Mars does not have a global magnetic field. Despite this, observations show that parts of the planet's crust have been magnetized. This suggests that polarity reversals have occurred in the past. This paleomagnetism is similar to the magnetic striping found on Earth's ocean floors. One theory is that these bands suggest plate tectonic activity on Mars four billion years ago, before the planetary dynamo stopped working and the planet's magnetic field faded.
A Martian day is called a "sol", and is a little longer than an Earth day. Mars rotates in 24 hours and 37 minutes. It rotates on a tilted axis, just like the Earth does, so it has four different seasons. Of all the planets in the Solar System, the seasons of Mars are the most Earth-like, due to their similar axial tilt. The lengths of the Martian seasons are almost twice those of Earth's, as Mars's greater distance from the Sun leads to the Martian year being almost two Earth years long.
Martian surface temperatures vary from lows of about (at the winter polar caps) to highs of up to (in equatorial summer). The wide range in temperatures is due mostly to the thin atmosphere which cannot store much solar heat. The planet is also 1.52 times as far from the Sun as Earth, resulting in just 43% of the amount of sunlight.
A 2015 report says Martian dark streaks on the surface were affected by water.
Liquid water cannot exist on the surface of Mars due to its low atmospheric pressure (there's not enough air to hold it in), except at the lowest elevations for short periods. The two polar ice caps appear to be made largely of frozen water. The amount of ice in the south polar ice cap, if melted, would be enough to cover the entire planet's surface 11 meters deep. A permafrost mantle stretches from the pole to latitudes of about 60°.
Geological evidence gathered by unmanned missions suggest that Mars once had much liquid water on its surface. In 2005, radar data revealed the presence of large quantities of water ice at the poles, and at mid-latitudes. The Mars rover "Spirit" sampled chemical compounds containing water molecules in March 2007. The "Phoenix" lander found water ice in shallow Martian soil in July 2008.
Landforms seen on Mars strongly suggest that liquid water at some time existed on the planet's surface. Huge areas of ground have been scraped and eroded.
Mars has two permanent polar ice caps. During a pole's winter, it lies in continuous darkness, chilling the surface and causing the deposition of 25–30% of the atmosphere into slabs of CO ice (dry ice). When the poles are again exposed to sunlight, the frozen CO sublimes (turns to vapor), creating enormous winds that sweep off the poles as fast as 400 km/h. Each season this moves large amounts of dust and water vapor, giving rise to Earth-like frost and large cirrus clouds and dust storms. Clouds of water-ice were photographed by the "Opportunity" rover in 2004.
The polar caps at both poles consist primarily of water ice.
Mars has a very thin atmosphere with barely any oxygen (it is mostly carbon dioxide). Because there is an atmosphere, however thin it is, the sky does change colour when the sun rises and sets. The dust in the Martian atmosphere makes Martian sunsets somewhat blue. Mars's atmosphere is too thin to protect Mars from meteors, which is part of the reason why Mars has so many craters.
After the formation of the planets, all experienced the "Late Heavy Bombardment". About 60% of the surface of Mars shows a record of impacts from that era. Much of the remaining surface is probably lying over the immense impact basins caused by those events. There is evidence of an enormous impact basin in the northern hemisphere of Mars, spanning , or roughly four times larger than the largest impact basin yet discovered. This theory suggests that Mars was struck by a Pluto-sized body about four billion years ago. The event is thought to be the cause of the difference between the Martian hemispheres. It made the smooth Borealis Basin that covers 40% of the planet.
Some meteorites hit Mars with so much force a few pieces of Mars went flying into space even to Earth! Rocks on Earth are sometimes found which have chemicals that are exactly like the ones in Martian rocks. These rocks also look like they fell really quickly through the atmosphere, so it is reasonable to think they came from Mars.
Mars is home to the highest known mountain in the Solar System, Olympus Mons. Olympus Mons is about 17 miles (or 27 kilometres) high. This is more than three times the height of Earth's tallest mountain, Mount Everest. It is also home to Valles Marineris, the third largest rift system (canyon) in the Solar System, 4,000 km long.
Our records of watching and recording Mars start with ancient Egyptian astronomers in the 2nd millennium BC.
Detailed observations of the location of Mars were made by Babylonian astronomers who developed methods using math to predict the future position of the planet. The ancient Greek philosophers and astronomers developed a model of the solar system with the Earth at the center ('geocentric'), instead of the sun. They used this model to explain the planet's motions. Indian and Islamic astronomers estimated the size of Mars and its distance from Earth. Similar work was done by Chinese astronomers.
In the 16th century, Nicholas Copernicus proposed a model for the Solar System in which the planets follow circular orbits about the Sun. This 'heliocentric' model was the beginning of modern astronomy. It was revised by Johannes Kepler, who gave an elliptical orbit for Mars which better fit the data from our observations.
The first observations of Mars by telescope was by Galileo Galilei in 1610. Within a century, astronomers discovered distinct albedo features (changes in brightness) on the planet, including the dark patch and polar ice caps. They were able to find the planet's day (rotation period) and axial tilt.
Better telescopes developed early in the 19th century allowed permanent Martian albedo features to be mapped in detail. The first crude map of Mars was published in 1840, followed by better maps from 1877 onward. Astronomers mistakenly thought they had detected the spectroscopic mark of water in the Martian atmosphere, and the idea of life on Mars became popular among the public.
Yellow clouds on Mars have been observed since the 1870s, which were windblown sand or dust. During the 1920s, the range of Martian surface temperature was measured; it ranged from 85 to 7 C. The planetary atmosphere was found to be arid with only traces of oxygen and water. In 1947, Gerard Kuiper showed that the thin Martian atmosphere contained extensive carbon dioxide; roughly double the quantity found in Earth's atmosphere. The first standard naming of Mars surface features was set in 1960 by the International Astronomical Union.
Since the 1960s, multiple robotic spacecraft and rovers have been sent to explore Mars from orbit and the surface. The planet has remained under observation by ground and space-based instruments across a broad range of the electromagnetic spectrum (visible light, infrared and others). The discovery of meteorites on Earth that came from Mars has allowed laboratory examination of the chemical conditions on the planet.
During the 1984 opposition, Italian astronomer Giovanni Schiaparelli used a telescope to help produce the first detailed map of Mars. What caught people's attention was that the maps had features he called "canali". These were later shown to be an optical illusion (not real). These "canali" were supposedly long straight lines on the surface of Mars to which he gave names of famous rivers on Earth. His term "canali" was popularly mistranslated in English as "canals", and thought to be made by intelligent beings.
Other astronomers thought they could see the canals too, especially the American astronomer Percival Lowell who drew maps of an artificial network of canals on Mars.