The "Red" Planet:
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, which is wrong.
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:
- craters: impact craters with heavy erosion due to atmosphere
- featureless terrain: large regions devoid of faults or craters (not maria since no young craters are found).
Observations by Viking Orbiter showed them to be deserts
- chaotic terrain: highlands and broken hills, probably old tectonic regions.
- Polar caps: change in size with seasons, but since the temperature is less than 0 degrees C at all times, the
ice is mostly CO2 ice with an H2O ice core. Viking
Lander confirmed that the atmospheric pressure goes up in the summer as the CO2 ice
- volcanoes: The Tharsis and Elysium regions are rich in old
cone volcanoes, averaging over 500 km across and 25 km high. The largest is Olympus Mons, shown above. These are
"hotspot" volcanoes like the Hawaii Islands. There extreme size due to the fact that there is no tectonic plate motion
- canyons: long, fractured regions. Most canyons on Earth are caused by either a) wind erosion, b)
H2O flow or c) lava flow. But, the atmosphere of Mars is too thin for wind erosion,
H2O is all ice, all volcanoes are inactive. Therefore, Martian canyons must be tectonic
features leftover from early epochs.
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 Mars' 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:
- The Noachian Era is the name given to the time spanning perhaps the first billion years of Mars' existence after the planet was formed 4.6 billion years ago. In this era, scientists suspect that Mars was quite active with periods of warm and wet environment, erupting volcanoes and some degree of tectonic activity. The planet may have had a thicker atmosphere to support running water, and it may have rained and snowed.
- In the Hesperian Era, which lasted for about the next 500 million to 1.5 billion years, geologic activity was slowing down and near-surface water perhaps was freezing to form surface and buried ice masses. Plunging temperatures probably caused water pooled underground to erupt when heated by impacts in catastrophic floods that surged across vast stretches of the surface -- floods so powerful that they unleashed the force of thousands of Mississippi Rivers. Eventually, water became locked up as permafrost or subsurface ice, or was partially lost into outer space.
- The Amazonian Era is the current age that began around 2 billion to 3 billion years ago. The planet is now a dry, desiccating environment with only a modest atmosphere in relation to Earth. In fact, the atmosphere is so thin that water can exist only as a solid or a gas, not as a liquid.
The following are the most recently completed Mars missions and what we have learned from them.
- Mars Pathfinder (December 1996 - March 1998): The first completed mission in NASA's Discovery Program of
low-cost, rapidly developed planetary missions with highly focused scientific goals, Mars Pathfinder far
exceeded its expectations and outlived its primary design life. This lander, which released its Sojourner
rover at the martian surface, returned 2.3 billion bits of information, including more than 17,000 images
and more than 15 chemical analyses of rocks and soil and extensive data on winds and other types of weather.
Investigations carried out by instruments on both the lander and the rover suggest that, in its past, Mars
was warm and wet, with liquid water on its surface and a thicker atmosphere. The lander and rover functioned
far beyond their planned lifetimes (30 days for the lander and 7 days for the rover), but eventually, after
about three months on the martian surface, depletion of the lander's battery and a drop in the lander's
operating temperature are thought to have ended the mission.
- Mars Global Surveyor (November 1996 - 2007): During its primary mapping mission from March 1999 through
January 2001, NASA's Mars Global Surveyor collected more information than any other previous Mars mission.
Today the orbiter continues to gather data in a second extended mission. As of May 1, 2003, it has completed
more than 20,000 orbits of Mars and returned more than 137,000 images, 671 million laser-altimeter shots and
151 million spectrometer measurements. Some of the mission's most significant findings include: evidence of
possibly recent liquid water at the martian surface; evidence for layering of rocks that points to widespread
ponds or lakes in the planet's early history; topographic evidence that most of the southern hemisphere is
higher in elevation than most of the northern hemisphere, so that any downhill flow of water and sediments
would have tended to be northward; identification of gray hematite, a mineral suggesting a wet environment
when it was formed; and extensive evidence for the role of dust in reshaping the recent martian environment.
Global Surveyor provided valuable details for evaluating the risks and attractions of potential landing sites
for the Mars Exploration Rover missions, and it served as a communications relay for the rovers as they descend
to land on Mars and afterwards. Contact with MGS was lost due to a software error in early 2007.
- Mars Odyssey (April 2001 - present): This orbiter's prime mapping mission began in March 2002. Its suite of gamma-ray spectrometer instruments has provided strong evidence for large quantities of frozen water mixed into the top layer of soil in the 20 percent of the planet near its north and south poles. By one estimate -- likely an underestimate -- the amount of water ice near the surface, if melted, would be enough water to fill Lake Michigan twice. Odyssey's infrared camera system has also provided detailed maps of minerals in rocks and soils. A layer of olivine-rich rock in one canyon near Mars' equator suggests that site has been dry for a long time, since olivine is easily weathered by liquid water. Nighttime infrared imaging by Odyssey's camera system provides information about how quickly or slowly surface features cool off after sunset, which gives an indication of where the surface is rocky and where it is dusty. Odyssey's observations have helped evaluate potential landing sites for the Mars Exploration Rovers. When the rovers reach Mars, radio relay via Odyssey will be one way they will return data to Earth.
The principal Mars' characteristcs are:
- Temperature always below the freezing point of water.
- Giant volcanoes (largest of any in the entire Solar System).
- Intense, large scale dust storms.
- Atmosphere that is much thinner than Earth or Venus, mostly CO2.
- Surface is red because of presence of iron oxide (rust).
- One Mars day is only slightly longer than an Earth day (24.6 hours).
- One Mars year is 1.88 Earth years.
- Polar caps of dry ice and water ice that change with the seasons.
- Frozen water in a permafrost layer (recently confirmed).
- About half the size of Earth.
- Two small, irregularly shaped moons—Phobos and Deimos.
- The most visited planet in our Solar System, including several recent missions, Mars Exploration Rovers Spirit and Opportunity and the Phoenix Lander.
On 19 June 2011, Mars Express pointed its high-resolution stereo camera at the Arabia Terra region of Mars, imaging the Danielson and Kalocsa craters.