Mars's valleys

Like Earth, Mars has valleys exhibiting complex geological histories, including flowing water, hill-slope processes, and structural control. Unlike Earth, yet similar to the Moon, Martian valleys may be as old as four billion years.

Overview

Valleys are low-lying, elongated troughs on planetary surfaces that are surrounded by elevated ground. On Earth, valleys often contain a stream with an outlet. About two hundred years ago, the origin of valleys on Earth was very controversial. In 1788, the Scottish naturalist James Hutton disputed the prevailing opinion that valleys formed by cataclysmic flooding, specifically the Noachian flood of biblical accounts. Hutton hypothesized that valleys formed gradually through the erosive action of the rivers and streams that lay on their floors. The fluvial origin of most valleys was subsequently demonstrated by detailed geomorphological work over the next century.

On the Moon, which lacks significant water and other volatile chemical components, valleys are very different. Valleys on the Moon are completely dry and are thought to have been created mostly by subsurface forces. Some lunar valleys may also form by chains of craters left by impacting meteors. Some, called sinuous rilles, have formed by the erosive action of lava. Others are structural depressions formed as surface blocks dropped between fractures. Such valleys also occur on Mars and Earth, but they are much less common than the fluvial forms.

In 1972, spacecraft images revealed the presence of channels and valleys on Mars that appeared very similar to those on Earth. On Mars, a very interesting inversion of scale occurs for channels and valleys. Channels are those troughs in which fluid flow once completely surrounded the depression that confined it. Martian channels are up to 200 kilometers wide and 2,000 kilometers long. The channels are much larger than networks of small valleys in the ancient, heavily cratered uplands of the planet. The valleys are typically a few kilometers wide and several hundred kilometers long. Both Martian channels and Martian valleys are extremely ancient by terrestrial standards but are comparable in age to many features on the Moon. The valleys formed early in the planet’s history—by analogy to the Moon, more than 3.5 billion years ago—when rates of impact cratering were much higher than they are at present. The channels are somewhat younger, extending in age from about three to 0.5 billion years ago.

Martian channels were formed by immense flows of fluid that emanated from zones of collapsed topography known as chaotic terrain. This fluid seems to have burst onto the surface as immense floods of water plus considerable sediment. In the extreme cold of the Martian environment, the water would have partly frozen to form ice jams driven by the turbulent water. Local blockades of ice may have induced secondary floods, and the ice itself could have flowed in a manner somewhat similar to terrestrial glaciers. Processes of cataclysmic water outbursts were probably repeated over long periods of time. These floods were probably generated from ground ice in the subsurface, perhaps heated by volcanic activity.

One of the enigmas about flowing water on Mars is the present surface environment of the planet. Surface atmospheric pressure is only about 0.7 percent of terrestrial atmospheric pressure at sea level. The temperatures measured at the Viking landing sites on the planet ranged from 243 kelvins by day to 193 kelvins at night. Subsequent landers verified the surface conditions first reported by the Vikings. Under these conditions, any standing body of water would rapidly vaporize and freeze. Rapid outbursts of water that formed the large channels, however, could have been maintained because of the relatively short duration of the flow events.

The existence of small valley networks of the heavily cratered terrains of Mars poses a problem for scientists. The valleys have short tributaries that end in abrupt valley heads, similar to the box canyons of the western United States. This valley form is believed to be caused by spring sapping. Sapping is the process whereby groundwater undercuts hill slopes; the groundwater apparently emerged as springs at the heads of the valleys, providing a subsurface source for flow.

One way to have maintained the groundwater flow that sustained Martian valley growth would have been for precipitation (rain or snow) to have fallen in the headwater areas. Water infiltrating the ground would then have recharged the groundwater flow system. This mechanism, however, requires Mars to have once had a very different climate from the one observed today. The atmosphere would have had to be warm and dense enough to hold considerable water. Atmospheric scientists have constructed theoretical models of such an ancient, hypothetical atmosphere for Mars. They conclude that increased amounts of carbon dioxide may have been present early in the planet’s history. The carbon dioxide could have contributed to a greenhouse effect, whereby the planetary surface is warmed by trapping incoming solar radiation as heat. An alternative mechanism for maintaining groundwater flow to the valley networks would have been for geothermal systems to drive the flow by convective, or circulating, heat. Subsurface volcanic rocks would supply the heat, circulating the groundwater in a manner similar to that in areas of hot springs, such as Yellowstone National Park. Water flowing in the valleys would cool, seep into the ground, and recharge a recirculating system driven by volcanic heat. This mechanism would not require a dense, warm atmosphere early in the planet’s history.

Both channels and valleys on Mars have been modified by many other processes besides water flow. Because these features have hill slopes, a variety of gravity-induced slope adjustment forms are present, called mass movements. These slope adjustments include landslides, flows of debris, and slow creep of slope materials. All these processes may have been facilitated by water and ice mixed with the rock materials. Movement of debris onto the channel and valley floors, in some cases, completely conceals evidence of fluvial action that originally cut into the landscape. The Mars Global Surveyor spacecraft provided images, taken years apart, that demonstrated slumping of walls, albeit in craters, that likely was the result of water under the surface diminishing the load-bearing capability of those walls.

Wind action is also facilitated by the confinement of valleys. Wind erodes fine sediment, producing a lineated topography that parallels prevailing directions. Eroded materials may locally accumulate as sets of sand dunes, or they may be more broadly distributed as sheets of deposited sand or dust.

Valleys also served as troughs along which erupted lavas descended from volcanic source areas. Indeed, Martian volcanoes serve as excellent sites in which to observe the evolutionary sequence of valley development on Mars. Volcanoes vary in age and in the character of their surfaces, thereby providing a kind of natural experiment on the formation of valleys. Studies of fluvial valley development on volcanoes indicate that incision by flowing water occurs only when the very permeable lava flows of the volcanoes are altered to have less permeable surfaces. Lowered surface permeability arises from volcanic ash that mantles local areas. Channels forming on this ash incise into the volcano. As valleys form by enlargement of these zones, the incision is able to tap groundwater in the permeable lava flows. This groundwater further sustains valley growth in a headward direction by sapping. Eventually, the volcano is dissected by a mature network of valleys adjusted to the water flow system that is sustaining its growth.

Many mysteries still surround the valleys on Mars despite a rich history of spacecraft imaging over four decades from Mariner 4, to the Mars Science Laboratory, to the Mars rovers, such as the Curiosity and Perseverance. While most valley networks are very old—older than most rocks on Earth—some are quite young. Very well-developed valleys occur on the relatively young Martian volcano Alba Patera. Valleys there are restricted to a local area of volcanic ash. It may be that water was introduced by local precipitation, perhaps related to outburst flooding in the large channels.

Mars has additional surprises, including an abundance of linear grooves and a lack of depositional forms. Also puzzling is the fact that the large channels on Mars contain landforms that are very different from landforms generally seen on Earth. The best Earth analogy to the outflow channels is a region called the Channeled Scablands in eastern Washington state. This area of flood-eroded basalt was generated by immense glacial lake outbursts during the ice ages. The Channeled Scablands are more similar to the Martian channels than any other region on Earth.

Methods of Study

The channels and valleys of Mars were discovered by remote-sensing observations generated from spacecraft. Despite American astronomerPercival Lowell’s accounts of Martian “canals” nearly one hundred years ago, telescopic views of the planet were inadequate to interpret the presence of channels and valleys. It was not until 1972, when the Mariner 9 spacecraft returned the first high-resolution pictures of them from orbit about Mars, that the importance of the valleys was realized.

The ages of the valleys are interpreted by the number of superimposed impact craters. Then, by analogy to known cratering rates and histories on the Moon, ages can be assigned to the Martian landforms. The genesis of the valleys must also be determined by analogy. An interpreter of the pictures of the valleys must have a broad familiarity with natural landscapes, which is used to infer causes for the combinations of features seen in the channels and valleys. Often, details of planetary landforms are somewhat different from what is generally known from terrestrial experience. On Mars, such lack of correspondence arises from lower gravitational acceleration, low surface pressure, and low temperature.

Details of Martian landforms are analyzed on special maps that show relationships and patterns. Geological maps elucidate the time sequence of development, and geomorphological maps show relationships among the planetary features. Quantitative measurements can be made of the landform shapes, which can be compared to measurements on similar-appearing terrestrial features.

Model building is the activity whereby an explanation of the valley is provided in a form that extracts significant elements from the natural complexity of the phenomenon. The model may be expressed in abstract mathematical terms; it can involve laboratory hardware, or it can simulate the sequence of landform development through various kinds of analogy. In all cases, however, the model is used to express in simple, predictable terms the complexity of the real-world system under investigation. Models are only as good as their correspondence to the natural system; however, successful model building requires an intimate knowledge of the system under investigation. This knowledge must be continually checked against new data about that system gained through ongoing investigation.

