It’s always sobering to realise that we have a more detailed view of the surface of Mars than we do of the floors of our own oceans. And, while the evidence for water’s role in the history of Mars and its geomorphology (or, I guess, more correctly, martiomorphology) is more or less accepted, the interpretation of that planet’s surface features is almost exclusively pursued through the spectacles of terrestrial (on-land) terrestrial processes. We look to the processes of rivers and floods on our continents to explain the surface features of Mars. So there’s a certain satisfaction when geologists break out of this mindset and propose ocean floor analogues from Earth to explain the – now definitely subaerial – topography of Mars.
Whether they are proved to be right or not really doesn’t matter at this point: Lorena Moscardelli and Lesli Wood have made the vital contribution of opening up a debate, stepping out of the box, challenging conventional wisdom. Both from the Bureau of Economic Geology at the University of Texas in Austin, they have published a paper in the July issue of Geology titled “Deep-water erosional remnants in eastern offshore Trinidad as terrestrial analogs for teardrop-shaped islands on Mars: Implications for outflow channel formation.” The “teardrop-shaped islands” are the TSIs and the ESRs are Erosional Shadow Remnants – but so what, what is this all about apart from more acronyms?
Well, first of all, let’s review what we know and what we don’t know about water on Mars. What we know is obviously that liquid water plays no role in today’s processes – there is evidence for water locked up in Martian minerals and the permanent polar ice caps contain frozen water (unlike the seasonal ones that form exclusively from dry ice, frozen carbon dioxide) – but a flash-flood is not one of the hazards that a Mars Rover faces. Then there is evidence of ancient shorelines – almost certainly lakes, possibly oceans. But when I say “ancient,” of course I mean serious deep-time – the idea of a warm and wet period on Mars refers to a time around three billion years ago. And finally we come to martiomorphology – there are endless features of the Martian landscape, imaged in superb detail, that look exactly like geomorphological hallmarks of rivers, floodplains, water-scoured gullies, deltas, alluvial fans and so on. There are always alternative explanations for individual features, but the collective evidence suggests flowing surface water.
But. detailed though our images are, and even though the Rovers Opportunity and Spirit (now, sadly, deceased) have, with unexpected grit and fortitude, explored and sampled a few square kilometres, a geologist or geomorphologist has yet to stride the field on Mars, and definitive interpretation eludes us. Which is where the fun lies.
One of the most closely scrutinised areas of Mars with respect to water-sculpted landforms is the gigantic valley of Ares Vallis – see, for example, this European Space Agency site (where the image at the head of this post came from). Like so many landforms on Mars, this is a seriously gigantic feature – a couple of thousand kilometres long, with valley walls a couple of thousand meters high. Its floor and the many tributary canyons provide enough for researchers to chew on for decades.
And that’s exactly what Moscardelli and Wood have been doing. Like their colleagues in the Martian scrutinising business, they must always bear in mind what they know of their own planet, but they have expanded this approach to considering the floor of our own oceans.
The topography of Ares Vallis and its tributary canyons bears many of the hallmarks of catastrophic flooding – canyon headwalls that seem to have been the site of massive cataracts that constantly eroded the cliffs backwards up the canyon and downstream from the cataracts, scoured floodplains. These landscapes are reminiscent of those of the so-called channelled scablands of Eastern Washington State, bizarre geomorphology carved by a series of gigantic floods released from glacial lakes as the climate warmed. Where, on Mars, all this water came from, is another matter, the subject of debate. The September 2010 issue of Geology contained a well-documented paper on the “Retreat of a giant cataract in a long-lived (3.7-2.6 Ga) martian outflow channel.” The research of a team from University and Imperial Colleges, London, argues that periodic groundwater release caused a long period of catastrophic flooding and erosion of the Ares Vallis region. And, when they say “long-lived,” they mean it – Ga is a billion years, so this period of landscape development lasted 1,100,000,000 years.
BUT. Among the distinctive and enticing landforms are TSIs - “teardrop-shaped islands,” clearly visible in the image below, taken from the paper by Moscardelli and Wood.
What could these forms be telling us? The teardrop – streamlined - shape is common in nature, and shows up in a variety of contexts on our own planet, not to mention in the aerodynamic profile of an airplane’s wing. Now, wonderfully, given that much of this work is available only to subscribers (of which I am one), Geology saw fit also to publish in the public domain, open-access, a short review of these landforms and their possible meaning on the floodplains of Mars. The illustration below is taken from that review (written by Devon Burr at the University of Tennessee), and it really is fascinating. For a start, it shows yardangs, on which Evelyn and I recently (and totally coincidentally) wrote a joint post.
Teardrop-shaped, streamlined, landforms are common terrestrially and extra-terrestrially, and can be shaped by water or wind – they seem to be one result of fluid flow, regardless of the fluid. And, as Burr explains, this is for good reason – the physics of drag:
Streamlined forms are shaped during flow as a tendency to minimize total drag. Total drag is the sum of form (pressure) drag and skin (friction) drag. Form drag around a blunt body arises largely from flow separation in the lee of an obstacle, so that elongation of the form through in-filling of the leeward separation zone reduces the form drag. Conversely, skin drag acting tangentially to the obstacle surface is minimized by reducing the surface area by making the feature geometrically more compact. Consequently, minimization of total drag is accomplished through a combination of elongation and compaction. The result of these countervailing tendencies is the streamlined form…. Both erosional and depositional streamlined forms are observed in terrestrial floodscapes.
And there’s a key point: such forms can result from erosion (as with yardangs), or deposition, in the downstream lee of an obstacle.
Cut to the deep ocean waters off the Orinoco delta. Using clever seafloor imaging, Moscardelli and Wood describe the features shown below (taken from their paper):
The dark circular features are mud volcanoes (liquefaction phenomena common in loose sediments in a tectonically active area) that seem to form the “anchor” for the TSIs – which are interpreted as ESRs; the mud volcanoes form the obstacle to massive flows down the sloping sea floor, leaving the streamlined form downstream from them protected from the erosional power of the flow and shaped by the physics of drag. Now, compare these with an example from Ares Vallis, where the TSIs are “anchored by impact craters:
Interesting, eh? As the authors conclude (after a detailed analysis documented in the paper) that “These observations suggest that the teardrop-shaped islands [on Mars] might have been formed as a result of catastrophic submarine mass movements similar to those documented within continental margins on Earth.”
There is, of course, one key difference between Earth and Mars that drives the drag equations: gravity. The gravity on Mars is significantly lower than that on Earth, and so such streamlined forms may result from different conditions of fluid flow. Given the fact that streamlined forms also occur on Mars in regions where an ocean is unlikely ever to have existed, and yet that water flow can be invoked as a mechanism for their formation, Burr concludes, “If the physical sedimentology in outflow channels on Mars is indeed similar to that in submarine environments on Earth (Komar, 1979), then the submarine analogy revived by Moscardelli and Wood might extend beyond the limited number of circum-Chryse TSIs. In other words, this analogy may argue for the effect of lower gravity producing submarine-style processes in Martian outflow channels generally, regardless of any hypothesized submarine context.”
So, who knows really what’s going on? The answer is, of course, nobody, but Moscardelli and Wood have opened up the debate, pointing the way to a broader understanding of sedimentary landforms in extra-terrestrial – and terrestrial – environments. As Burr sums it up:
Testing between these two interpretations—that certain streamlined forms on Mars formed in a submarine environment, or that all streamlined forms on Mars formed in an outflow environment in which particle physics mimics that in submarine environments on Earth—will require morphometric data from ESRs for comparison to Martian streamlined forms, as well as examination of the geologic context for the Martian examples. The question extends beyond Mars to other worlds. Titan has even lower gravity than Mars, a 10-times thicker atmosphere than Earth, and also shows streamlined forms (Fig. 1E). Could subaerial processes on Titan mimic outflow processes on Mars and submarine processes on Earth? Pinning down the true cause for the similar appearance between terrestrial ESRs and Martian TSIs would contribute to understanding the effect of reduced effective gravity on sedimentary landforms.
I’ll leave (assuming that anyone has made it this far) with a favourite image that raises a favourite question: what planet are you from?
[The paper by Moscardelli and Wood was picked up by Wired Science; the full reference is: Lorena Moscardelli and Lesli Wood, Deep-water erosional remnants in eastern offshore Trinidad as terrestrial analogs for teardrop-shaped islands on Mars: Implications for outflow channel formation, Geology 2011;39;699-702; for the cataracts analysis, Nicholas H. Warner, Sanjeev Gupta, Jung-Rack Kim, Shih-Yuan Lin and Jan-Peter Muller, Retreat of a giant cataract in a long-lived (3.7-2.6 Ga) martian outflow channel, Geology 2010;38;791-794. Burr’s open-access summary is here. For the origin of my final image, have a browse around the Mars HiRise image collection. The image of Ares Vallis at the head of this post is from the European Space Agency Mars Express site.]