As on the Earth, layering - which gives rise to stratification - can be caused by several processes. On Mars, layering from dust deposits laid down in the present and by inference in the past and layering from impact ejecta build-up (probably the dominant process) are established mechanisms that account for perhaps the majority of stratified sequences. As we will see on this page, there is now strong evidence for the action of water from streams and other outpouring mechanisms operating in the martian past.
Some students of the martian surface argue that at one or more time(s) in the past, there were lakes and perhaps even oceans on Mars that existed when the martian climate was warmer than today - gone now because most of the water has either dispersed into outer space or become frozen in the upper reaches of deposits near the surface. James Head III and his colleagues at Brown University have interpreted evidence, including low elevations, drainage patterns, and cliffs and terraces that may indicate shoreline erosion, of at least two stages of oceanic concentrations of water in the northern lowlands. (Needless to say, their proposal has stirred up disagreements.) Here is their map and a pictorial showing the emplaced ocean:
Studies reported in 2008 present evidence from terrain analysis around Valles Marineris that the period in which water was present on the martian surface may have lasted until about 3 billion years ago. The water was responsible for producing Valles Marineris and most other channels. But, this claim is moot as to whether the water could (or did) collect in a standing "ocean".
There is greater certainty that ancient water on Mars had collected in lakes. This next pair of images show dessication cracks in terrestrial lake beds on the left and similar features on Mars on the right. The spacing of these cracks is too wide to be accounted for by ice phenomena.
The occurrence of stratified rocks would substantiate the evidence that water had once collected into bodies of standing water. Many Mars images have been acquired that show distinct layering, as will be displayed in a number of images on page 19-13. We will preview those images by showing here just one typical set of layers exposed in West Candor Chasm, an auxiliary canyon off Valles Marineris:
Various explanations are put forth as to their nature: Are they marine sedimentary, lake beds, volcanic flows, volcanic ash deposits, or other unknown types, or as seems likely a combination of these modes of layering? The occurrence of marine sedimentary layers on Mars is still much debated, with as yet no direct proof. But, like the Earth and its Moon, layered materials found on the martian surface follow the Law of Superposition, which defines the relative ages of overlapping deposits. And, like the Moon and some other planetary bodies, the relative densities of craters have been counted over its surface, which has led to a broad stratigraphic division of the principal geologic units that are recognized on Mars:
This subdivision yields three broad Eras. The oldest, the Noachian, extends from the formation of the planet to a time estimated to be 3.8-3.5 billion years ago. Crater-forming bombardment was maximum during this time, producing Hellas and other basins. Both extensive lava flows and perhaps a broad regional ocean developed in this time, and the dichotomy of older high plains in the southern hemisphere and lower, more lava-covered plains in the northern hemisphere began in the Noachian. The Hesperian Era lasted from the 3.5 b.y. time period until about 1.8 billion years ago. Crustal fissures allowed widespread volcanic flows to emplace. The Elysium volcanoes formed and those in the Tharsis region began to develop. Again, water may have been active in modifying the landscape. It probably acted upon a large fissure that grew into Valles Marineris. The Amazonian Era continues to the present. Water erosion/deposition may still have played a role but wind now was the principal agent of surface change. During this time Olympus Mons grew as did the Tharsis volcanoes.
The global surface of Mars can be displayed on a map in terms of these relative crater densities, which like the Moon, provide (along with characteristics of volcanic surface) the principal means for distinguishing age units, as shown:
A general "stratigraphic units" map of Mars , produced by standard photogeologic methodology, is shown below along with a units key :
The U.S. Geological Survey has produced quadrangle maps of most of Mars by now, as illustrated by this one which shows the Hesperia Planum and Tyrrhenia Terra regions of Mars.
