Aeolis Serpens, Mars’ longest sinuous ridge, is an ancient riverbed

Posted on May 17, 2013 by rburnham

A linear ridge that winds for more than 200 kilometers (120 miles) through part of South Australia was a river channel roughly 10 million years ago. After the paleoriver stopped flowing, silica-rich groundwater seeped into the riverbed, cementing its sediments. Then erosion stripped away the softer surrounding rocks, leaving the former watercourse as a flat-topped ridge (known as the Mirackina ridge) snaking across the ground north of Coober Pedy, South Australia.

MEANDERING OVER MARS, Aeolis Serpens winds as a ridge across Aeolis Dorsa. But the ridge was once a river channel. After stream flow ceased, groundwater cemented the riverbed sediments. When the surrounding ground eroded, the former watercourse was left standing high. (Image is taken from Figure 1 in the paper.)

Such a “landform inversion” is also the origin for Aeolis Serpens, the longest sinuous ridge known on Mars, says a team of geologists led by Rebecca Williams (Planetary Science Institute). Reporting in Icarus, the scientists note that the Aeolis ridge changes character along its length and is not completely continuous. This they attribute to the cementation varying in effectiveness, a feature seen in portions of the South Australia paleoriver as well.

Using images from the CTX and HiRISE cameras on the Mars Reconnaissance Orbiter, the group traced the Aeolis Serpens ridge for a distance of approximately 500 kilometers (300 miles). They note that the southernmost section may not simply dwindle away, but could be buried by later sediments, and thus the Aeolis channel may have been even longer originally.

The team also determined elevations along the ridge using stereographic pairs of images. These led the scientists to construct profiles and cross-sections for the meandering paleochannel at stages along its path. They found that the ridge adopts “a number of forms including trough, trace, single-ridge and double-ridge morphologies.” These, they explain, likely reflect changes in the groundwater’s ability to cement riverbed sediments, as well as the kinds of sediments in the channel.

Regarding the ridge’s place in Martian geologic history, the scientists note, “Aeolis Serpens is one of a cluster of sinuous ridges located between the two westernmost lobes of the Medusae Fossae Formation.”

This enigmatic formation has numerous suspected origins, although most scientists currently lean toward it being volcanic ash. The team writes, “Prior studies concluded that the ignimbrite [volcanic ashflow] hypothesis matches well all of the major observations.” Because the formation’s material is porous, groundwater could easily migrate through it, with both stream flow and groundwater cementation being concentrated near the lowest layers of the formation.

“Based on the stratigraphic relationships,” the team concludes, “Aeolis Serpens formed early in the emplacement of the Medusae Fossae Formation.” They add that the river probably flowed at a rate of 100 to 1,000 cubic meters per second. Yet the team cautions that this is only a rough estimate.

Because cementation varied along the paleoriver’s length, so does the shape and profile of the ridge. As a result, they note, in this case, “meander wavelength and radius of curvature can be reliably measured, but flow width and gradient may not be preserved or accurately determined.”

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Most deltas on Mars created by short, catastrophic floods

Posted on April 22, 2013 by rburnham

Rivers that run into lakes and other standing bodies of water drop sediment where the flow slackens as it enters the body of water. Over time, the accumulating material builds a delta — a wedge of sediment whose form can tell scientists about both the river’s activity and that of the body of water.

DOWN IN THE DELTA. Laboratory experiments show that most deltas on Mars form inside crater lakes, and were likely built by short-duration floods. The biggest factor controlling the delta's form is the water level in the lake while the delta is building. Falling water levels cause channel erosion, rising levels cause multiple back-stepping lobes of sediment, and steady levels cause outward growing deltas, with or without multiple surface channels. (Image taken from Figure 1 in the paper.)

Three planetary scientists at Utrecht University, Netherlands, modeled the development of different kinds of Martian deltas using a laboratory stream table. They found that most deltas on Mars formed by short, catastrophic floods, rather than (as is common on Earth) longer periods of fluvial activity. Germari de Villiers led the group, which reports on their findings in the Journal of Geophysical Research.

