Before photography, the only recorded observations were drawings. Below, Old Mars displayed many features which were later shown not to exist.
Up till the early 1920's, we thought Mars looked like the drawings above and below. Note the ``canals'', which originally described as ``channels'' in Italian, but then was badly translated to ``canals'' which implied they were built by intelligent beings.
And even today many important people believe in canals on Mars.
Today, Mars looks like this from the Hubble Space Telescope
Even before 1800's, it was known that Mars had some large surface features (grey-green regions between larger red regions). And it was know that Mars had seasons because the size of the polar caps changed.
Since 1976, we have send several probes to Mars. Orbital probes have produced detailed visual and radar maps of the surface. Some of the most notable surface features on Mars, such as:
valleys and canyons
The above image is a mosaic of the Valles Marineris hemisphere of Mars. The center of the scene shows the entire Valles Marineris canyon system, more than 3,000 kilometers long and up to 8 kilometers deep. Although it appears to be a canyon formed by water, in fact, Valles Marineris is a deep crust fracture. Many huge ancient river channels begin from the chaotic terrain and north-central canyons and run north. Many of the channels flowed into a basin called Acidalia Planitia, which is the dark area in the extreme north of this picture. The three Tharsis volcanoes (dark red spots), each about 25 kilometers high, are visible to the west. Very ancient terrain covered by many impact craters lies to the south of Valles Marineris.
Although Valles Marineris originated as a tectonic structure, it has been modified by other processes. This image shows a close-up view of a landslide on the south wall of Valles Marineris. This landslide partially removed the rim of the crater that is on the plateau adjacent to Valles Marineris. Note the texture of the landslide deposit where it flowed across the floor of Valles Marineris. Several distinct layers can be seen in the walls of the trough.
The water that carved the channels to the north and east of the Valles Marineris canyon system had tremendous erosive power. One consequence of this erosion was the formation of streamlined islands where the water encountered obstacles along its path. This image shows two streamlined islands that formed as the water was diverted by two 8-10 kilometer diameter craters lying near the mouth of Ares Vallis in Chryse Planitia. The water flowed from south to north (bottom to top of the image). The height of the scarp surrounding the upper island is about 400 meters, while the scarp surrounding the southern island is about 600 meters high.
This image of the head of Ravi Vallis shows a 300-kilometer (186-mile) long portion of a channel. Like many other channels that empty into the northern plains of Mars, Ravi Vallis originates in a region of collapsed and disrupted ("chaotic") terrain within the planet's older, cratered highlands. Structures in these channels indicate that they were carved by liquid water moving at high flow rates. The abrupt beginning of the channel, with no apparent tributaries, suggests that the water was released under great pressure from beneath a confining layer of frozen ground. As this water was released and flowed away, the overlying surface collapsed, producing the disruption and subsidence shown here. Three such regions of chaotic collapsed material are seen in this image, connected by a channel whose floor was scoured by the flowing water. The flow in this channel was from west to east (left to right). This channel ultimately links up with a system of channels that flowed northward into Chryse Basin.
This image shows the south polar cap of Mars as it appears near its minimum size of about 400 kilometers. It consists mainly of frozen carbon dioxide. This carbon dioxide cap never melts completely. The ice appears reddish due to dust that has been incorporated into the cap.
Mars also has two small moons, Phobos and Deimos (Fear and Panic). They are not regular moons like our Moon, but rather irregular-shaped objects, which probably means that they are captured asteroids.
Other Surface features:
Water on Mars:
There is a great deal of evidence of the existence of liquid water on Mars, at least in the far past.
First, the secondary atmosphere for all terrestrial worlds is rich in CO2, H2O and SO2. On Earth, the temperature is just right for H2O to rain out and form oceans. On Venus, the temperature is too hot and H2O stays as a vapor to be destroyed by photodisintegration.
On Mars, it is too cold for large amounts of liquid water (i.e. oceans and lakes). Currently, all the H2O is locked up in permafrost and subsurface ice reservoirs. But notice that most of the water flow features (e.g. islands) are near the base of old volcanoes or impact craters. It implies that these features were caused by past events that heated the subsurface ice to produce a short-lived flow of liquid H2O. In addition, Mars Pathfinder found numerous pebbles rounded by water flow.
Mars did have a warmer, wetter past. Was this enough to start life?