In addition to the quadrangle maps produced by the U.S.Geological Survey (see above), the USGS has now prepared maps of the entire planet at several scales. The best known of these, at 1:25,000,000, is shown in an overview image below, but none of the text or legend are legible in this version. You can track down a readible version on the Internet by clicking on Mars map kept online by the Lunar and Planetary Institute (note: it is a slow load).
Here is a variant of most of that map but with a different color scheme, shown without a legend or annotation:
It should by now be evident that Mars has a diverse and intriging history, with a wide variety of surface features. These are examined in pages 19-12ff.
Before looking at some representative images of the Red Planet, we set up a physiographic framework for Mars: From Earth-based telescopic observations, astronomers suspected Mars had oxidized, iron-rich materials at its surface, was subject to dust storms, and had ice caps at both poles that alternately expanded and contracted over a martian year (687 Earth days). The storms implied at least a thin gaseous envelope (out to about 125 km). Spectroscopic measurements indicated CO2 and maybe nitrogen. The Mariner/Viking missions greatly modified our concepts of the martian landscapes. Consider the generalized landforms maps of the two martian equatorial hemispheres, drawn on a Lambert equal-area projection from those mission results, as shown here:
From T.A. Mutch et al., The Geology of Mars, © 1976. Reproduced by permission of the Princeton University Press, New Jersey.
Much of the upper parts of the two globes (left and right hemispheres in the diagram) have plains units (p) (in white), many thought to be volcanic flows (pv), and volcanic constructs (v) (such as shield volcanoes), some of whose ages could be less than one-half billion years.The bottom polar hemisphere in each globe contains cratered terrain (cu) (dark gray), considered to be several billions of years old (and the likely source of the 3.5 b.y. old martian meteorite containing organics, found in Antarctica), where the bulk of the larger impact structures survive. Other subdivisions include cratered plains (pc) and moderately cratered plains (pm). Unusual or specialized units consist of channel deposits (c), grooved terrain (g), knobby hummocky terrain (hk), fretted hummocky terrain (hf), chaotic hummocky terrain (hc), and mountainous terrain (m). Units confined to the polar regions are: permanent ice (pi), layered deposits (id) (thought to be layers of windblown dust interspersed with ice), and etched plains (ep). As with Venus, structural signs of plate tectonics are absent.
Compositional data (discussed on pages 19-13 and 19-13a) suggest that the darker shaded southern part on the Mars maps is underlain by basalt whereas parts of the northern hemisphere contain andesites as well as basalts.
Many features in these categories are also landmarks and physiographic provinces. In the left hemisphere, the three volcanoes in a row (Ascraeus Mons [top], Pavonis Mons [center], and Arsia Mons [bottom]) are in the Tharsis Montes group. The two large ones above and to the left are Olympus Mons (largest volcano in the Solar System) and Alba Patera (also a caldera-capped volcano). The long, curvilinear feature (c) near the equator in the left hemisphere is the great Valles Marineris, many times longer and much deeper than the Grand Canyon, with associated chasmas or tributary canyons. In the right hemisphere, the large area (p) in the southern half is Hellas Planitia, site of the biggest impact basin on Mars, cut into ancient terrain and backfilled with lava.
Further knowledge of martian physiography can be gained from relief (elevation difference) maps of Mars. Several of these for selected regions appear in the next three pages; such maps can be constructed from laser altimeter and radar data, as provided by Mars Global Surveyor and other spacecraft. Shown below is a global shaded relief map of Mars, made from MGS data. Note the higher elevation (red) region around Olympus Mons and the Tharsis volcanoes; the low (blue) depression that constitutes the Hellas Basin; and the generally low state of the northern polar region (James Head's ocean basin). Note also that the southern 60% of Mars is significantly higher (except the Hellas Basin) than the northern part.
The next map, made from MGS MOLA data, focuses in on what is probably the most interesting part of the martian surface - the high terrain that includes Olympus Mons; the other three "Mons", and Noctis Labyrinthus:
The Voyager and Mariner missions confirmed the thin martian atmosphere consists of 95.3% CO2, with the remainder being mostly N2 (2.7%), argon (1.6%) and minor O2, CO, and traceable water vapor. Atmospheric pressure averages 7 millibars or about 0.7% of Earthís. This thin atmosphere is the survivor of what once may have been a much denser atmosphere. That thicker atmosphere was eventually blown away by the solar winds. For a while the atmosphere may have been protected by a strong magnetic field (which on Earth prevents the solar wind from removing its atmosphere). But the magnetic field eventually shut down (see page 19-13), allowing the solar wind to remove most of the original atmosphere. Evidence for this primitive atmosphere is indirect and inconclusive but an early atmosphere is consistent with some of the signs of water transport responsible for sedimentary layers that are abundant at various locales on Mars (see 20-13ff). The question is important insofar as it bears on possibilities for life.
Despite the meagre present day atmosphere in terms of density, what is there has some dramatic effects on the martian surface. Windstorms in the current atmospheric envelope can reach speeds greater than 200 km/hr (124 mph). These give rise to dust storms and to aeolian deposits.
Clouds of water-ice are fairly common in season, especially near the poles. This Viking Orbiter image shows clouds above the martian limb and perhaps clouds over the land.
The presence of small amounts of oxygen has been confirmed by X-ray emissions monitored by the Chandra X-ray telescope. O2 is excited by X-rays accompanying the solar wind. The amounts seem to vary, as seen in this Chandra image:
Both Mariner 10 and the Viking Orbiters gathered many images showing various aspects of atmospheric circulation, such as this view of spiraling clouds above the martian North Polar region; note also the bright patches of ice.
Clouds reminescent of cumulus types on Earth are seen in the atmosphere. These are evident in this MERS Opportunity (see page 19-13a) image which also show cloud bands formed by atmospheric diversion by the Crater Mie:
The martian atmosphere is generally hazy, resulting from suspended dust and possibly some water condensate. Instead of the blue sky of Earth, one peering up from the Mars surface would see its sky as grayish-yellow to reddish, depending on the time of the martian day. Although Viking Landers gave some indication of these colors, the next image, from Mars Pathfinder's camera looking well up into the sky, is a good rendition of color near sunrise; the clouds are of the cirrus type as known on Earth.
There are indications that Mars had a much thicker atmosphere one or more times in its past, especially during the first billion years. This is tied into evidence for extensive water on Mars, as will be discussed in subsequent pages. Most of the earlier atmosphere was lost, possibly by escape but more likely by being blown away by the solar wind. Unlike Earth, whose atmosphere is protected from the solar wind by its magnetic field, Mars may never have had a global magnetic field, implying that its core is either not iron or if so, then is not molten now (but possibly in the past).
The presence of sedimentary layers on Mars (to be documented later) implies lakes or even seas in martian history. This condition suggests milder climates in the past, during which water could remain unfrozen. There may even be cyclic climate change. Mars wobbles much more than Earth (Earth's wobble is dampened by the gravitational action of its large Moon). The poles may dip as much as 40⪚ from time to time. This could lead to periodic warming.
Thus, Mars is now known to not be DEAD as a planet. As we shall see, there may be active water transfer, buildup of ice caps, and possibly volcanic eruptions.
As further proof that Mars is meteorologically quite active despite the thinness of the atmosphere, an extensive dust storm was imaged from an Orbiter in the act of happening:
Dust storms commonly begin to form in the martian Spring. Here is a series of images obtained during a single Earth day that show a full-fledged dust storm in the high southern latitudes and dust clouds of lesser intensity over much of the remainder of the planet:
When enough of these regional dust storms form, they coalesce and sometimes cover the entire planet for many (earth) months. More than half the planet was engulfed in a widespread dust storm that began in June of 2001 and started to diminish in September. Compare these "before" and "after" Hubble images of minimal and maximal dust coverage; note how the surface features in the left image are masked by the pervasive, near global dispersal of dust into the atmosphere as seen in the right image.