The scientists explain that deltas on Earth are shaped by upstream factors such as climate, geology, and vegetation, and by factors in the downstream receiving basin, such as basin morphology, waves, tides, and sea level fluctuations.

Not all such factors apply to Mars, as the researchers note. For example, “Martian deltas commonly formed in impact crater lakes with average diameters of around 40 kilometers [25 miles], so waves were probably insignificant.” Likewise, Mars has no moon big enough to raise tides and no vegetation to interfere with erosion patterns.

To investigate in the lab, the team used a flume filled with sand and water flowing into a circular crater-like basin. By varying the flow and the level of water in the basin, they simulated various geological situations and climate changes.

“The experiments show that delta formation occurs by a sequence of events beginning with high-discharge flows of relatively high sediment concentration,” say the reseachers. What happens then depends on the level of water in the lake or basin, which becomes gradually filled by the flood. “The behavior of the downstream water level is the most important factor determining delta morphology.”

If the water level in the crater lake rises during the flow, the delta will build numerous lobes of sediment that step backward toward the incoming flow. If the water level remains constant, then the delta grows and may or may not develop channels on the delta. And if the water level falls, the delta deposit becomes incised by channels that cut down into its sediments. This last case is seen in the experiments, but not with Martian deltas.

From their experiments, three implications for Martian deltas emerged, according to the scientists. First, the early stages of delta building involve a highly energetic, hyper-concentrated flow — like a flash flood — which deposits most of the sediments. Second, channel incision in a delta is focused and occurs rapidly. This finding led the researchers to believe that after the high-discharge event no modifications of the delta body took place, which in turn suggests that the crater lake dried out slowly and that the delta was formed by a single event.

Third, the experiments indicated that water soaking into the subsoil of the channel and crater lake probably leads scientists to underestimate the total volume of water.

A flood making a Martian delta likely ended about as abruptly as it began, the team says. If there was significant late-stage flow, the water would cut into the existing delta — and they note, “almost all deltas on Mars show no large-scale erosion by post-formation flow over their surface.”

This led the team to an observation about climate. “The preservation of numerous deltas on Mars, mostly without indications of fluvial erosion, leads us to argue that the climate may not have been warm and wet and sustaining long-duration hydrological activity throughout Mars’ history.”

More likely, they say, a given catastrophic event led to instant production of flood waters which quickly filled a crater basin. The floods that formed the delta ended abruptly, and the crater lake soon dried up, leaving the delta exposed.

“We therefore conclude that rapid formation by single, high discharge events was most likely responsible for most of these delta deposits on Mars.”

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Are brines actually needed to make recurring slope lineae flow?

Posted on April 5, 2013 by rburnham

Recurring slope lineae (RSL) are finger-like dark lines on steep slopes that appear and grow longer during the warmest time of year, then fade and disappear over winter. They repeat the following Mars year in the same places. While scientists have not pinpointed the fluid responsible for the flows, the betting runs heavily on brines: water full of salts that lower the freezing point.

BASKING IN WARMTH. Recurrent slope lineae (RSL) lengthen only at times of year when ground temperatures rise above freezing (blue horizontal line). This suggests that RSL flows do not require salt-laden brines and that fresh water alone is sufficient. (Image taken from the online abstract.)

Brines may be involved, says David Stillman (Southwest Research Institute, Boulder), but fresh water can do the job on its own. Reporting (PDF) on the work of a team of scientists, Stillman spoke at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas.

The team used images from the HiRISE and Context (CTX) cameras, and derived ground temperatures using data from the Thermal Emission Spectrometer (TES), Mars Climate Sounder (MCS), and Thermal Emission Imaging System (THEMIS). The survey revealed the stages the RSL typically go through each Mars year.

