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Space Topics: Planetary AnalogsStars Above, Earth BelowAstronomy and Space Exploration in America's National Parks
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Red Rock Planet?
The mountains and plains of Mars as revealed by NASA's Spirit rover are reminiscent of the American desert Southwest. Would an arch on the horizon look too out of place? Credit: NASA / JPL / Cornell / with addition by Tyler Nordgren |
The low average rainfall in the southwest (and non-existent rainfall on Mars) severely limit the growth of plant life, so, unlike other places on Earth, in the southwest we have a clear and unobstructed look at the planet and its history recorded in stone. The red of the revealed rocks is from iron-oxide (rust) coating the landscape. On Earth the red is only skin deep, where the weathered rock surfaces come in contact with the air. On Mars, it’s in the dust that coats the global surface. The mesas, buttes, hoodoos, and pinnacles common to the southwest (and familiar to everyone who has ever seen a Warner Brothers Road Runner cartoon) also exist on Mars.
On Earth, the towering monoliths are formed from millions of years of alternating shallow seashores, blowing sand dunes, and deep alluvial deposits from nearby mountains eroded to dust by rain and ice. These sedimentary deposits are compressed into stone by the pressure of their burial until planetary tectonics thrusts them up into the sky to become worn down by the action of water flowing downhill to the sea. Plateaus develop gullies, which develop channels that form canyons, which separate mesas, which are worn down to the narrow buttes that are ubiquitous to the area. The Arches around me in the park that bears their name form when weak carbonic acid in the water dissolves the bonds that hold the sand grains together in stone. “All this is the music of waters,” said John Wesley Powell, the geologist and explorer of the Colorado River region in 1895.
Relics of a warmer, wetter Mars
Viking imagery plus Mars Global Surveyor altimetry data reveal vast and ancient river or outflow channels. The environmental conditions under which these channels could have formed are very different from conditions on Mars today. Current Mars exploration is driven, in part, to understand more about this warmer, wetter, period in Martian history. Credit: NASA / JPL / USGS |
Erosion of canyon walls
Water seeps from the curved walls at the head of a small canyon on Earth. Over time the channel lengthens as the rock walls are undercut by erosion. Note the fallen debris against the canyon's walls. Credit: Tyler Nordgren |
Rainfall and the freezing and thawing cycle of water are obvious sources for the erosion of the southwest. But what of Mars? Orbital images show narrow gullies, networks of small branching valleys, and immense braided flood channels originating in chaotic regions of innumerable mesas and buttes. The temperature and atmospheric pressure there are both too low for liquid water to exist on the surface today. Analysis of gully patterns and computer models of past climatic conditions indicate that, even in a past warmer, wetter Mars, rain probably never fell.
Jeff Kargel in his book Mars: A Warmer, Wetter, Planet (full disclosure: I have the same publisher as he) describes the current hypothesis that subsurface liquid water and ice are responsible for these features on Mars. He describes a scenario in which, in an earlier, warmer Mars, groundwater burst from springs in canyon walls, washed away sediments, and undercut the rock layers above. Eventually the high walls above collapsed, and the debris washed downstream. Circular alcoves formed, and, over time, these washes and channels worked backwards into plateaus, giving rise to branching networks of canyons.
The process Kargel discusses is on display before me, and takes place throughout the southwest. In the heart of Arches, I follow an old Park Service maintenance trail that takes me into a wash behind the iconic Delicate Arch. There I stand at the circular head of the wash --a natural amphitheater --and, out of the rock wall before me, a spring of fresh water flows out of a boundary layer in the rock and trickles down the red rock.
Next door to Arches National Park is its less visited sibling, Canyonlands
National Park (as well as the neighboring Dead Horse Point State Park). These
parks sit on massive peninsulas of stone jutting out into the vast canyon networks
of the merging Colorado and Green Rivers. From the many overlooks, the history
of the region can be read in the differing layers of rock, worn away at different
rates depending upon their composition and chemistry. Orbital images of Mars
show similar layering within regions of chaotic terrain as revealed by the
Mars Reconnaissance Orbiter’s HiRISE camera. It, too, has been worn away
by erosion, and I can imagine the time to come when people will walk these
Martian Monument Valleys and read the history in their walls.
Layered canyons: Earth
Layered canyons of the Green River as seen from Utah's Dead Horse Point State Park just outside Canyonlands National Park. Note the mesas and buttes. Credit: Tyler Nordgren |
Layered canyons: Mars
Layered canyons in a region of chaotic terrain on Mars. Note the mesas and buttes. Credit: NASA / JPL / U. Arizona |
As all those who have been following NASA’s Opportunity Mars rover know, this story is just now beginning to be read in the exposed bedrock of crater walls in Meridiani Planum. This fact leads me to a strange place within the Island in the Sky District of Canyonlands. Here, in the middle of red rock country, I come to the edge of a great circular basin surrounded by concentric rings of rock called Upheaval Dome. The late planetary scientist Gene Shoemaker mapped the site for the U.S. Geological Survey in the 1950s and is one of several scientists who, in the 1980s and onward, showed it to be a heavily eroded ancient meteorite impact crater (a competing theory put forward by petroleum geologists is that it is the eroded remains of a salt dome). Within the red crater walls are a myriad layers, all turned up with the rest of the countryside from the force of the impact.
Upheaval Dome
Upheaval Dome in Canyonlands National Park may be the heavily eroded remnants of an asteroid impact. High winds and dust devils blow along its rim. Credit: Tyler Nordgren |
If this truly is an impact crater, then it is the only one preserved within the National Park System. Crater Lake in Oregon and all those Craters of the Moon in Idaho are volcanic calderas. Meteor Crater in Arizona is privately owned. But here in Canyonlands I can stand upon the rim of this giant bowl and marvel at the force that must have made it. My picture of Mars is complete as I am reminded of Opportunity sitting on the rim of Victoria Crater -- the diameter is about the same, more or less, and all around the rim are similar bays and prominences. Like at Victoria, strong winds blow up and out of the crater, and as I watch a dust devil forms and blows over me. The Spirit and Opportunity science teams called these “cleaning events.” For me, however, I now have sand in every orifice exposed to the air. Cleaning indeed; I wonder if Opportunity ever got sand up its nose.
From here I head down to Flagstaff, where the folks at Lowell Observatory have agreed to help me look through what Percival Lowell had to say -- from his ideas on Earthly analogs to what he thought Mars to be. From the 21st century, the 19th is only a five hour drive away. After that, I am off to Yosemite in May.
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