Mars studies can be separated into two distinct categories. During the initial discovery phase, Mars was visited by Mariner spacecraft. Mariner 4 flew by, snapping a small number of pictures in 1965 that revealed a moon-like character to the surface of the Red Planet. It was something of a disappointment that the more sophisticated Mariner 6 and 7 flyby missions in 1969 verified the arid, cratered character indicated by the previous Mariner spacecraft. Then, Mariner 9 orbited Mars and revealed valleys and river features as well as giant volcanoes, indicating that Mars had a dynamic past. The Viking landers examined the plains and found ambiguous soil analysis results that could not definitely answer the question about life on Mars. These missions completed the discovery phase.

Beginning with the failed Russian Phobos missions and followed by the National Aeronautics and Space Administration’s (NASA’s) robust sequence of spacecraft—Mars Observer (unsuccessful), Mars Pathfinder (a rover), Mars Global Surveyor, Mars Climate Orbiter (unsuccessful), Mars Polar Lander (unsuccessful), Mars Odyssey, the Mars Exploration Rovers, Mars Reconnaissance Orbiter, Mars Phoenix, Mars Science Laboratory, Mars Curiosity and Perseverance rovers—along with the European Space Agency’s Mars Express, a second phase of focused Mars studies examined planetary features in large part seeking an answer to one fundamental question in particular: the fate of Mars’s water.

A third phase of Mars exploration will begin once humans develop a capability to journey to Mars ,complete research in situ, and travel across the Martian surface with sophisticated instruments. Various space agencies, from the ESA, NASA, and the Japanese Space Agency have plans to send astronauts to Mars by the 2030s.

Context

The valleys of Mars reflect immense environmental changes that occurred on the planet. Surface conditions presently are too cold and the atmosphere too thin for water to exist in its liquid state. However, in the past, during selected epochs of its planetary history, Mars seems to have been able to sustain an active hydrological cycle that produced river valleys similar to those on Earth. Because the small valleys occur throughout the heavily cratered terrains of the planet, the scale of environmental change must have been global. In July 2008, the Mars Phoenix lander sampled subsurface water ice near the northern polar region, providing the first direct evidence of water still on Mars. Scientific discovery pointing to evidence Mars once had surface water continued with future NASA rovers.

In 2008, data from the Mars Reconnaissance Orbiter led to the conclusion that the Mars of the past possessed a wet environment, something that had been suspected after the tantalizing images from Mariner 9 led scientists to alter their assessment of the planet Mars after the disappointing dry, cratered surface revealed by the Mariner 4, 6, and 7 flyby missions. Current studies strongly indicated that ancient highlands on Mars, which constitute nearly half the planet’s surface, contain certain types of clay minerals that need water in order to form. This suggests that Mars once had large lakes, flowing rivers, and smaller wet environments for prolonged periods of time in the distant past. Water erosion moved the clay minerals into delta formations. The crater Jezero appears to have once confined a lake that was later breached; its water carried the clay minerals out into a fan-shaped structure. In 2012, Mars Science Laboratory's Curiosity rover analyzed rounded pebbles in Gale Crater whose shape indicate they occupy a dry streambed; based on the shape and size of the rocks, scientists estimate the stream to have flowed from ankle to hip deep at a rate of at least one meter per second. Scientific discovery pointing to evidence Mars once had surface water continued with future NASA rovers. In 2023, the Mars Perseverance rover took images of a delta on a crater floor of the planet which showed signs it once held flowing water.

Earth has also been affected by global environmental change. Numerous times over the past several million years, the planet has experienced decreased global temperatures with associated glaciation. During glacial advances, hydrological conditions were profoundly changed. Most recently, public interest on Earth has become focused on the warming associated with artificially increased levels of carbon dioxide and other trace gases in the atmosphere. Thus, like Mars, Earth oscillates between periods of increased warmth and cold. By comparing theories that explain such cycles, scientists hope to understand exactly how environmental change occurs and thus develop a means of predicting future change that will affect humanity on its home planet.

In a broad sense, the development of humankind has been linked to discoveries associated with exploration and with the migration of peoples to new lands; the space program is the most modern manifestation of such trends. When the channels and valleys of Mars were discovered, they stimulated an immense scientific effort to explain the conditions on Mars that made flowing water possible in the past. Scientists generally agree that most of the water on Mars is locked up in its subsurface, frozen as ground ice in layers of thick permafrost. There is speculation that if Mars has enough water and it can be accessed, it could help sustain a future population of emigrants from Earth.

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