“RSL start lengthening when near-maximum (roughly 2 p.m.) temperatures reach 296 Kelvin (73° F),” the scientists note, “and they stop lengthening at 289 K (61° F).” THEMIS mid-afternoon surface temperatures indicate that RSL lengthen only when the temperature is above 273 K, or the freezing point of pure water. Water-saturated soil can be expected to be above freezing to a depth of about 10 centimeters (4 inches) for several hours a day, the team says.

Mid-latitude RSL appear to be associated with surface features suggesting buried ice (for example, ice-rich latitude-dependent mantled units, pedestal craters, concentric crater fill, etc.). The team notes, “This ice is more than 400,000 years old and would likely maintain a saturated subsurface atmosphere.”

Stillman’s group says, “We suggest that the reason RSL emanate from bedrock outcrops is because bedrock has a thermal conductivity about 40 times greater than regolith (dry soil) does.” This allows the wave of annual heat to penetrate in bedrock to a depth of several meters (yards).

During winter, the subsurface temperature of outcropping bedrock will fall below the frost point of the subsurface atmosphere. Consequently, water vapor will condense into the bedrock unit throughout the winter. In spring, subsurface temperatures begin to rise, ultimately melting the ice in the bedrock, allowing it to flow.

Finally, the team concludes, “RSL lengthen when surface temperatures rise above 273 K [32° F]. This suggests high concentrations of brine are not necessary to generate the RSL. Our proposed flow mechanism explains the repeatability of RSL and allows vapor-deposited ice to recharge bedrock, even at topographic highs such as crater central peaks.”

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‘Faint young Sun paradox’ a problem for Mars (and Earth, too)

Posted on April 3, 2013 by rburnham

Astronomers say that billions of years ago when the Sun was young, it shone with only 70 percent its current brightness, notes Robert Craddock (Smithsonian Institution). If that were true of today’s Sun, he explains, Earth’s surface would freeze over, and conditions on Mars would be much colder and drier than they are now.

RUNNING WATER. Fluvial valley networks on early Mars show rainfall and erosion comparable to Arizona's Grand Canyon, seen bottom center at the same scale. This was at a time when the young Sun was too faint to keep temperatures above freezing. (Image taken from the online abstract.)

Craddock, the head of team of scientists, reported (PDF) on the history of water on early Mars at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas.

Given a faint young Sun, “Earth and Mars should have been much too cold for liquid water to be present on the surface of either planet, creating a paradox,” he says. Yet on Earth, the geologic record from the early Precambrian era contains fluvial sediment, indicating surface conditions warm enough for flowing water.

Mars also presents the same paradox. The team says, “There’s a variety of geologic evidence indicating that early Mars supported rainfall as well as an advanced hydrologic cycle.”

Recent work shows that this “climatic optimum” occurred not throughout the Noachian period, the earliest in Martian geologic history, but rather came towards the end of the period. Nonetheless, the researchers say, “the scale of erosion represented by many valley network systems is immense, and can rival the Grand Canyon on Earth.” These systems likely operated over hundreds of thousands of years at a minimum.

In addition to fluvial valley networks, the scientists note, there are widespread areas of impact craters showing evidence of modification by erosion processes that softened and weathered landscape features on a global scale.

Sharpening the paradox, the researchers note that an atmosphere of carbon dioxide can’t stay warm enough to provide above-freezing temperatures, even if the surface pressure were 10 times greater than Earth’s.

Yet “on Mars, valley networks and modified impact craters both attest to the fact that Mars experienced periodic if not sustained rainfall during the Noachian and through the Hesperian,” the team says. “In addition, there are the outflow channels that also required catastrophic releases of liquid water, and there’s compelling evidence for an ocean in the northern hemisphere.”

If a pure CO2 atmosphere does not provide clement conditions regardless of its surface pressure, the scientists say, then some other combination of atmospheric conditions and gas constituents is needed. Here, more modeling of ancient climates should help.

Another possibility is that the earliest stages of sunlike stars run differently from what astrophysicists currently think. The team suggests that new data on extrasolar planetary systems from the Kepler Observatory may resolve the problem.

“Ultimately, reconciling the astronomy, geology, and meteorology rests with better climate models,” the team says. And the problem goes beyond just Mars, they add. “The implications are not only for understanding the history of water on Mars, but the origin of life on Earth as well as habitable zones and extrasolar planets.”

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Gale’s winds sculpted the Mt. Sharp mound as they built it

Posted on April 1, 2013 by rburnham

The major reason for sending the Mars Science Laboratory rover Curiosity to Gale Crater is the five-kilometer (three-mile) high layered mound, dubbed Mt. Sharp, that looms at the crater’s center. The lowest layers have been altered by water and perhaps higher layers as well, possibly by lakes that have collected in the crater.

GROWTH BATTLED EROSION to make the mound in Gale Crater in the Kite team's computer simulation. Debris fell from the sky onto the growing mound, draping across layers already there. Then winds coming down the mound whittled at the new-deposited material as they met winds descending the crater's walls. The red line shows the final mound profile and its layering. (Image is taken from the online abstract.)

But where did the mound’s material come from — and what was its original extent? Did its sediments once fill all or most of Gale?

The mound’s origin was material that fell out of the air or was blown along the surface, says Edwin Kite (Caltech), who led a team of scientists in reconstructing the likely way the mound grew. Kite reported (PDF) on the group’s findings at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas. (On the heels of the meeting, the same team published a paper in Geology that extends the argument in more detail.)

“Although mound is widely considered to be an erosional remnant of a once crater-filling unit,” the team notes that they found that layers in the mound showed bedding planes that dip down and outward. These suggest that the mound’s current form is close to its largest extent.

As the Kite group explains, “We propose that the mound’s structure, stratigraphy, and current shape can be explained by growth in place near the center of Gale Crater, mediated by feedback between winds and the crater’s topography.”

They add, “Our model shows how sediment can initially accrete near the crater center far from crater-wall downslope winds. Eventually the increasing relief of the resulting mound generates mound-flank slope-winds strong enough to erode the mound,” thus limiting its growth.

The group notes that the key observation was that most mound layers tilt outward. If the mound were a remnant of a thick deposit laid across the crater, it would have compacted in the center, tilting layers inward. The scientists conclude that the mound material didn’t come from lakebed sediments, for example, but eolian (wind-driven) ones.

As the mound grew, eventually it reached a size where downslope winds from the walls and the mound itself swept away the latest additions to the mound.

“Slope-wind erosion of indurated or lithified eolian deposits cannot explain our data,” the team says, “unless the topographic depression surrounding the mound existed during mound growth.” This requirement links the mound layer orientations, slope winds, and the mound’s size.

The team suggests its findings indicate “The mound grew with its modern shape, and that the processes sculpting the modern mound may have molded the growing mound.” They add that Curiosity can start testing this model as soon as it reaches the foot of Mt. Sharp.

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Dust drifts: new windblown feature on Mars?

Posted on March 27, 2013 by rburnham

Mars has ample loose material blowing around on its surface, a fact which has been known and studied for decades and more. However scientists have paid little attention to sedimentary deposits of dust. New work using detailed images from the HiRISE camera on Mars Reconnaissance Orbiter has recently identified a new type of windblown deposit on Mars: the dust drift.

STREAKY DUST. Bright dust streamers extend like tufts of whiskers downwind from raised features on the rim of this old eroded crater. The dust collects behind the features, where the wind abruptly slackens and the dust falls out of suspension in the air. (Image taken from the online abstract.)

Unlike active sand dunes, which appear dark because they are made of grains of basalt, a dark volcanic rock, the dust drifts appear light in tone, matching that of dust itself.

The discovery was announced by a team of scientists led by Paul Geissler (U.S. Geological Survey). Colin Dundas (also USGS) presented the report (PDF) at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas.

“Bright tapered deposits were first noticed in a HiRISE image of the west flank of Alba Mons,” the team notes. “The dust appears to be temporarily trapped in the lee of crater rims, both inside the craters and along the outside rims where they form streamers.” The geologists add that the deposits don’t fill the smooth crater floors, which suggests that the streamers or drifts need a topographic feature such as an elevated crater rim to form.

Arsia Mons and other volcanos are thickly covered in dust, so the discovery was not too surprising. Then the team found similar features in a region, Solis Planum, where dust is far scantier and surfaces are frequently swept clean. Looking upwind, the researchers spotted a location, Syria Planum, that has stable dust deposits.

The team says, “At first sight, we interpreted these deposits as erosional features, remnants of a formerly extensive layer of dust.” But on closer examination, the drifts’ overall shape didn’t fit with known eroded surfaces. “The deposits are also sinuous in plan,” they noted, which required winds from multiple directions.

“We suggest instead,” they explain, “that the tapered deposits are dust drifts, accumulated during periods of strong surface winds that were heavily laden by dust.” The MER rovers have seen such winds during global dust storms.

“Dust is caught up by topographic obstacles and sticks to the surface, piling up on the downwind rims of the craters and partially infilling the upwind sides,” the researchers say. “Between dust storms, the features must undergo erosion, so they may currently be in an erosional state in spite of the fact that they were formed by dust deposition.”

If this interpretation is correct, they note, dust drifts may let scientists track the direction of windblown dust wherever these features are found.

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Icy jets erupt from north polar dunes in spring

Posted on March 27, 2013 by rburnham

Jets of gas erupting in the springtime from beneath slabs of carbon dioxide ice at the Martian south pole was a dramatic finding in 2006. It explained the mysterious “spiders” which came and went each year. Now the same mechanism working on a smaller scale has been proposed to explain grooves that appear on north polar sand dunes.

CRYO-VENTS on north polar dunes shoot sediment into the air from under a thin CO2 ice layer. The escaping gas erodes furrows under ice. (Image is taken from the online abstract.)

Mary Bourke (Trinity College Dublin and Planetary Science Institute) reported on the discovery (PDF) at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas. Her report presented evidence from HiRISE images showing long furrows on sand dunes.

“These are shallow and narrow forms which can extend up to 300 m (1,000 feet) along the dune surface,” she says. The furrows are typically about 25 centimeters (10 inches) deep and some 1.5 meters (49 in) wide. They may be straight or highly sinuous, and network patterns vary — radial, rectilinear, tributary, and distributary all occur.

“These are important dune surface features,” she explains, adding that they can be seen almost all HiRISE images of north polar dunes and have been detected on south polar dunes.

Details of the furrow patterns – such as radiating upslope – indicate that they are caused by something flowing, but not simply downhill under gravity. “This suggests that the formative fluid is likely to be a pressurized gas,” Bourke explains.

Adapting the model for south polar gas jets to the smaller scale of the dunes, the same basic process appears to be at work, she says, calling the process “cryo-venting.”

It works as follows. When north polar autumn descends into winter, a seasonal CO2 ice layer forms on the north polar dunes. As spring sunlight arrives, it triggers the CO2 to begin sublimating at the bottom of the ice layer, raising the gas pressure underneath the seasonal ice. The gas buildup lifts the overlying ice, flexing it, and causing stress cracks.

As cracks open, gas escapes, along with sub-ice sand and sediment caught up in the gas. The sediment flows erode the furrows, and the jetted sand falls back in fans, spots, and grain flows on top of the ice. When the ice disappears, all that’s left are the furrows.

Gas jets are spaced roughly 5 meters (16 ft) apart, and cryo-venting carries significant amounts of sediment, Bourke notes. She estimates that the process can move the volume of a small dome dune (about 500 cubic meters) each season.

“The cryo-venting lasts for about 40 sols [Martian days],” Bourke says. And unlike the south polar gas jets, the dune vents don’t erupt at the exact same locations on the dunes year after year. This could be due to seasonal variations in both the dune and the ice cover alike.

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Gale Crater’s Lake District

Posted on March 26, 2013 by rburnham

Mars rover Curiosity landed on, or just beyond, the far end of an alluvial fan — rocks, gravel, and sand washed down by the Peace River from the north rim of Gale Crater. The rover has driven for 200 sols (Mars days) across a landscape that was shaped and mineralogically altered by water, and which lies near the lowest part of Gale Crater’s floor.

LAKE GALE. High-resolution data and images reveal geological details that point to a former lake inside Gale Crater. According to the evidence, the lake's water level stood for a while at several elevations. The images show this lake at two levels: –2277 meters (where it joins a proposed ocean in the northern lowlands) and –3377 m. (Images from the abstract.)

What are the odds that Gale Crater didn’t contain standing water — a lake — during its 3.8 billion year history?

A group of scientists led by William Dietrich (University of California, Berkeley) use new topographic data to argue that Gale shows evidence for lakes at three different elevation levels, plus a few possible shallow lakes near the landing site. The team reported its results (PDF) at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas.

The geologists examined high-resolution digital elevation models to identify likely ancient shorelines, fluvial deltas, and other features of former lake levels. They write, “We describe the possible lake levels from highest to lowest, and hypothesize that this also records a time progression in lake levels because of preservation patterns.”

The highest lake level lies at about -2100 meters (-6900 feet) below Martian “sea level.” Images showing broad terrace-like features about the same height as Gale’s northern rim flank its giant mound, Mt. Sharp. These show features in HiRISE images consistent with being deposited underwater.

If so, the team notes, the lake level would have been higher than the northern rim, and very roughly coincident with a proposed northern ocean at a level of –1848 m (–6100 ft). In this model, Gale would have been an inlet or bay on the ocean’s shore.

The next lower lake level lies at about –3300 m (–10,800 ft). This roughly corresponds to the lower ends of channels on the south side of Mt. Sharp and to a distinct bench on the northeastern side of the crater. The scientists note that a lake at this level would have had an average depth of 650 m (2100 ft).

The third and lowest lake level (–3780 m / –12,000 ft) is the best defined through a bench that can be traced along the crater wall and on Mt. Sharp. This lake would have been confined to the northern and eastern sides of Mt. Sharp, and its average depth would have been about 170 m (560 ft).

As the lake shrank, it would have ended as shallow separate pools on the floor. The team notes that Peace Vallis built an alluvial fan that descends into a closed basin toward the Glenelg rover site, which is 670 m (2200 ft) below the third lake level. “It seems highly probable that shallow lakes may have formed there episodically,” they say.

“The simplest interpretation of this succession of lake levels,” explains the Dietrich team, “is that the crater filled with water and then the lake level progressively fell, perhaps stalling at two levels long enough to create a topographic record of the shoreline.”

The three high lake levels would have saturated deposits in Mt. Sharp and in the crater walls. Deposits in Mt. Sharp — at least the lower levels — are Curiosity’s prime geological targets.

“As lake levels in Gale decreased, the low area immediately adjacent to the Curiosity landing site would have been one of the last areas to dry out,” they note.

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Does Arabia show a new style of Martian volcanism?

Posted on March 25, 2013 by rburnham

Arabia is one of the largest and oldest regions of Mars. Among its varied rocks are widespread deposits of soft and easily eroded ones which may have a volcanic origin. However, Arabia has no obvious volcanic vents or sources. So are its volcanos so old they have been eroded into invisibility — or does Arabian volcanism look different from more familiar Martian volcanic styles?

Eden Patera

THIS EDEN WAS NO GARDEN. Superficially, Eden Patera resembles an impact crater, but it lacks characteristic impact features such as an ejecta blanket, central peak, and raised rim. Instead, it's more likely to be a complex volcanic caldera that formed by collapse as magma withdrew, perhaps following explosive eruptions. (Image taken from Figure 2 in the abstract.)

The latter idea is being put forward by a group of researchers led by Joseph Michalski (Planetary Science Institute). They have identified a number of features and structures in northern Arabia that may be volcanic in type, but which have passed unnoticed because at a glance they resemble impact craters. Michalski’s team reported (PDF) on their findings at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas.

