Periodic Bedrock Ridges

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Periodic Bedrock Ridges (PBRs) are features of the surface geomorphology of Mars and formerly unknown on Earth, first described in a paper in the Journal of Geological Research - Planets, in March 2012, [1] by Professor David R. Montgomery, Joshua L. Bandfield, and Scott K. Becker of the University of Washington. Periodic Bedrock Ridges (PBRs) have also been identified at the ExoMars 2022 landing site, Oxia Planum, which show that the landing site experienced multiple climatic changes in the Amazonian. [2]

Evidence for sediment transport and erosion by wind is widespread over the surface of Mars today and was likely a major geomorphic process for much of its geological past. Although Martian surface features resembling aeolian dunes and ripples have been recognized since the Mariner and Viking missions, such features have been interpreted previously as active, indurated, or exhumed sedimentary forms. [1]

The authors reported evidence based on High Resolution Imaging Science Experiment images flown on the Mars Reconnaissance Orbiter, that show some megaripple forms are eroded into cohesive substrate rather than being composed of loose granular material or fossilized dunes. Exposure of stratigraphic continuity within layered, cohesive material extending crest to trough through features with mean wavelengths of 18 metres (59 ft) to 51 metres (167 ft) demonstrates the primarily erosional formation of what they termed periodic bedrock ridges (PBRs). Thus, some surfaces on Mars previously considered to be covered by wind-deposited material are actually wind-carved exposures that offer insights into Martian history. [1]

PBRs lack the distinctive streamlining associated with wind-parallel yardangs, and comparison of PBR orientation to yardangs, megayardangs, and active sedimentary dunes in the same vicinity confirm that these PBRs formed transverse (across, or 90 degrees) to prevailing winds. Observed wavelengths of PBRs are comparable to those predicted by a simple model for erosional wavelengths of periodic transverse bed forms owing to the spacing of flow separations within the flow. Recognition of these transverse aeolian erosional forms brings up the question of how widespread Martian PBRs are and how many have been misinterpreted as active or indurated (fossilized) sedimentary dunes. [1]

PBRs are not known on Earth. Montgomery has said that there could be landforms on Earth that are somewhat similar to PBRs, but to date there's nothing exactly like it, largely because there are not many bedrock landscapes on Earth in which wind is the main erosion agent. "There are very few places ... where you have bedrock exposed at the surface where there isn't also water that is carving valleys, that's shaping the topography. Mars is a different planet, obviously, and the biggest difference is the lack of fluvial action, the lack of water working on the surface." [3]

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Ventifact Rock that has been eroded by wind-driven sand or ice crystals

A ventifact is a rock that has been abraded, pitted, etched, grooved, or polished by wind-driven sand or ice crystals. These geomorphic features are most typically found in arid environments where there is little vegetation to interfere with aeolian particle transport, where there are frequently strong winds, and where there is a steady but not overwhelming supply of sand.

Aeolian processes Processes due to wind activity

Aeolian processes, also spelled eolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth. Winds may erode, transport, and deposit materials and are effective agents in regions with sparse vegetation, a lack of soil moisture and a large supply of unconsolidated sediments. Although water is a much more powerful eroding force than wind, aeolian processes are important in arid environments such as deserts.

Yardang Streamlined aeolian landform

A yardang is a streamlined protuberance carved from bedrock or any consolidated or semiconsolidated material by the dual action of wind abrasion by dust and sand and deflation Yardangs resemble the 'ridge and furrow' landscape of zeugen. Yardangs become elongated features typically three or more times longer than wide, and when viewed from above, resemble the hull of a boat. Facing the wind is a steep, blunt face that gradually gets lower and narrower toward the lee end. Yardangs are formed by wind erosion, typically of an originally flat surface formed from areas of harder and softer material. The soft material is eroded and removed by the wind, and the harder material remains. The resulting pattern of yardangs is therefore a combination of the original rock distribution, and the fluid mechanics of the air flow and resulting pattern of erosion.

Amazonis Planitia Planitia on Mars

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Dreikanter Type of rock

A Dreikanter is a type of ventifact that typically forms in desert or periglacial environments due to the abrasive action of blowing sand.

Margaritifer Terra Terra on Mars

Margaritifer Terra is an ancient, heavily cratered region of Mars. It is centered just south of the Martian equator at 4.9°S 25°W and covers 2600 km at its widest extent. The area reveals "chaos terrain", outflow channels, and alluvial plains that are indicative of massive flooding. Wind erosion patterns are also in evidence. A region within the terra shows some of the highest valley network densities on the planet. Ares Vallis is another notable feature, where the flood and flow patterns are in evidence; it was the landing site of the Soviet Mars 6 lander and NASA's Mars Pathfinder. It is also one of several proposed landing sites for the Mars 2020 Rover.

Erg (landform) Broad area of desert covered with wind-swept sand

An erg is a broad, flat area of desert covered with wind-swept sand with little or no vegetative cover. The word is derived from the Arabic word ʿarq (عرق), meaning "dune field". Strictly speaking, an erg is defined as a desert area that contains more than 125 km2 (48 sq mi) of aeolian or wind-blown sand and where sand covers more than 20% of the surface. Smaller areas are known as "dune fields". The largest hot desert in the world, the Sahara, covers 9 million square kilometres and contains several ergs, such as the Chech Erg and the Issaouane Erg in Algeria. Approximately 85% of all the Earth's mobile sand is found in ergs that are greater than 32,000 km2 (12,355 sq mi). Ergs are also found on other celestial bodies, such as Venus, Mars, and Saturn's moon Titan.

Ripple marks Wave structures created in sediments by bottom current

In geology, ripple marks are sedimentary structures and indicate agitation by water or wind.

Terra Sabaea Terra on Mars

Terra Sabaea is a large area on Mars. Its coordinates are 2°N42°E and it covers 4,700 kilometres (2,900 mi) at its broadest extent. It was named in 1979 after a classic albedo feature on the planet. Terra Sabaea is fairly large and parts of it are found in five quadrangles: Arabia quadrangle, Syrtis Major quadrangle, Iapygia quadrangle, Ismenius Lacus quadrangle, and Sinus Sabaeus quadrangle.

Olympia Undae Martian dune field

Olympia Undae is a vast dune field in the north polar region of the planet Mars. It consists of a broad "sand sea" or erg that partly rings the north polar plateau from about 120° to 240°E longitude and 78° to 83°N latitude. Stretching about 1,100 km (680 mi) across and covering an area of 470,000 km2, Olympia Undae is the largest continuous dune field on Mars. It is similar in size to the Rub' Al Khali in the Arabian Peninsula, the largest active erg on Earth.

Medusae Fossae Formation

The Medusae Fossae Formation is a large geological formation of probable volcanic origin on the planet Mars. It is named for the Medusa of Greek mythology. "Fossae" is Latin for "trenches". The formation is a collection of soft, easily eroded deposits that extends discontinuously for more than 5,000 km along the equator of Mars. Its roughly-shaped regions extend from just south of Olympus Mons to Apollinaris Patera, with a smaller additional region closer to Gale Crater.

Amazonis quadrangle Map of Mars

The Amazonis quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The Amazonis quadrangle is also referred to as MC-8.

Margaritifer Sinus quadrangle One of a series of 30 quadrangle maps of Mars

The Margaritifer Sinus quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The Margaritifer Sinus quadrangle is also referred to as MC-19. The Margaritifer Sinus quadrangle covers the area from 0° to 45° west longitude and 0° to 30° south latitude on Mars. Margaritifer Sinus quadrangle contains Margaritifer Terra and parts of Xanthe Terra, Noachis Terra, Arabia Terra, and Meridiani Planum.

The common surface features of Mars include dark slope streaks, dust devil tracks, sand dunes, Medusae Fossae Formation, fretted terrain, layers, gullies, glaciers, scalloped topography, chaos terrain, possible ancient rivers, pedestal craters, brain terrain, and ring mold craters.

Transverse aeolian ridges (TARs) are visually bright features commonly found in topographic depressions on Mars. These small-scale and relict bedforms were first seen in narrow-angle images from the Mars Orbiter Camera (MOC) and were called “ridges” to preserve both dunes and ripples as formative mechanisms. While TARs are widespread on Mars, their formation, age, composition, and role in past Martian sediment cycles remain poorly constrained.

Arsinoes Chaos

Arsinoes Chaos is a chaos terrain in the Margaritifer Sinus quadrangle on Mars. It is 200 km in diameter. Its location is 7.66 °S and 27.9 °W. Arsinoes Chaos was named after Arsinoe, a queen of ancient Egypt, daughter of Ptolemy and Berenice.

Yardangs on Mars

Yardangs are common in some regions on Mars, especially in the Medusae Fossae Formation. This formation is found in the Amazonis quadrangle and near the equator. They are formed by the action of wind on sand sized particles; hence they often point in the prevailing direction that the winds were blowing when they were formed. Because they exhibit very few impact craters they are believed to be relatively young. The easily eroded nature of the Medusae Fossae Formation suggests that it is composed of weakly cemented particles, and was most likely formed by the deposition of wind-blown dust or volcanic ash. Yardangs are parts of rock that have been sand blasted into long, skinny ridges by bouncing sand particles blowing in the wind. Layers are seen in parts of the formation. A resistant caprock on the top of yardangs has been observed in Viking, Mars Global Surveyor, and HiRISE photos. Images from spacecraft show that they have different degrees of hardness probably because of significant variations in the physical properties, composition, particle size, and/or cementation.

Oxia Planum Plain on Mars

Oxia Planum is a 200 km-wide clay-bearing plain located on the planet of Mars inside the Oxia Palus quadrangle on the eastern border of Chryse Planitia. The plain lies between the Mawrth Vallis outflow channel to the north-east and the Ares Vallis outflow channel to the south-west. In 2019, the International Astronomical Union Working Group for Planetary System Nomenclature officially approved Oxia Planum as a feature on the surface of Mars.

Hyperboreae Undae Martian dune field

Hyperboreae Undae is one of the largest and densest dune fields of Planum Boreum, the Martian North Pole. It is named after one of the classical albedo features on Mars. Its name was officially approved by IAU in 1988. It extends from latitude 77.12°N to 82.8°N and from longitude 302.92°E to 316.02°E. Its centre is at latitude 79.96°N, longitude 49.49°W, and has a diameter of 463.65 kilometres (288.10 mi).

References

  1. 1 2 3 4 Periodic bedrock ridges on Mars, David R. Montgomery, Joshua L. Bandfield, Scott K. Becker, JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, E03005, 12 PP., 2012 doi:10.1029/2011JE003970, accessed 28 March 2012
  2. ↑ Periodic Bedrock Ridges at the ExoMars 2022 landing site: Evidence for a Changing Wind Regime, S. Silvestro A. Pacifici F. Salese D.A. Vaz A. Neesemann D. Tirsch C.I. Popa M. Pajola G. Franzese G. Mongelluzzo A.C. Ruggeri F. Cozzolino C. Porto F. Esposito, GEOPHYSICAL RESEARCH LETTER, doi:10.1029/2020GL091651, accessed 12 January 2021
  3. ↑ Geologists discover new class of landform - on Mars, Vince Stricherz, Seattle WA (SPX), MarsDaily.com, 23 March 2012, accessed 28 March 2012
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