What is the evidence that Mars has water now and had more in the past? Certainly, the water ice seen at the polar caps, the clouds seen in the atmosphere around the large volcanoes, and the fog and frost that is seen in the low elevation valleys in the Martian morning, and some evidence for gullies in canyons, testify to the presence of some water on Mars today. But not much water. Is this all the wate Mars has and ever had?
Mars has surface geologic features that have been termed valley networks. These features look like river valleys on Earth, i.e., fluvial features formed by water flowing for long periods of time, slowly carving out valleys out of the surrounding rock. Valley networks appear to require a warm, wet Mars, a Mars in which water flowed on the surface (without freezing or evaporating quickly) for 100s of millions of years.
The other surface feature apparently indicating the presence of water on Mars in the past are the outflow channels. Outflow channels have been interpreted as the result of catastrophic melting or flooding which consequently carved out deep, wide channels. This interpretation of outflow channels does not require that water was flowing on Mars for extended periods of time, only that large volumes of water were present and available for flooding when released.
Valley networks are found only on the ancient cratered terrains of Mars (southern hemisphere; ages of 3.5 - 4 BY). The valley networks, first observed by Mariner 9 in 1972, provided the first evidence that Mars had liquid water running across its surface 4 billion years ago. Outflow channels are largely in the northern lowlands, near the equator, on much less cratered, and therefore much younger, terrain (perhaps of ages 2 BY).
Map of outflow channels and valley networks (from Mike Caplinger, Malin Space Science Systems). On this map, outflow channels are colored red, and valley networks are colored yellow. Note that the Valley networks are dominantly in the southern hemisphere and all are in ancient cratered areas.
The term outflow channel was coined in 1975 to denote the large channels, several tens of km across, that start full size at discrete sources. Outflow channels have few tributaries but often branch downstream (the opposite pattern of a river system). The term channel implies a conduit for the flow of water that was filled with water.
This large field image shows several outflow channels in the vicinity of Valles Marineris: Kasei Vallis region. Most outflow channels are more than 100 km wide, and often are 1000s of km long.
The Tiu Vallis region emerged from the region called Hydaspis Chaos, which appears to be a region about 100 km x 200 km that released an enormous quantity of water that then carved out the Tiu valley.
Features known as teardrop islands, here seen in Ares Vallis, are generally thought to be higher elevation regions not carved out by the enormous outflows because the walls of craters protected the land immediatley downstream.
The Mangala Vallis outflow channel appears to have started in a deep crack (a graben).
When did the outflow channels form? All of them date from the Hesperian period on Mars, which postdates the era of the great bombardment and probably covers the time period from about 2-4 BY ago. They required enormous volumes of water likely locked in permafrost or in underground water reservoirs. The water would have been released catastrophically by an impact or the breakage of a permafrost seal over a water reservoir by tectonic event. Whatever the mechanism of release, almost certainly, climactic conditions must have been similar to those on Mars now.
How did outflow channels form? Probably through the sudden heating and catastrophic release of large quantities of water stored in or near the surface. The heating might be related to any number of causes, from internal tectonics to externally triggered heating by an impact. The best example of such an event on Earth is are the Channeled Scablands of the state of Washington. The Scablands were carved out when the glacier that plugged the Pleistocence era lake, Lake Missoula, collapsed. Lake Missoula had covered parts of Idaho and western Montana. Estimates for the volume of discharge necessary to carve out the Martian outflow channels run as high as 1 billion cubic meters per second (for comparison: the average rate of discharge of the Mississippi River is 20,000-30,000 cubic meters per second, so the Martian discharge rate would be equivalent to more than 30,000 times the flow of the Mississippi! the estimated discharge rate for the Channeled Scablands is about 10 million cubic meters per second). Thus, the floods generated by the formation of the Martian channels may have carried more than 100 times as much water as the largest known floods on Earth.
The minimum estimates for the total volume of water necessary to have
carved the Martian outflow channels is at least 6 million cubic km, enough
to make a global ocean 40 m deep.
Valley networks are the most common drainage system on Mars. The formation of such valley networks normally assumes warm and wet climatic conditions, but this may be much to simplistic. Valley networks exist in the cratered uplands; they are branching networks in which tributaries converge downstream (the opposite of outflow channels); however, most are far less complex than terrestrial networks. Most valley networks are no wider than a few km. Most (92%; 759 out of 827 identified) of the valley networks are on parts of Mars called the Noachian, which predates the Hesperian, i.e., they formed before the outflow channels during the great bombardment era. About 4% (34) may be Hesperian and another 4% may be younger.
Most valley networks appear to have formed by the process of groundwater sapping, in which the water source is underground and the valleys from by the collapse of the surface as it is undermined by the movement of water underground. Thus, it is possible that few or none of these valleys require a warm, wet Mars.
One valley network on Mars is Nirgal Vallis, as seen by Viking and later by the Mars Global Surveyor. (labeled image)
Another is found in the Thaumasia region.
Terra Meridiani valley network near the martian Equator. This valley is in a heavily cratered region. shows many impact craters. The brightness of the valley is due to the reflectivity of sand deposits.
Viking and MOC images of Nanedi Vallis
Recent results suggest that the uplift
of the Tharsis Bulge is strongly correlated with
the topography of the valley networks. The study of the Tharsis region indicates that it was already a permanent feature of Mars 4 BY ago. This study suggests that the eruption an emplacement of the Tharsis bulge requires the equivalent of 10,000 Manua Loa's worth of magma (300 million cubic km). Such an eruption would have release a tremendous amount of carbon monoxide (CO) and/or carbon dioxide (CO2), sufficient to produce an atmosphere of 1.5 bars, thereby producing a greenhouse effect, a global ocean 120-m deep, and a warm, wet Mars for a long period of time, at least long enough to carve the valley networks. Figure 4, found on the web link noted above, shows the general topographic gradients of the valley networks, revealing that they largely flow to the north, as a result of the upward stretching of the southern highlands in response to the Tharsis uplift.
Gusev Crater and Ma'adim Vallis. Gusev Crater is approximately 150 kilometers (93 miles) across. Ma'adim Vallis is the nearly straight canyon that enters Gusev Crater from the lower right. (Viking orbier image). Close-up MGS image of lower region where Ma'adim Vallis enters Gusev Crater. Was Gusev Crater an ancient lake? There is no evidence of ancient shorelines, either because no lake ever existed, because wind erosion has erased the shoreline evidence, or because such features never formed.
The Elysium Basin and Marte Vallis, Mars, Viking image and a close-up Viking image of the region of the MGS images. There were two competing ideas about the Elysium Basin. One hypothesis held that the depression was once the site of a vast lake approximately 1,500 meters (4,900 feet) deep. Because the floor of Elysium Basin has very few small, fresh impact craters, it was proposed that this lake dried up relatively recently in martian history--that is, the lake would have been younger than most of the volcanoes, craters, and even the Ares Vallis flood channel in which is located the Mars Pathfinder landing site. At some point, the lake in Elysium Basin was thought to have reached such a depth that it began to spill over a rise on its east end. The water spilling out the east end of Elysium Basin was thought to have created Marte Vallis--a channel containing streamlined islands that stretches for hundreds of kilometers (miles) to the northeast. The lake bed and channel, it was proposed, might make good places to land future rovers that could travel around and collect samples that might contain evidence of past martian life.
The other hypothesis held that the Elysium Basin floor was covered with flows that were emplaced as extremely fluid lava (molten rock). It was suggested that a lake of water could have been in the basin long, long ago, but that the most recent geologic events had erupted huge volumes of very fluid lava across the basin floor. Some of this lava was proposed to have even poured out of the basin and travelled down Marte Vallis. In this hypothesis, it was assumed that Marte Vallis--named for the Spanish word for "Mars"--was first carved by water, and then was a conduit for lava from volcanic eruptions. The lavas were proposed to have been very fluid--behaving almost like water. Such fluid lavas are known on Earth to result from molten rock that has a low concentration of silica, a high temperature, and/or a high eruption rate.
Close-up image shows the textured surface in Elysium Basin. The texture seen here--one of dark, flat fragments separated by bright cracks--is termed platey, and indicates a surface where the dark areas, or plates, separated and moved apart. Such a texture occurs when the rigid surface of a once-fluid material (for example, ice over water, or the cooled surface of a lava flow over the molten interior) is broken up and moved about by the underlying fluid. In this case, the relationships strongly suggest lava, which today is hardened to rock throughout. If Elysium was an ancient lake, all evidence of materials deposited at the lake bottom appear to have been thoroughly covered over by recent (millions but not billions of years ago) lava flows.
Deuterium on Mars
The deuterium to hydrogen ratio in the atmosphere of Mars is 8 x 10-4, which is 5.2 times larger than the ratio of 1.6 x 10-4 on Earth. This implies that Mars has lost a good deal of its water, as has Venus!
Mars Image Gallery from the Mars Orbiter Camera
1997-1998 MOC Images Listed By Theme or Topic
MGS MOC Captioned Images Listed By Release Date
Mars Global Surveyor---Mars Orbiter Camera (MOC)