SLIDE 2 For fifteen years, I have taught the world’s only Geography of Mars course, an odd outgrowth of one of my hazards research projects (Rodrigue 2001). This has introduced my students and me to a different universe of software, often purpose-built programs released by NASA into the public domain.

SLIDE 3 One of these is the Goddard Space Flight Center's Gridview, a program for analysis of data from the Mars Orbiter Laser Altimeter (MOLA). Operating from 1997 to 2001, MOLA took 670 million readings (Planetary Data System 2022; Neumann n.d). These were calibrated and integrated into the most detailed, precise, and accurate digital elevation model for any planet, including Earth! (MOLA Team 2020).

SLIDE 4 The program was last updated in 2011. It remains freely available from Goddard, where it provides access to several MOLA datasets through a quirky, yet easy to use interface.

SLIDE 5 The program opens an azimuthal projection of Mars at coarser resolutions and, as you see here, quadrangles or gores for finer resolution. It allows globe rotation, zooming, profile construction, area and volume calculations, geographic grid and elevation readings, and even fly-throughs.

SLIDE 6 It also allows the construction of shaded relief maps with control over hypsometric color, greyscale, vertical exaggeration, and altitude and azimuth of illumination source. This talk reports findings of massive linear structures on Mars using this now rather antique bit of NASA software.

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Data and Methods

I use Gridview in labs for my Geography of Mars class and for supervising thesis research set on Mars. In this latter capacity, I used shaded relief to exaggerate surfaces my students were mapping to help me follow their geomorphic decision-making. I built a dozen of these covering the globe at 15° S using an azimuthal projection centered every 30° of longitude. They are done in greyscale, with vertical exaggeration and lighting at 15° above local horizons and azimuths 30° apart. All of these used the densest MOLA database available at global scale: MOLA32, or pixel resolution at 1/32nd degree.

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Findings

These Gridview visualizatons have brought out a number of unusual and undiscussed features on the surface of Mars. Two of them are introduced here, using two of the animations. A map of place names might help!

SLIDE 7 Analysis refers to the gigantic Tharsis Rise volcanic province and the crustal dichotomy between the Southern Highlands and Northern Lowlands and focusses on the regions of Terra Cimmeria and Terra Sirenum.

SLIDESET 2: centered on 210° W

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Feature 1: Long Ridges in Terræ Cimmeria and Sirenum

Andrew Siwabessy (2021) was mapping Terra Cimmeria for his thesis in 2019-21, originally to learn if the massive Tharsis Rise volcanic platform had exerted far-field tectonic stresses nearly half the planet west of the Rise (Anderson et al. 2001). Detailed mapping did not reveal the expected structures, such as graben and basins-and-ranges. Alternate tectonic strains were found, however.

My Gridview visualizations had illuminated several irregular long wavelength northeast-southwest ridges in and near this region that pop out at certain azimuths

run SLIDEset 2 through all azimuths . The proposed basement fractures could have accommodated uplift in the pre-Noachian period, that is, before 4.1 billion years ago, to form the western Cimmerian uplands in Mr. Siwabessy’s study area. As he states (2021, p. 180), “Rather than tectonic plate boundaries, it is hypothesized that the proposed global fracture network serves as the fundamental model for tectonic expression on Mars,” accommodating the stresses of later events. These include the massive impact that probably created the crustal dichotomy between Mars Southern Highlands and the Northern Lowlands about 4.5 billion years ago. Later stresses would include the giant impacts from the Solar System’s Late Heavy Bombardment of roughly 4.1 to 3.7 billion years ago and the uplift of the Tharsis Rise beginning perhaps 4.2 billion years ago (Williams et al. 2008).

SLIDESET 3: centered on 180° W, open at 0° azimuth

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Feature 2: The “Zipper”

The Gridview visualization centered on 180° W and -15° shows another odd feature. This is a hemispheric scale linear structure that stretches from northwest Terra Cimmeria, across Terra Sirenum, passing south of Argyre Planitia.

run SLIDEset 3 through all azimuths It seems to be a wide ridge of locally elevated terrain with a graben-like area of depressed topography running along much of its spine, pocked in several places by large craters and basins. I sometimes refer to it as “The Zipper.”

SLIDE 8 Examination of the most recent free air gravitational anomaly maps for Mars (Genova et al. 2016) show the lineation as a slight ridge of positive anomaly flanked by areas with lower or even negative values, which coïncide with the Gridview visualization.

SLIDE 9 The most recent map of crustal thickness variations on Mars came out in 2022 (Wieczorek et al. 2022). It also shows a faint thickening running in a line in the same pattern.

SLIDE 10 Maps of remanent magnetic anomalies are coarser in scale (Connerney et al. 1999, 2005) and, so, harder to link with the “Zipper.” It is notable, however, that the location of this feature marks the region where the crisp linear alternating bands of magnetic anomalies give way to a splotchier and less zonal pattern found outside of southern terræ Cimmeria and Sirenum.

One possible origin for this structure has been discussed for other solar system bodies, a zonal partition of strains associated with despinning and contraction of oblate ellipsoids. Polar areas develop extensional features, such as graben, and equatorial areas develop compressional uplift and ridges. The most extreme version is on Enceladus in the Saturn system, with a 20 km high ridge girdling this small body’s equator.

Mars’ “Zipper,” however, is tilted roughly 20 or 30° with respect to its current equator. Resolution of this paradox might lie in Mars’ capacity to move its entire lithospheric and mantle shell as a single unit over its core. This movement, true polar wander, has been proposed as a mechanism allowing Mars to stabilize its rotational axis by centering the developing Tharsis bulge over its equator (Bouley et al. 2016). In a manner of speaking, Tharsis provides the kind of axial stabilization services that our own Moon does! And, indeed, the earth has also experienced true polar wander! (Wang and Mitchell 2023).

SLIDE 11 Is it possible that the “Zipper” is a fossilized expression of Mars’ primordial equator, the ridge forming from the compressive stresses of Mars’ despinning and contraction?

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SLIDE 12

Conclusions

Anyone who scrolls through any of these twelve Gridview visualizations or who downloads Gridview and starts experimenting with their own visualizations is going to find many other such lineations. These are nearly imperceptible in any other MOLA-based map, such as Google Mars, because of the map viewers’ inability to change the azimuth and height of illumination.

Similar experiments on a larger scale analysis of a smaller region can be expected to yield similar surprises as discussed here and help with geological and geomorphological mapping. Altimetric grids are available at the Gridview site and the NASA PDS Geosciences Node, not only for Mars but for the Moon as well. The PDS also offers altimetric data for Mercury and Venus. Happy sleuthing through the Solar System!

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