Mars is another example of a secondary atmosphere from outgassing (therefore, we know that Mars must have had an early epoch of tectonic activity).
Unlike the Earth or Venus, the atmosphere of Mars is very thin, about 1% the mass of Earth's atmosphere. Its composition is 95% CO2, 3% N2, 2% Ar and less than 1% O2. A high noble gas content implies that Mar's atmosphere was much thicker in the past (noble gases do not react with other elements and are heavy enough to stay within the gravitational field of Mars).
The climate on Mars is desert-like due to its thin atmosphere. There is too little mass in the atmosphere to hold in heat (very little greenhouse effect). The warmest daytime temperatures are around 50 degrees F, but the nighttime temperatures plunge to -170 degrees F. Other Martian weather features are dust storms and occasional CO2 fog in the canyons.
Viking Lander 1 Viking Lander 2
Our first views of the Martian surface came from the Viking Lander 1 and Viking Lander 2. They indicated a surface and soil that is mostly old impact debris with sand-blasted gravel. The soil is rich in Si and Fe, Fe oxide (rust) given Mars its red color.
Viking also showed that Martian winter is think with frost, but that this frost was mostly CO2 ice.
In the summer of 1997, Mars Pathfinder landed on the surface of Mars. The lander contained cameras and meteorological instruments, and also carried a robot rover (shown below) whose job was to take soil and rock samples.
As the chart above shows, the Martian soil and rocks have a high abundance of iron and calcium, such as iron, compared to Earth rocks. Note also that the squares, Martian meteorites, proves they fell on the Earth from Mars.
The low surface gravity of Mars has produced a crust and soil that is not as chemically differentiated as the Earth's crust. Meaning that the soil of Mars is rich in heavy metals such as Fe. This was show with the magnetic results from Pathfinder.
The Three Ages of Mars:
Based on what we have learned from spacecraft missions, scientists view Mars as the "in-between" planet of the inner solar system. Small rocky planets such as Mercury and Earth's Moon apparently did not have enough internal heat to power volcanoes or to drive the motion of tectonic plates, so their crusts grew cold and static relatively soon after they formed when the solar system condensed into planets about 4.6 billion years ago. Devoid of atmospheres, they are riddled with craters that are relics of impacts during a period of bombardment when the inner planets were sweeping up remnants of small rocky bodies that failed to "make it as planets" in the solar system's early times.
Earth and Venus, by contrast, are larger planets with substantial internal heat sources and significant atmospheres. Earth's surface is continually reshaped by tectonic plates sliding under and against each other and materials spouting forth from active volcanoes where plates are ripped apart. Both Earth and Venus have been paved over so recently that both lack any discernible record of cratering from the era of bombardment in the early solar system.
Mars appears to stand between those sets of worlds, on the basis of current yet evolving knowledge. Like Earth and Venus, it possesses a myriad of volcanoes, although they probably did not remain active as long as counterparts on Earth and Venus. On Earth, a single "hot spot" or plume might form a chain of middling-sized islands such as the Hawaiian Islands as a tectonic plate slowly slides over it. On Mars there are apparently no such tectonic plates, at least as far as we know today, so when volcanoes formed in place they had the time to become much more enormous than the rapidly moving volcanoes on Earth. Overall Mars appears to be neither as dead as Mercury and our Moon, nor as active as Earth and Venus. As one scientist quips, "Mars is a warm corpse if not a fire-breathing dragon." Thanks to the ongoing observations by the Global Surveyor and Odyssey orbiters, however, this view of Mars is still evolving.
Mars almost resembles two different worlds that have been glued together. From latitudes around the equator to the south are ancient highlands pockmarked with craters from the solar system's early era, yet riddled with channels that attest to the flow of water. The northern third of the planet, however, overall is sunken and much smoother at kilometer (mile) scales. There is as yet no general agreement on how the northern plains got to be that way. At one end of the spectrum is the theory that it is the floor of an ancient sea; at the other, the notion that it is merely the end product of innumerable lava flows. New theories are emerging thanks to the discoveries of Mars Odyssey, and some scientists believe a giant ice sheet may be buried under much of the relatively smooth northern plains. Many scientists suspect that some unusual internal process not yet fully understood may have caused the northern plains to sink to relatively low elevations in relation to the southern uplands.
Scientists today view Mars as having had three broad ages, each named for a geographic area that typifies it: