Mars

The fourth planet from the sun, Mars is the second-smallest planet in our solar system (only larger than Mercury). With a radius of 2,106 miles, it is roughly half the size of Earth. Mars is known for its distinctive red color caused by iron-rich minerals in its regolith that have oxidized or rusted. Mars is a dusty, cold, desert world with a thin atmosphere mostly of carbon dioxide, argon, nitrogen, and small amounts of oxygen and water vapor. A dynamic planet, Mars has seasons, polar ice caps, canyons, and extinct volcanoes. Evidence suggests the planet was even more active in its past. Mars has two moons named Phobos and Deimos.

Image of Mars (showing the Valles Marineris) from a mosaic of 102 Viking Orbiter images.

Mars is one of the most explored bodies in our solar system. Missions to Mars began in the 1960s, with the first successful flyby on July 14th, 1965, by NASA's Mariner 4 probe. On December 3rd, 1971, the Soviet Union's Mars 3 lander was the first spacecraft to complete a soft landing successfully on another planet, but was lost nearly immediately after. In 1976, NASA's two Viking missions reached Mars, each with an orbiter and lander. It represented the first extended exploration of Mars, with each probe operational for years and transmitting considerable data back to Earth. February 2021 saw the arrival of three separate missions to Mars from three different nations—the Perseverance rover from Nasa, the Hope orbiter from the United Arab Emirates, and China's Tianwen-1 mission. As part of the Tianwen-1 mission, the Zhurong Mars rover landed on Mars in May 2021, making China the second nation to successfully land a probe on Mars.

Mars was named by the Romans after their god of war, due to its red color. The ancient Greeks had previously named the planet Ares, also after their god of war. Other civilizations have named the planet based on its color. The Egyptians called it Her Desher, meaning "the red one," and ancient Chinese astronomers referred to it as the fire star. Mars's two moons, Phobos and Deimos, were named for the twin sons of Ares.

Mars formed with the rest of the solar system, roughly 4.6 billion years ago. However, the process by which it formed remains unclear. There are currently two leading theories:

• Core accretion
• Disk instability

The more widely accepted core accretion theory better describes the formation of the inner terrestrial rocky planets (Mercury, Venus, Earth, and Mars), but struggles to model the formation of the more distant giant planets (Jupiter, Saturn, Uranus, and Neptune). The disk instability theory of planet formation better accounts for these giant planets, but struggles to explain the inner terrestrial planets.

The core accretion theory describes the solar system starting as a large cloud of cold gas and dust, known as a solar nebula. Gravity caused this nebula to collapse in on itself and flatten into a spinning disk, and matter was drawn into the center of this disk forming the sun. Other matter formed clumps called planetesimals that combined to form planets, moons, asteroids, and comets. However, solar wind (charged particles emitted from the sun) caused the lighter elements (hydrogen and helium) to the outer regions of the solar system, leaving behind smaller rocky worlds.

Observations of exoplanets support the core accretion theory. In 2005, NASA discovered an exoplanet orbiting the sun-like star HD 149026. The exoplanet had a mass similar to Saturn, but a much small diameter. Models of the planet showed it had a solid core approximately 70 times the mass of Earth. Other theories suggest planets form through the rapid collapse of a dense cloud. However, the large rocky core of this planet suggests it could not have formed through cloud collapse, instead requiring the core to have grown first (core accretion) before acquiring gas.

Problems with the core accretion model remain. These include an explanation of how gas giants evolved fast enough to contain the significant mass of lighter gasses. Simulations cannot account for the rapid formation needed, with the process occurring over several million years. Other issues include migration, where young planets are predicted to spiral into the sun.

Disk instability is a newer theory that suggests clumps of dust and gas were bound together during the early solar system. These clumps compact into giant planets. Unlike core accretion, disk instability predicts much faster planetary formation (thousands of years), which allows them to trap the amount of lighter gas we see in current giant planets. The faster formation also leads to achieving orbit-stabilizing mass sooner, preventing newly formed planets from spiraling into the sun. If disk instability dominates how planets form, we should see a large number of giant planets.

Mars completes one rotation every 24.6 hours (24hr 37min), making a martian day (called sols, short for solar day) very similar to an Earth day (23.9 hours). Mars orbits the sun once every 687 days or 669.6 sols (martian year). The martian orbit has a semimajor axis of 1.524 astronomical units (AU), or 142 million miles, and an eccentricity of 0.0934. This eccentricity is greater than that of every other planet except Mercury and causes a significant difference between the aphelion (1.6660 AU) and perihelion (1.3814 AU) distances. Over the course of its orbit, Mars travels 9.55 AU at an average orbital speed of 14.5 miles per second.

Depiction of the four inner planets' orbits.

With respect to its orbital plane around the sun, Mars's axis of rotation (obliquity) is tilted 25 degrees, which is another similarity to Earth (23.4 degrees). This obliquity gives Mars distinct seasons. As it is further from the sun and takes longer to complete an orbit (martian year), Mars's seasons are longer in comparison with Earth. Also, compared with Earth, Mars's greater orbital eccentricity (more significant change in distance from the sun) means its seasons vary in length. The longest season is spring in the northern hemisphere (autumn in the southern), lasting 194 sols. Autumn in the northern hemisphere (spring in the southern) is the shortest, at 142 sols. The winter/summer seasons are 154 sols for the northern winter (southern summer) and 178 sols for the northern summer (southern winter).

Unlike Earth, Mars's axis of rotation changes on a shorter timescale (hundreds of thousands to millions of years). This change in tilt is caused by the influence of gravitational torques from other planets. Evidence suggests Mars's obliquity has changed significantly over its history, reaching greater than 60 degrees and lower than 10 degrees. With its current tilt of 25 degrees, ice is present in relatively modest quantities at the north and south poles. However, with dramatically different obliquity in the past, ice may have built up near the equator.

Close approach between Mars and Earth (when the two planets are nearest to each other during their orbits) occurs roughly every twenty-six months. At this time, Earth and Mars are approximately 33.9 million miles apart. As Earth and Mars have elliptical orbits (not perfectly circular) the minimum distance between them varies. Plus, the gravitational effects of other planets continually have small effects on their orbits, in particular Jupiter's gravitational effect on the orbit of Mars. In 2003, Mars made its closest approach to Earth in almost 60,000 years. Missions to Mars take advantage of the close approach, planning launch windows for the shortest trip.

Observations of Mars's orbit by Danish astronomer Tycho Brahe were key to Johannes Kepler developing his laws of planetary motion. Compared with other planets, the predicted position of Mars had the largest errors. Analyzing Brahe's data, Kepler was able to show the orbit of Mars was an ellipse.

Our understanding of the internal structure of Mars is limited to existing research. This includes measurements of the planet's rotation using probes on the surface (InSight's RISE experiment), measurements of the varying gravitational field from orbit, Mars's lack of a magnetic field, and information from martian meteorites and rocks on its surface. This information helps us understand how mass is distributed inside the planet and provides estimates for the presence and thickness of different regions, the lack of a molten metal core, and mineralogical data. Data shows the internal structure of Mars has the same three components as Earth (core, mantle, and crust).

On July 22nd, 2021, three papers were published in Science based on data from the NASA InSight lander seismometer called the Seismic Experiment for Interior Structure (SEIS). The data provided details on the depth and composition of the planet's crust, mantle, and core, proving Mars's core is molten. Up to that point, the instrument had recorded 733 distinct marsquakes with roughly 35 (all with magnitudes between 3.0 and 4.0) analyzed for the papers. Unlike earthquakes that are caused by tectonic plate movement, Mars has no tectonic plates and marsquakes are caused due to rock fractures forming in the crust due to stresses from the planet continuing to cool and therefore shrink.

Each paper focused on a different layer by analyzing the speed and shape of seismic waves inside the planet. Data shows:

• The crust is thinner than expected and may contain two or three sublayers. The two sublayers go as deep as 12 miles (20 km), and if there are three sublayers it extends to 23 miles (37 km)
• The mantle extends 969 miles (1,560 km) below the surface.
• The martian core has a radius of 1,137 miles (1,830 km)

Illustration of Mars's structure with a core, mantle, crust, and atmosphere.

Mars has a significant difference in elevation between the northern and southern hemispheres. Known as the martian dichotomy, the northern hemisphere is significantly lower in elevation. Although the cause is unknown, the leading theory is the northern hemisphere was struck by an asteroid at a low angle early in the planet's history.

Topographic map of mars.

Mars has many volcanoes, Tharsis is an area in the western hemisphere that contains the largest volcano in the solar system, Olympus Mons. Elysium in the eastern hemisphere also contains volcanos, with the largest called Elysium Mons. The surface of Mars is filled with impact craters, in particular the older southern highlands.

The ice cap in the north pole region is significantly larger than the south pole. The north polar ice cap contains spiral-shaped troughs due to climate changes. Observations from the Shallow Radar instrument (SHARAD) aboard the Mars Reconnaissance Orbiter suggest the total volume of water ice in the northern ice cap is 821,000 cubic kilometers (30% of Earth's Greenland ice sheet).

Many of the names for geographical regions on Mars are based on classical names given by the astronomer Giovanni Schiaparelli.

Mars consists of minerals containing silicon and oxygen, metals, and other elements. The surface is primarily composed of tholeiitic basalt, although parts are more silica-rich than typical basalt and are potentially more similar to andesitic rocks on Earth or silica glass. Much of the surface is covered by iron(III) oxide dust. Low albedo regions show concentrations of plagioclase feldspar, with northern low albedo regions displaying higher than usual concentrations of sheet silicates and high-silicon glass. In regions of the southern highlands, detectable amounts of high-calcium pyroxenes have been discovered. Localized concentrations of hematite and olivine have also been found.

There is no evidence of Mars currently having a structured global magnetic field. However, evidence shows parts of the planet's crust have been magnetized and that polarity reversals of its dipole field have alternated in the past. The paleomagnetism of magnetically susceptible minerals shows properties similar to the alternating bands found on the ocean floors of Earth. A theory suggests these bands were caused by plate tectonics on Mars in the distant past, before the planet’s magnetic field faded away.

According to ESA, the martian atmosphere is composed of the following:

• 95.32% carbon dioxide
• 2.7% nitrogen
• 1.6% argon
• 0.13% oxygen

The martian atmosphere also contains a significant amount of dust that colors photos taken from the surface. The atmospheric pressure on Mars's surface is 6.35 mbar (less than 1% of Earth). Mars has seasonal variations in pressure due to the atmospheric changes in the level of carbon dioxide. The cold southern polar winter draws carbon dioxide from the atmosphere as it freezes onto the polar cap. The highest pressures occur in the southern hemisphere summer months, with the lowest pressures in the northern winter months.

With the lack of oceans, atmospheric circulation on Mars is simpler than on Earth. Hadley cell motion dominates at low latitudes with warmer air around the equator flowing northward to roughly 30° latitude where it cools, sinks, and flows back toward the equator at the surface. This flow is not directly north-south due to Mars's rotation and surface landforms that direct the flow locally. In the northern hemisphere, the prevailing direction of surface winds is from the northeast. At higher latitudes, polar air masses dominate with a series of high and low-pressure areas sweeping around the planet from west to east. Sharp weather fronts and storms occur when these areas interact with Hadley cell movements. Due to the lower pressure atmosphere, these storms tend to be less violent than storms on Earth.

The martian atmosphere does not have an ozone layer and ultraviolet radiation harmful to organic compounds reaches the surface unhindered. Mars has clouds of both water ice and carbon dioxide ice particles. These typically form near the large volcanoes as winds rise over them causing condensation. During winter clouds also form over the polar regions. The Phoenix spacecraft that landed in the north pole region of Mars (68^o latitude) detected carbon dioxide snow falling. The snow sublimated before reaching the ground. Other missions have detected water ice frost on cold mornings, that quickly disappears after sunrise.

Mars is the only rocky terrestrial planet with more than one moon. The two bodies orbiting Mars are among the smallest moons in the solar system. They were first discovered by American astronomer Asaph Hall in 1877, within six days of each other. Hall named the moons for the mythological sons of Ares, the Greek counterpart of the Roman god of war, Mars. Phobos means fear, and Deimos means dread. The closely orbiting moons had previously been hidden in the glare of the planet.

Phobos is the closer and larger moon, compared with Deimos. Both are made of material that resembles type I or II carbonaceous chondrites, the same substance asteroids are made of, mixed with ice. The moons also have non-spherical, lumpy, elongated shapes that make them look more like asteroids than moons. They are heavily cratered, covered in dust and loose rocks, and are among the darker objects in the solar system. Phobos and Deimos always present the same face to Mars, like Earth's moon.

Mars's moons have been discussed as possible bases for missions to Mars. They could potentially allow astronauts to observe Mars and launch robots to the surface while protected from cosmic rays and solar radiation for two-thirds of every orbit.

Due to their composition and shape, it was thought Phobos and Deimos were captured asteroids. The original theory suggested the two objects were pushed out of the asteroid belt by Jupiter's gravity and began orbiting Mars. However, the stable, almost circular orbit of the two moons contradicts this theory. Other theories suggest the moons formed from leftover debris during the creation of Mars, or they were the result of a major collision with Mars early in the solar system.

No missions have successfully explored Phobos or Deimos as the primary objective. However multiple spacecraft have performed flybys, the first being NASA's Mariner 9 probe in 1971. The following are other probes orbiting Mars that have performed long-range observations:

• NASA's Viking orbiters
• Soviet Phobos 2 mission
• NASA's Mars Global Surveyor
• ESA and NASA's Mars Express mission
• NASA's Maven

Russia attempted to send a mission to Phobos. However, the mission (called Phobos-Grunt) became stuck in terrestrial orbit and fell back to Earth.

Photo of Phobos taken by the HiRise camera on NASA's Mars Reconnaissance Orbiter.

Phobos orbits 3,700 miles above the surface of Mars. Phobos's orbit is closer to Mars than any other known moon's orbit is to its planet. The larger of the two moons, Phobos is 17 x 14 x 11 miles (27 x 22 x 18 km) in diameter. It completes its trip around Mars once every 7 hours and 39 minutes with an equatorial orbit that is almost circular. Phobos's orbital period is three times faster than the martian rotation period (three orbits per sol), giving the unusual result of the moon rising in the west and setting in the east as seen from Mars. Its orbit is also so close to the surface, the curvature of the planet obscures its view when standing in the polar regions. Phobos is slowly spiraling into Mars, getting roughly six feet (1.8 m) closer each century. Within 50 million years, Phobos will crash into Mars or break apart to form a ring around it.

The most prominent feature on Phobos is a 6-mile (9.7 km) crater called Stickney. The impact caused streak patterns across the Phobos's surface, and the Mars Global Survey has shown Stickney to be filled with fine dust with evidence of boulders falling down its sloped surface. Phobos has no atmosphere. Temperature measurements show large variations between the sunny and dark sides of the moon due to the fine dust on Phobos's surface, which cannot retain heat. With high temperatures hitting 25 degrees Fahrenheit (-4 degrees Celsius) and low temperatures of -170 degrees Fahrenheit (-112 degrees Celsius).

In 1988, Soviet spacecraft Phobos 2 detected outgassing from Phobos. However, it could not determine its nature due to a malfunction, during which a planned lander was also lost.

Color-enhanced image of Deimos taken by HiRISE camera on NASA's Mars Reconnaissance Orbiter.

