Mars

**Mars** is the fourth planet from the Sun and the second-smallest terrestrial planet in the Solar System, being larger than only Mercury. In English, Mars carries the name of the Roman god of war and is often referred to as the "**Red Planet**". The latter refers to the effect of the iron oxide prevalent on Mars's surface, which gives it a reddish appearance distinctive among the astronomical bodies visible to the naked eye. Mars is a terrestrial planet with a thin atmosphere, with surface features reminiscent of the impact craters of the Moon and the valleys, deserts and polar ice caps of Earth. However, it is half the size of the Earth, with a size of 6,780 kilometres.

The days and seasons are comparable to those of Earth, because the rotational period as well as the tilt of the rotational axis relative to the ecliptic plane are similar. Mars is the site of Olympus Mons, the largest volcano and highest known mountain on any planet in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the Northern Hemisphere covers 40% of the planet and may be a giant impact feature. Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. Mars has a diameter of 6,779 kilometres, which is half of the Earth, and merely a mass of 6.410 x 10^23 kg, ten times lighter than Earth.

## Physical Characteristics

### Orbit

Mars is the fourth planet from the Sun, with a distance orbiting to the Sun of 227,940,000 kilometres. Its orbit is not parallel to the orbit of Earth, as there are considerable fluctuations between Mars and Earth. They are in closest proximity when they are in opposition. This will likely result in Sun-Earth-Mars forming a line. Mars' orbit is more elliptical than circular as there is variation in distance during opposition. During opposition at the point of greatest distance from Sun, known as aphelion, most recently on 12 February 1995, the distance between Mars and Sun is 249,830,000 kilometres (about 100 million kilometres to Earth). During the opposition at the point of shortest distance from the Sun on 7 November 2005, known as perihelion, the distance between the Sun is 197,470,000 km, about 55.8 million kilometres to Earth. As a result, Mars orbits the Sun 687 days (~2 Earth years) to complete 1 Martian year.

### Rotation

A Martian day has 24 hours and 37 minutes, as how long it takes for the planet to rotate its axis. The rotational axis is tilted 24 degrees towards the orbital plane. An axial tilt, comparable to that of Earth, is 1.7 degrees (about 1.8 degrees to ecliptic), causes seasons. During a given season, certain areas are also twice as long. It takes Mars 687 Earth days (1 Martian year) to orbit the Sun.

### Surface

The most noticeable characteristics of the Martian surface is reddish tint. This is caused by iron oxidation in the rocky sediment and dust, referring as "Red Planet" or "Dusty Planet". Mars has diverse landscapes, with cratered plains and mountains, with candidate of deserts, icy polar caps, huge faults, and volcanoes. The craters were created by meteorites, in which some of the craters are partially eroded because of the of the strong winds and dust, as these form the reddish tint in the Martian atmosphere. Sand storms are the most common storms in this planet. However, there are no continental plates that can collide or drift apart. The Martian crust consists of a single plate. This facilitated the formation of extremely high mountains or volcanoes. The highest mountain (or volcano), is Olympus Mons, it is the highest mountain in the solar system, with a length of 600 kilometres, and height of 22 kilometres. It is three times larger than Mount Everest, as a highest mountain on Earth. Near the equator, there is a huge plain, the Valles Marineris. It contains canyon-like ravines that are three times deeper and ten times long as the Grand Canyon. However, the maximum temperatures can rise above 10 °C. The ice on the south pole disappears almost entirely, because there are considerable amount of liquid water on Mars. The permafrost layers on Martian landscape is comparable to Earth tundras.

## Moons

Mars has two moons named Phobos and Deimos, named from two companions Terror and Fear in fiction. Asaph Hall discovered these moons in 1877. Their peculiar shape earned the satellites the nickname 'potato moon'. Their appearance suggests that they have been originated in the Kuiper belt and were captured by Mars. Deimos has a diameter of 12 kilometres. Its dark rock has only a few craters and its surface was smoother than Phobos, while the mean distance to Mars is 23,500 kilometres, corresponding to a 30-hour orbital period. Phobos is the largest of the two moons in Mars (also known as Martian Moons), with a diameter of 22 kilometres, as it orbits Mars in a mere 7 hours and 40 minutes. It is literally littered with craters. The largest of these, called Stickney, has a diameter of 9 kilometres. However, it is predicted that Phobos is moving closer to the surface of Mars, it will finally hit the planet 50 to 100 million years from now.

## Exploration

Dozens of crewless spacecraft, including orbiters, landers, and rovers, have been sent to Mars by the Soviet Union, the United States, Europe, India, the United Arab Emirates, and China to study the planet's surface, climate, and geology.

Once spacecraft visited the planet during NASA's Mariner missions in the 1960s and 1970s, many previous concepts of Mars were radically broken. After the results of the *Viking* life-detection experiments, the hypothesis of a hostile, dead planet was generally accepted. The data from *Mariner 9* and *Viking* allowed better maps of Mars to be made, and the *Mars Global Surveyor* mission, which launched in 1996 and operated until late 2006, produced complete, extremely detailed maps of the Martian topography, magnetic field and surface minerals. These maps are available online at websites including Google Mars. Both the *Mars Reconnaissance Orbiter* and *Mars Express* continued exploring with new instruments and supporting lander missions. NASA provides two online tools: Mars Trek, which provides visualizations of the planet using data from 50 years of exploration, and Experience Curiosity, which simulates traveling on Mars in 3-D with *Curiosity*.

As of 2021, Mars is host to fourteen functioning spacecraft. Eight are in orbit*: 2001 Mars Odyssey*, *Mars Express*, *Mars Reconnaissance Orbiter*, *MAVEN*, *Mars Orbiter Mission*, *ExoMars Trace Gas Orbiter*, the *Hope* orbiter, and the *Tianwen-1* orbiter. Another six are on the surface: the *InSight* lander, the Mars Science Laboratory *Curiosity* rover, the *Perseverance* rover, the *Ingenuity* helicopter, the *Tianwen-1* lander, and the *Zhurong* rover.

The *Rosalind Franklin* rover mission, designed to search for evidence of past life, was intended to be launched in 2018 but has been repeatedly delayed, with a launch date pushed to 2024 at the earliest. The current concept for the Mars sample-return mission would launch in 2026 and feature hardware built by NASA and ESA. Several plans for a human mission to Mars have been proposed throughout the 20th and 21st centuries, but none have come to fruition. The NASA Authorization Act of 2017 directed NASA to study the feasibility of a crewed Mars mission in the early 2030s; the resulting report eventually concluded that this would be unfeasible. In addition, in 2021, China was planning to send a crewed Mars mission in 2033.

### Astronomy on Mars

With the presence of various orbiters, landers, and rovers, it is possible to practice astronomy from Mars. Although Mars's moon Phobos appears about one-third the angular diameter of the full moon on Earth, Deimos appears more or less star-like, looking only slightly brighter than Venus does from Earth.

Various phenomena seen from Earth have also been observed from Mars, such as meteors and auroras. The apparent sizes of the moons Phobos and Deimos are sufficiently smaller than that of the Sun; thus, their partial "eclipses" of the Sun are best considered transits (see transit of Deimos and Phobos from Mars). Transits of Mercury and Venus have been observed from Mars. A transit of Earth will be seen from Mars on 10 November 2084.

## Viewing

The mean apparent magnitude of Mars is +0.71 with a standard deviation of 1.05. Because the orbit of Mars is eccentric, the magnitude at opposition from the Sun can range from about −3.0 to −1.4. The minimum brightness is magnitude +1.86 when the planet is near aphelion and in conjunction with the Sun. At its brightest, Mars (along with Jupiter) is second only to Venus in luminosity. Mars usually appears distinctly yellow, orange, or red. When farthest away from Earth, it is more than seven times farther away than when it is closest. Mars is usually close enough for particularly good viewing once or twice at 15-year or 17-year intervals. As Mars approaches opposition, it begins a period of retrograde motion, which means it will appear to move backwards in a looping curve with respect to the background stars. This retrograde motion lasts for about 72 days, and Mars reaches its peak luminosity in the middle of this interval.

The point at which Mars's geocentric longitude is 180° different from the Sun's is known as opposition, which is near the time of closest approach to Earth. The time of opposition can occur as much as 8.5 days away from the closest approach. The distance at close approach varies between about 54 and 103 million km (34 and 64 million mi) due to the planets' elliptical orbits, which causes comparable variation in angular size. The most recent Mars opposition occurred on 13 October 2020, at a distance of about 63 million km (39 million mi). The average time between the successive oppositions of Mars, its synodic period, is 780 days; but the number of days between the dates of successive oppositions can range from 764 to 812.

Mars comes into opposition from Earth every 2.1 years. The planets come into opposition near Mars' perihelion in 2003, 2018 and 2035, with 2020 and 2033 being close to perihelic opposition. Mars made its closest approach to Earth and maximum apparent brightness in nearly 60,000 years, 55,758,006 km (0.37271925 AU; 34,646,419 mi), magnitude −2.88, on 27 August 2003, at 09:51:13 UTC. This occurred when Mars was one day from opposition and about three days from its perihelion, making it particularly easy to see from Earth. The last time it came so close is estimated to have been on 12 September 57,617 BC, the next time being in 2287. This record approach was only slightly closer than other recent close approaches.

### Mars monolith

The **Mars monolith** is a rectangular object (possibly a boulder) discovered on the surface of Mars. It is located near the bottom of a cliff, from which it likely fell. The Mars Reconnaissance Orbiter took pictures of it from orbit, roughly 180 miles (300 km) away. The HiRISE camera that was used to photograph the monolith has a resolution of approximately 1 foot or 30 centimeters per pixel. Around the same time, the Phobos monolith made international news.

## See Also

* Olympus Mons
* Valles Marineris
* Phobos
* Deimos