“We have identified a new type of ancient volcanic construct within Arabia Terra,” they note. “The features are characterized by the presence of large collapse structures, with low overall topographic relief, and they are associated with fine-grained deposits and ridged-plains lava flows.”

Northern Arabia Terra contains many examples of these features, they report, each of which likely produced huge volumes of explosively erupted (pyroclastic) materials and lava. Given the large estimated eruption volumes, evidence for collapse, and other features, these volcanos resemble “supervolcanos” on Earth, such as the Yellowstone volcanic region.

The geologists identified several examples, including Eden Patera, Siloe Patera, Euphrates Patera, Ismenia Patera, and Oxus Patera. They also include Semeykin Crater, as it is surrounded by volcanic deposits and shows volcanic features inside as well.

“The discovery of a new type of volcanic feature within the ancient crust of Mars,” says the team, “fundamentally changes the view of ancient Martian volcanic processes, and expands known volcanic source regions and processes.”

Scientists have known that volcanism early in Martian history was typically more explosive than it has been in more recent times, and the new finding adds evidence to support this view.

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Developing a stratigraphic column for Gale Crater’s floor

Posted on March 19, 2013 by rburnham

After more than 100 sols (Martian days) in Gale Crater, NASA’s Curiosity rover has driven some 500 meters (1,600 feet), traversing several rock units. A sketch of the crater floor’s geological history is emerging.

HIGHER IS YOUNGER and lower is older: Curiosity descended in elevation and time as it drove eastward from Bradbury Landing. (Image from abstract.)

On its half-kilometer drive from its landing site on Bradbury Rise eastward toward Yellowknife Bay, Curiosity descended about 15 m (50 ft) in elevation. While much of the drive was across loose sand and gravel sediments, Curiosity also passed several outcrops of bedrock. These gave scientists a look at the rock units making up part of the Gale Crater floor.

A team of researchers led by Kathryn Stack (Caltech) reported (PDF) on the first drafts for a stratigraphic column for Gale’s floor at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas.

“This study presents an inventory of outcrops targeted by Curiosity, grouped by lithological properties observed in Mastcam and Navcam imagery,” the researchers say. The team identified potential outcrop locations in color Mastcam and Navcam images using thickness, orientation, and similarity to nearby rock fragments so as to distinguish outcrops from “float,” rocks separated from their original location.

They then sorted and classified the outcrops by tone and color, presence of bedding, grain-size, and weathering style into several types. These types included conglomerates, fine-grain beds, and thickly bedded and fractured ones.

“Creating stratigraphic columns from a 2D rover traverse cross-section requires several assumptions,” the team cautions. These were that the outcrops represent layers located where they formed, that they were deposited horizontally and are still horizontal so that outcrop elevations reflect their stratigraphic position, and finally, that younger units lie above older ones.

THREE MODELS for the local geology at the Curiosity landing site. In each, coarse outcrops of gravel and pebbles ("conglomerates") lie on top. These represent sediments deposited by ancient stream flow. Thickly bedded mudstones on the bottom reflect fine-grain sediments, perhaps deposited in at the far end of an alluvial fan. (Image from K. Stack.)

Model 1 assumes a “layer-cake” stratigraphy where fine-grained cross-stratified sandstones are interbedded with the Bradbury gravels. In this model, the rock units of Yellowknife Bay are older than those exposed on Bradbury Rise.

In Model 2, the Yellowknife Bay units represent a distinct sequence of rocks from those deposited on Bradbury Rise. In this scenario the rocks at Yellowknife Bay are in on-lap relationship with those of Bradbury Rise meaning that the Yellowknife Bay sequence may be younger or equivalent in age to the rocks of Bradbury Rise.

Model 3 considers the Bradbury conglomerates to be the youngest unit in the sequence, deposited unconformably  as a thin surficial deposit on Bradbury Rise.

“It may not be possible to distinguish between the three stratigraphic models at this point in the mission,” the team acknowledges. But they note that stratigraphic frameworks such as these can help guide the selection of targets for camera observations, drilling, and detailed geochemical analysis.

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