water ice. White arrows point to anomalies resulting from depth correction algorithm where MOLA topography is missing (for further discussion, see Supplementary Information). Vertical exaggeration is about 90:1. b, Expanded view of troughs (box in a) showing discontinuities and V-shaped structures in the upper 500m. T1 and T2 (arrowed) are troughs 1 and 2, also shown in Supplementary Fig. 2. c, Same as Fig. 2b but with interpretation of data to delineate structures.

  3. Stratigraphy and layer thickness changes resulting from different proposed mechanisms of trough formation/migration.
    Figure 3: Stratigraphy and layer thickness changes resulting from different proposed mechanisms of trough formation/migration.

    a, No trough migration. Layers are truncated by erosion but otherwise uniform in thickness and sub-horizontal. b, Trough migration caused by ablation of material on the equator-facing slope2. The trough migrates upward and northward, but layer thickness varies only on the south-facing slope. c, Wind-dominated stratigraphy. (1) Scouring on south-facing slope initiated at scarp. (2) Material transported southward assuming no material is lost to sublimation. (3) Small mound (or undulation) builds downwind, creating a secondary downwind face. (4) V-shape forms during subsequent deposition over the area between the leeward face of an undulation and farther downstream. d, Relative thickness of black layer in c, showing lateral change with respect to trough. e, SHARAD data from orbit 5192_01 (location indicated in Fig. 1b). Sub-horizontal layers (orange) are broken by sloping discontinuities and are vertically separated across the boundaries by approximately 150m. Stratigraphic layers below the discontinuities (light blue) are more continuous and uniform in thickness. Vertical exaggeration is about 90:1. f, Plot of separation distance between reflector pairs shown in e. The change in thickness of the upper pair (orange) is similar to the pattern predicted by wind transport in d. For each trough, positions are indicated by vertical lines. Smaller variations in the lower layer (blue) may be related to regional deposition patterns or data processing algorithms.

  4. Image of trough and effect of katabatic winds.
    Figure 4: Image of trough and effect of katabatic winds.

    Portion of THEMIS image V12432001 (ref. 30), showing laminar flow scouring the south-facing slope of a trough on the main lobe and moving material southward/downwind where it is then deposited. This image may be an uncommon example of extreme wind transport or a frequent, seldom observed occurrence. Location indicated in Fig. 1a.

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Author information

Affiliations

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

    • Isaac B. Smith &
    • John W. Holt

Contributions

I.B.S. interpreted the data, created figures, and wrote the paper. J.W.H. wrote and edited the paper and figures and assisted in interpretation.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

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

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Supplementary information

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  1. Supplementary Information (606K)

    This file contains Supplementary Notes S1-S5, Supplementary Figures S1-S5 with legends and References.

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