Did ice and dust make layered deposits in Valles Marineris?

Posted on May 15, 2012 by rburnham

Vast mounds of layered material lie in numerous places throughout the giant canyon system of Valles Marineris, and especially in Candor Chasma, Ophir Chasma, and Melas Chasma. The origin of these “interior layered deposits” (ILDs) have been debated since they were discovered in the early 1970s. Theories include lakebed deposits, sub-ice eruptions, and groundwater rising to alter beds of wind-blown volcanic ash and other materials.

DUST AND ICE. A mixture of ice, dust, and sulfur-laden aerosols could have created layered deposits in Valles Marineris. As the ice in the deposits evaporated and sublimation, the dust would lose cohesion and erode. (Image taken from Figure 3 in the paper.)

Two scientists — Joseph Michalski (Planetary Science Institute) and Paul Niles (NASA Johnson Space Center) — now suggest in Geology that the ILDs formed by a climate-change driven process combining ice, dust, and volcano-generated sulfuric acid. (The two also recently advocated a related origin for the giant mound in Gale Crater.)

“We propose,” the scientists write, “that the ILDs are remnants of sediments originally composed of dust, ice, and acidic aerosols that were concentrated into discrete deposits at low latitudes during periods of high obliquity.” Models of the Martian climate over millions of years show that during such times when the rotation axis tilts more with respect to the Martian orbit, ice and snow accumulate in the equatorial regions instead of the poles.

The researchers continue, “Recent spectroscopic results show that these materials contain coarse-grained hematite, sulfates, and clays. These suggest that the layered deposits are fundamentally similar to layered sulfate deposits seen elsewhere on Mars, and are therefore a key piece of the global aqueous history of Mars.”

A problem with previous explanations for making the ILDs through groundwater or lakebed deposits is that these theories require unrealistically large quantities of material to be deposited and eroded.

The atmospheric deposition model avoids these requirements, they note. “Many of the complications of explaining the ILDs as sourced from groundwater or standing bodies of water are nonfactors if the sediments originated through atmospheric sources.

“We favor an alternative model in which the ILDs form in a configuration similar to what is observed today through atmospherically driven deposition of ice, dust, and volcanogenic sulfuric acid.”

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Mars dunes move as much as Earth dunes

Posted on May 10, 2012 by rburnham

Scientists have known for years that Martian sand dunes and ripples move as wind blows over them. But for the most part they thought the motion was small because the atmosphere is thin and high-speed winds are rare.

CREEPING SANDS. HiRISE images from 2007 and 2010 let scientists measure the motion of sand ripples and dunes in Nili Patera. The results indicate that for Nili at least, the sand movements are comparable to dune movements on Earth. The wind-rose at upper right shows prevailing winds blow from the northeast, driving the dunes to the southwest. (Image taken from Figure 1 in the paper.)

Now new research using before-and-after images taken by the HiRISE camera on NASA’s Mars Reconnaissance Orbiter shows that the amount of sand movement in one place at least is comparable to what’s seen on Earth. These reveal that entire dunes as much as 60 meters (200 feet) thick have moved as a unit.

After studying the dune field in Nili Patera, Nathan Bridges (Johns Hopkins University Applied Physics Laboratory) and colleagues write in Nature that, “The dunes are near steady state, with their entire volumes composed of mobile sand.” The images were taken in 2007 and 2010.

“We chose Nili Patera because we knew there was sand motion going on there, and we could quantify it,” says Bridges. “The Nili dunes also are similar to dunes in places like Antarctica and to other locations on Mars.”

The movement of sand ripples amounted to as much as 4.5 meters (about 15 feet). The scientists also noted that ripples moved faster as they rose up the wind-facing side of the dunes. By correlating ripples’ movement to their position on the dune, the analysis determined the entire dunes are moving. This let the scientists estimate the volume, or flux, of moving sand.

How much did they move? The team calculates that if you stood in the Nili Patera dunes and measured across a one-yard width, you would see more than two cubic yards of sand, about as much as in a child’s sandbox, pass by during an Earth year.

This conflicts with previous views, says the team. “One view of Mars has been that conditions since the end of the Hesperian period, 1.8 billion years ago, have been fairly static, with very low erosion rates. This study shows that this is not the case at Nili Patera, and probably not at other areas of Mars where there are significant gusts of sand and wind.”

Yet the new results help explain a geological puzzle, they say. “Vast areas of the Martian surface show evidence of erosion and removal, including of mantle materials for which the processes and agents of exhumation have been a mystery — yet these places also contain fields of large dunes that migrate at relatively slow rates.

“Over long time periods, it may be that much or all of Mars has been subjected to large sand fluxes, with associated erosional modification of the landscape.”

Says Bridges, “No one had estimates of this flux before. We had seen with HiRISE that there was dune motion, but it was an open question how much sand could be moving. Now we can answer that.”

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Mars Rover Opportunity explores Cape York

Posted on May 4, 2012 by rburnham

Mars Exploration Rover Opportunity reached the south end of Cape York, a segment of the rim of Endeavour Crater about 700 meters (2,300 feet) long, on August 9, 2011. Scientists and engineers examined several targets there before driving the rover to an over-wintering site at the north end of the rim segment. Opportunity has been spending the Martian winter at Greeley Haven.

TISDALE. A false-color Pancam image shows Tisdale, a foot-high block of ejecta from the small crater Odyssey. A close-up view (B) shows it's made of rocky fragments of all sizes welded together by impact melt. This probably indicates an origin in the impact that formed Endeavour Crater. Image C shows yard-wide Kidd Creek, another block of breccia. (Image taken from Figure 3 in the paper.)

Endeavour Crater spans 22 kilometers (14 miles) rim to rim and dates to the Noachian Era, the oldest period in Martian geologic history, more than 3.7 billion years ago. Project scientists led by Steven Squyres (Cornell University) have now published (May 3, 2012) a report in Science which gives details about the rocks of the Shoemaker Formation, which make up Cape York.

This formation is an impact-shattered rock unit (dubbed a breccia), which is a common feature at impact craters. The breccia is made of fragments (called clasts) of the target rock, the ancient basalts of Meridiani Planum, mixed with impact-melted rock.

As noted back in December, the geological story of Cape York combines rocks, impact energy, and groundwater. Scientists identified several rocks and outcrops that embodied parts of the story. The new report extends the findings.

“We suggest that Tisdale [a foot-high rock] may represent the main breccia unit of the rim,” the team writes. “And Chester Lake [a yard-wide rock] and the rocks near Greeley Haven were emplaced later in the impact flow.”

Making a gently sloping bench all around Cape York are sandstones that belong to the same flat-lying sedimentary rocks that Opportunity landed on in January 2004 — and on which it spent its entire mission up until it arrived at Cape York. Cutting into this bench are several thin bright veins. Opportunity studied one of these veins, dubbed Homestake,  and found it was almost pure gypsum.

The scientists say, “The gypsum veins at Cape York provide clear evidence for relatively dilute water at moderate temperature, perhaps supporting locally and transiently habitable environments.

“More broadly,” they continue, “rocks at Cape York appear to record early events in a transition from (commonly) hydrothermal waters that altered basaltic crust to phyllosilicates to sulfate-charged ground waters that generated salt-rich sandstones deposited widely over the Meridiani plains and elsewhere.”

Summing up, the team says, “The ubiquity of impact breccia at Cape York contrasts with the only other Noachian terrain explored in situ, the Columbia Hills in Gusev Crater. The rover Spirit encountered great lithologic diversity there, including materials interpreted as impact ejecta. However, none were breccias, and none had the lateral extent of the Shoemaker formation.

“We suggest that the difference can be attributed to Opportunity’s sampling of the rim deposits of a single large crater, rather than Spirit’s sampling of more distal ejecta from multiple impacts.”

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Lava coils: new form of flow discovered on Mars

Posted on April 26, 2012 by rburnham

High-resolution photos of Martian lava flows show coiling spiral patterns that resemble snail or nautilus shells. Such patterns have been found in a few locations on Earth, but never before on Mars. The discovery appears in a paper in Science published by Andrew Ryan and Philip Christensen (both Arizona State University).

CURLICUES OF ROCK. Never before seen on Mars, lava coils can occur when lava flows are pulled in two directions at once while the lava is still soft and plastic. The scale bar is 82 feet long. (Image taken from Figure 2 in the paper.)

The new result came out of research into possible interactions of lava flows and floods of water in the Elysium volcanic province of Mars.

“Athabasca Valles has an extensive literature,” Ryan says,  “as well as an intriguing combination of seemingly fluvial and volcanic features.” Among the features are large slabs or plates that resemble broken floes of pack ice in the Arctic Ocean on Earth. In the past, a few scientists have argued that the plates in Elysium are in fact underlain by water ice.

Assessing claims that ice was present today beneath the lava plates drove Ryan to study the area. This led him to look closely at every available image of the region, with an emphasis on those from the HiRISE camera on Mars Reconnaissance Orbiter.

“I first noticed puzzling spiral patterns in an image near the southern margin of Cerberus Palus,” Ryan explains. “The coils become noticeable in the full-resolution HiRISE image only when you really zoom in. They also tend to blend in with the rest of the light-gray terrain until you stretch the contrast a bit.”

On Earth, lava coils can be found on the Big Island of Hawaii, mainly on the surface of ropey pahoehoe lava flows. They have also been seen in submarine lava flows near the Galapagos Rift on the Pacific Ocean floor.

As Ryan explains, “The coils form on flows where there’s a shear stress — where flows move past each other at different speeds or in different directions. Pieces of rubbery and plastic lava crust can either be peeled away and physically coiled up — or wrinkles in the lava’s thin crust can be twisted around.”

Similarly, he notes, scientists have documented the formation of rotated pieces of oceanic crust at mid-ocean ridge spreading centers. “Since the surface of active lava lakes, such as those on Hawaii, can have crustal activity like spreading centers do, it’s conceivable that lava coils may form there in a similar way, but at a smaller scale.”

The size of Martian lava coils came as a surprise. “On Mars the largest lava coil is 30 meters across — that’s 100 feet. That’s bigger than any known lava coils on Earth,” he says. Ryan and Christensen’s work has inventoried nearly 200 lava coils in the Cerberus Palus region alone.

Looking ahead, Ryan says, “Lava coils may be present in other Martian volcanic provinces or in outflow channels mantled by volcanic features. I expect that we’ll find quite a few more in Elysium as the HiRISE image coverage grows over time.”

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Sea cliffs in Libya Montes?

Posted on April 24, 2012 by rburnham

New work suggests that three possible shorelines from ancient lakes or seas in Isidis Planitia lie in Libya Montes.

SHORELINES? Distinct levels which may represent ancient shorelines appear in central Libya Montes. Possible coastal cliffs appear at –3600 and –3700 meters, indicating two distinct surface levels for bodies of water. Elevations are in meters relative to the mean radius of Mars. (Image taken from Figure 4 in the paper.)

These mountains form the southern rim of Isidis, a Noachian-age impact basin 1,225 kilometers (760 miles) in diameter. They lie along the highland/lowland boundary and consist mainly of mountainous massifs and ridges, mixed with remnants of impact craters. In numerous places the montes show evidence of flowing water: fluvial channels, deltas, and alluvial fan deposits.

Now a team of scientists led by Gino Erkeling (University of Münster) is proposing that they have identified old shorelines or sea cliffs in Libya Montes. If true, these strengthen the case for possible sea-scale standing bodies of water in the Isidis basin and other depressions on Mars in the past. The new work appears in a recent paper in Icarus.

“At the Libya Montes/Isidis Planitia boundary, we identified landforms at three different elevation levels,” says the team. “The landscape features show evidence of intense fluvial activity, standing bodies of water, alteration by water, wave-cut action, and distinct water levels caused by freezing and sublimation of a cold ocean.”

In an unnamed crater 60 kilometers (45 miles) wide, the team found the first features at elevations between –2500 and –2800 meters relative to the Martian datum. They include valleys, terraces, delta deposits with hydrated minerals, and an outlet in the crater rim. These all point to a standing body of water in the crater, say the scientists.

About one kilometer (3,300 ft) lower lie shoreline features consisting of cliffs and terraces. The researchers write, “Most conspicuous are a series of candidate coastal cliffs of the Arabia shoreline that coincide with the –3700 meter elevation.” The cliff landforms possibly resemble terrestrial sea cliffs eroded by wave-cut action and could have formed during sea-level variations of an Isidis sea.

The lowest feature the scientists identify is the –3800 meter Deuteronilus contact. It is likely the result of standing water — or an ice sheet — that filled the northern lowlands of Mars.

The landscape features are consistent, the team says, with a global change in climate from warm and wet conditions to cold and dry ones.

“Because the possible shorelines appear close to each other in the Libya Montes,” they say, “we propose this site as a new candidate landing site for potential future missions after MSL Curiosity.”

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