Using the Thermal Emission Spectrograph on Hubble, a sequence of images taken between June and August, 2001 shows the variations in density of dust (shown in red) in the martian atmosphere.
Earth-based telescopes can spot developing dust storms almost as soon as they start. The yellow-brown area in this next full Mars image made when that planet was at its closest distance to Earth (55,000,00 km [34,000,000 miles]) in the last hundred years is an organizing dust storm imaged through a ground-based telescope in Arkansas:
Dust is in the atmosphere most of the time, including periods when dust storms have subsided. This dust can obscure a scene, giving the impression that the image is not quite in focus, as shown here for an MRO image of the Pavlonis Mons region:
Certainly, as evident from the previous images, its thin atmosphere is remarkably active, with fast-moving winds picking up and redepositing surficial particles in dunes and other aeolian features. Here is a "candid camera" image of what on Earth are called dust devils - tornado-like wind swirls that pick up surface fines to create a tubular or thin funnel-shaped cloud that sweeps across the surface. This Mars Orbiter Camera (MOC; see page 19-13) image shows a thin dark trace along a crater wall made when the dust-devil (actually moving; on top, at the left end of the streak) removed material from the wall:
Thin dark markings, often found in swarms, on parts of the martian surface, have now been identified as trails caused by numerous dust devils ("mini-tornadoes") scouring the loose surficial material. Here is a striking example:
Dust devils must be commonplace on Mars. In a single season, the number of trails like those shown below, can be many. The winds seem to destroy earlier ones, then fresh trails are created that expose the dark surficial materials below a new covering of lighter-toned dust:
In parts of Mars that have dark surfaces, the dust devil trails appear as lighter streaks, implying the underlying near surface contains lighter materials exposed when the whirlwind picks up the dark fines covering the surface. Thus:
Individual dust devils have been imaged as they actually passed by landers on Mars. The Rover Spirit caught this image of a nearby whirlwind:
The martian dust and sand is frequently spread out as wind streaks.
Despite the very low density of today's martian atmosphere, its high speed winds can cause erosive sculpturing, and can deposit uplifted dust and sand into vast dune fields. The Viking Orbiter image below contains transverse (elongated, or parallel) and barchan (crescent) dunes laid down in a plains setting.
A second image shows parallel dunes on the left and barchan dunes on the right.
More recent imagery shows the splendor of martian dune fields, such as this one in Ius Chasma:
Here are longitudinal dunes adjacent to the Schiaparelli crater:
Distinctive ripple dunes can develop within shallow troughs or linear valleys. This is a good example:
In contrast to dunes on Earth that usually have a light tone because they are made up of mostly clear to cloudy white quartz, dunes on Mars usually are much darker because the grains making them up are either basaltic fragments or darker hematite. This is a typical set of darker dunes:
Although uncommon, some places on Mars show two sets of dunes - one dark and the other light in tone. This dual occurrence is hard to explain and suggests two different processes (or time intervals) at work:
Martian dunes can have a regular, criss-crossing pattern (lattice-like) that is uncommon on Earth where strong prevailing winds tend to favor a single alignment (cross-winds would likely destroy the reticulation). Here is an example from the martian polar region:
More striking close-up views of dunes have been imaged by the Mars Global Surveyor (MGS; MOC) (see page 19-13). Crescent dunes very similar to those found on Earth also develop on Mars, as seen in this MGS ) image:
A variant of this type leads to dunes that approach crude circles in shape.
In this next image, dunes having shapes similar to those just above have formed but they are far more widely spaced and tend to be round. Some "wags" have named them "martian cookies". They are most common in the polar regions where their dark materials stand out against a frosty surface.
A MOC close up view of barchan dunes appears next:
The martian dunes come in a wide variety of forms. This MOC image shows dunes with a broad, gentle slope and steep, ragged foreface: