Most likely, you saw the news splash in late September announcing water on Mars. I considered discussing this finding, but there are numerous articles about it for the non-scientist already. Instead, I decided to dive into a much lesser known feature of Mars: its TARs.
TARs stand for ‘transverse aeolian ridges’ and is actually a purposefully unhelpful name because the formation mechanism of TARs are not fully known. Transverse simply has to do with the direction these features are oriented with respect to the wind (perpendicular), aeolian is an adjective saying that it relates to wind, and ridges suggest a shape.
Discovered originally in the early 1980s by Viking missions, TARs are usually less than 7 meters (23 feet) in height and spaced 10 to 60 meters (33 to 197 feet) apart. Symmetrical, sometimes seen with sand dunes, TARs are always bright, sometimes even brighter than the surface they stand upon. While found at all elevations, they are often seen in lower areas, such as the centers of craters, and mostly in latitudes between 35 North and 55 South–the tropics of Mars. They appear inactive and are littered with craters and boulders in some places. They seem older than sand dunes and have slopes under 28 degrees.
All that was known but we still had a question: how did they get there?
Today’s article, “The birth and death of transverse aeolian ridges on Mars” by Paul E. Geissler, was published by the American Geophysical Union’s Journal of Geophysical Research: Planets in late 2014. The author takes us through two hypotheses about TARs before presenting his own.
Two main ideas about TARS are that they are either a type of ripple, or are a form of sand dunes. Formed by similar processes but with differences such as size, neither of these quite fit TARS. Here’s why:
Granule ripples (the type of ripple TARS have been suggested to be) sort out fine particles and coarser particles remain. This mechanism does not care about the color of the particles or what they are made of–yet TARs are always bright.
Sand dunes have a similar issue; the wind that forms sand dunes does not discriminate based off particle color.
TARs also cannot be sand dunes covered in dust, as TARs appear bright in places without dust–they are actually bright, not just covered in something bright.
So then…what are TARs? How do they form? The author of today’s article has a hypothesis: they are old dust deposits, shaped by millennia of sandblasting. Sandblasting is the repeated impact of sand particles picked up by the wind and impacting an object, slowly wearing away at it.
In this hypothesis, dust is carried in suspension by the wind, high above the surface of Mars. With high topography, such as mountains, these drop from the sky, the air too thin to carry them. They slide downslope, may be carried up by more winds, and in the end, there are a variety of different deposits formed all over the tropics of Mars. Some collect on the lee side of formations raised above the ground, such as crater rims. Others form pyramid-like shapes, with one end trailing longer than the others; in a line, these blend and form dune-like features. TARs, then, are perhaps these same formations, but etched furthers by years of sandblasting. The different original dust deposits lead to different variations in the present TARs.
This hypothesis actually explains the features of TARs. Here:
TARs are always bright: pulverized sand turns to the bright dust of TARs. We know that TARs are being sandblasted and eroded today.
TARs are bright in dark regions and are found with no local supply of dust: dust carried in suspension is a global source, meaning all the dust that forms TARS came from a global source, with a uniform composition and therefore, consistent color. Mars is known to have global dust storms.
Based on cratering, TARs near sand dunes are younger than other TARS: sand dunes going over TARs have been observed. When unburied again, the TARs are further eroded. This erosion and sandblasting will also erase craters, making TARs seem younger than they are when age is judged by crater count. Sand dunes likely provide extra erosion when they cross TARS, making them appears younger than their counterparts that are not near sand dunes.
The previous estimate of the age of TARs put them at 1 to 3 million years old at most; the author of this study puts them at perhaps 5 million years old. Why? Erosion makes the TARs appear younger than they are. Why 5 million specifically? 5 million years ago, the axial tilt of Mars was likely different. Earth’s is a familiar 23.5 degrees or so, and is stabilized fairly well by the Moon. Mars’ axial tilt has varied from 15 to 35 degrees, and 5 million years ago, was likely near 45 degrees. This tilt would have given way to much different atmospheric conditions, which would have been more likely to produce TARs, as they likely formed with much stronger winds than currently found on Mars. Unless more TARs are being produced, the current ones will eventually vanish, eroded into nothingness. Arguably, we live in an interesting time in terms of Mars: modern TARs, and as September showed, even seemingly barren, desert planets can have water.
Geissler, Paul E. “The birth and death of transverse aeolian ridges on Mars.” Journal of Geophysical Research: Planets 119.12 (2014): 2583-2599.
Today’s main article.
Nadia Drake. “NASA Finds ‘Definitive’ Liquid Water on Mars.” National Geographic. N.p., 28 September 2015. Web.
An articled discussing September’s water on Mars announcement.
NASA/JPL/University of Arizona. “HiRISE: Field of Transverse Aeolian Ridges in Proctor Crater (ESP_024449_1320).” N.p., 14 October 2011. Web.
The image used in this post.
Stephen Marshak. Earth: Portrait of a Planet. Third Edition. New York: W. W. Norton & Company, 2007. Print.
My geology textbook, used mostly for definitions and to refresh my memory.