The smaller moon Deimos is 9 x 7 x 6.8 miles in size (15 x 12 x 11 km). It orbits Mars every 30 hours at a distance of 12,470 miles (20,069 km). Similar to Phobos, Deimos is a small, lumpy, and heavily cratered object. Deimos's craters are generally smaller than 1.6 miles (2.5 km) in diameter, and the moon lacks the grooves and ridges seen on Phobos. Meteorite impacts throw material up and out of the resulting crater. Typically this material falls back to the surface around the crater. However, these deposits are not seen on Deimos. Possibly due to its low gravity, ejecta deposits instead escape into space. Material is not seen to be moving down slopes on Deimos. The moon has a thick regolith, perhaps as deep as 328 feet (100 m). Unlike Phobos, Deimos is slowly drifting away from Mars.

Missions to Mars have been launched regularly since the 1960s. Roughly half of all missions have failed, although the percentage has improved in recent years. As of June 2021, there are three rovers and one lander in operation on Mars:

• Curiosity Rover, part of NASA's Mars Science Laboratory mission
• NASA's InSight (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport) Lander
• Perseverance Rover, part of NASA's Mars 2020 mission
• Zhurong rover, part of CNSA (China National Space Administration) Tianwen-1 mission

The first missions to Mars were launched by the Soviet Union in the 1960s. However, the Korabl, Zond, and early Mars probes from the Soviet Union failed due to a range of issues, including failing to reach Earth orbit, communication errors, spacecraft breaking apart, and launch vehicle failures.

The first successful missions to Mars were part of NASA's Mariner program. As part of the program, NASA designed and built ten spacecraft at the Jet Propulsion Laboratory (JPL) to explore the inner solar system (Venus, Mars, and Mercury). All were relatively small spacecraft weighing less than half a ton (without onboard rocket propellant) and launched on an Atlas rocket with either an Agena or Centaur upper-stage booster.

Depiction of a Mariner probe.

Although Mariner 3 failed due to the shroud encasing the spacecraft atop its rocket failing to open properly, Mariner 4 performed the first successful flyby of Mars on July 14th, 1965, returning close-up photos of another planet for the first time. Mariner 4 survived much longer than the eight months it took to reach Mars, lasting roughly three years in solar orbit, providing data on solar wind and making coordinated measurements with sister probe Mariner 5, which launched to Venus in 1967.

Mariner 6 and 7 were the first dual mission to Mars, flying over the equator and south polar region in 1969. The probes analyzed the atmosphere and surface with remote sensors and returned hundreds of images. These images showed the dark features were not canals as previously interpreted in the 1800s.

Mariner 8 failed during launch while Mariner 9 became the first artificial satellite of Mars, successfully orbiting the planet for almost a year. Launched on May 30th, 1971, the rocket propellant needed to control the spacecraft into martian orbit nearly doubled the launch mass. The probe achieved orbit in November 1971, and its final transmission was on October 27th, 1972. Upon arrival, Mariner 9 observed a dust storm obscuring the entire planet for a month. Once the dust from the storm settled, the probe began compiling a mosaic of high-quality images covering 100% of the martian surface. These revealed Mars's giant volcanoes and a grand canyon stretching 3,000 miles across its surface. They also showed a dry and dusty planet with relics of ancient riverbeds carved through the landscape. The probe also provided the first close-up pictures of both martian moons.

The USSR's Mars 3 mission in 1971 consisted of an orbiter that returned eight months of data and a lander that became the first probe to safely land on Mars. However, it only returned data for 20 seconds until becoming unresponsive. The Mars 5 mission in 1973 returned 60 images over nine days of operation. The Mars 6 orbiter/lander, also launched in 1973, produced data from the occultation experiment. The lander failed during its descent to Mars.

NASA's two Viking missions became the first US probes to land safely on the surface of Mars. The two identical spacecraft each consisted of a lander and an orbiter. Viking 1 launched on August 20th, 1975, and Viking 2 launched on September 9th, 1975. The spacecraft took almost a year in transit with Viking entering Mars orbit on June 19th, 1976, and Viking 2 on August 7th, 1976.

After studying photos from orbit, the original landing site for Viking 1 was deemed unsafe and a new location nearby was chosen. Viking 1 landed on Mars on July 20th, 1976 on the western slope of Chryse Planitia at 22.3 degrees north latitude, 48.0 degrees longitude. The site certification team also decided Viking 2's planned landing site was unsafe after examining high-resolution photos from orbit. A new site was chosen and Viking 2 landed on Mars on September 3rd, 1976, at Utopia Planitia, at 47.7 degrees north latitude and 48.0 degrees longitude.

Full-scale (10 x 10 x 6ft) proof-test model of the Viking landers.

While the Viking mission was planned to continue for ninety days after landing, it lasted considerably longer and became the first mission to observe Mars for an extended period of time. The first spacecraft to cease operations was the Viking 2 Orbiter on July 25th, 1978, when it used all the gas in its attitude-control system keeping the solar panels pointed at the sun. NASA engineers were able to maintain attitude-control gas on the Viking 1 orbiter until August 7th, 1980 with reduced operations for its final two years. Lander 2 sent its last data on April 11th, 1980 with Lander 1 making its final transmission to Earth on November 11th, 1982.

As well as taking photographs and collecting other data on the martian surface, the two Viking landers performed three biology experiments looking for possible signs of life. While the experiments discovered some unexpected chemical activity in the martian soil, they found no clear evidence of living microorganisms in the soil near the landing sites. The lander data suggests the high solar ultraviolet radiation environment combined with the dryness and oxidizing nature of the soil chemistry prevents the formation of living organisms.

Depiction of Mars Pathfinder's airbag landing.

Launched on December 4th, 1996, Mars Pathfinder was the first-ever robotic rover on Mars. Landing at Ares Vallis on July 4th, 1997, the mission was designed as a technology demonstration showing a new way to deliver an instrumented lander. However, Mars Pathfinder outlived its primary goal and delivered a significant amount of data until the mission ended on September 27th, 1997. The mission consisted of a lander and a 23-pound (10.6 kg) rover, named Sojourner, carrying instruments for scientific observations and to provide engineering data for the development of new technologies. The Pathfinder mission landed on Mars using a parachute to slow descent in the thin atmosphere and a large system of airbags.

1997 Image of Sojourner using its APXS instrument.

The science instruments included the following:

• Alpha Proton X-ray Spectrometer (APXS), determining the elemental composition of rocks and soil
• three cameras, providing images of the terrain for geological studies and documenting the performance and operating environment for Pathfinder technologies
• atmospheric Structure Instrument/Meteorology Package, performing measurements on the martian atmosphere during descent and at the lander

Key discoveries from Mars Pathfinder include:

• observations of rounded pebbles and cobbles, suggesting conglomerates that formed in running water during a warmer past with liquid water
• radio-tracking precisely measuring Mars's pole of rotation, suggesting the planet has a metallic core between 800 miles (1,300 km) and 1,250 miles (2,000 km)
• magnetic airborne dust, potentially the magnetic mineral maghemite
• observations of dust devils and water ice clouds in the early morning

NASA's Mars Pathfinder & Sojourner Rover (360 View)

Launched before Pathfinder, on November 7th, 1996, the Mars Global Surveyor entered orbit on September 12th, 1997. The probe spent a year and a half trimming its orbit from a looping ellipse to a circular path before starting its mapping mission in March 1999, observing Mars from a low-altitude nearly polar orbit until the end of its mission in November 2006. The orbiter has studied the entire surface of Mars as well as its atmosphere and interior. During its mission, Mars Global Surveyor has observed the following:

• repeatable martian weather patterns, including dust storms in the same location for a few weeks every year
• high-resolution images showing gullies and debris that suggest occasional sources of liquid water similar to an aquifer.
• Mars's lack of global magnetic field
• temperature and close-up images of Phobos
• changes in radio transmissions providing a vertical profile of atmospheric temperature and pressure.
• gravitational effects on spacecraft acceleration as a result of the internal structure of Mars

Launched on April 7th, 2001, NASA's 2001 Mars Odyssey mission is still operational and is the longest-lasting Mars spacecraft. Odyssey arrived at Mars and entered orbit on October 24th, 2001. Its primary mission was to make the first global map of the chemical elements and minerals (makeup and distribution) that make up the martian surface. This mission began in February 2002 and was completed in August 2004. The mission identified regions with buried water ice measured the surface temperature and topography and measured the radiation in the low-Mars orbit to be twice that of the low-Earth orbit. The orbiter has gone on to become a communication relay for later rovers and landers including the Mars exploration rovers, the Phoenix lander, and the Curiosity rover, as well as helping to identify potential landing sites for future missions.

A joint mission between NASA and the European Space Agency (ESA), Mars Express launched on June 2nd, 2003, arriving at Mars on December 25th, 2003. The spacecraft has been in operation analyzing the atmosphere and surface of Mars in polar orbit since. The mission also carried a small lander called Beagle 2, which was lost on arrival in December 2003. The main objective of the mission is to search for subsurface water from orbit. The orbiter has seven scientific instruments investigating the geology, atmosphere, surface, history of water, and potential for life on Mars. The science payload was derived from the European instruments lost on the failed Russian Mars '96 mission.

The MER mission consisted of two rovers, Spirit and Opportunity. Launched in the summer of 2003, they landed on opposite sides of Mars in January 2004. The two rovers had identical scientific instruments and far greater mobility compared with the previous Pathfinder rover. The mission's primary scientific goal was to search for and characterize a wide range of rocks and soils for information related to past water activity on Mars. Both rovers far exceeded the planned 90-day mission lifetime, continuing to explore Mars and return valuable scientific data for many years.

Depiction of the MER rovers.

The a variety of scientific instruments were utilized for the MER mission:

• panoramic camera
• miniature thermal emission spectrometer
• Moessbauer spectrometer
• alpha particle x-ray spectrometer
• microscopic imager

These instruments were supplemented by rock abrasion tools for removing weathered surfaces of rocks as well as engineering tools for use in the operation of the rover.

Each rover that landed on the surface of Mars was protected by airbags. After rolling, the airbags deflated and the landing craft opened. The rovers began by taking panoramic images to give scientists information to select promising geological targets that the rovers then drove to. Both rovers have sent hundreds of thousands of high-resolution, full-color images of martian terrain as well as detailed microscopic images of rocks and soil surfaces. Data from the rovers have allowed scientists to reconstruct the ancient past of Mars when it had liquid water. Both Spirit and Opportunity found evidence for past wet conditions that possibly could have supported microbial life.

Spirit landed inside Gusev Crater, a possible former lake in a giant impact crater on January 4, 2004.

Spirit's landing site inside Gusev crater, taken with the rover's panoramic camera before leaving the lander.

Spirit's initial travels in Gusev Crater revealed a more basaltic setting. Upon reaching the "Columbia Hills," the rover found a variety of rocks suggesting early Mars was characterized by impacts, explosive volcanism, and subsurface water. Unusual-looking bright patches of soil were found to be extremely salty and affected by past water. At "Home Plate," a circular feature in the "Inner Basin" of the "Columbia Hills," Spirit discovered finely layered rocks. Spirit lasted twenty times longer than its original design, and its final communication to Earth was on March 22, 2010.

Opportunity landed at Meridiani Planum, a place with mineral deposits suggesting Mars had a wet history, on January 25, 2004.

Opportunity's landing site inside "Eagle Crater," taken from the rover leaving the crater showing the empty lander.

Studying "Eagle" and "Endurance" craters revealed evidence for past inter-dune playa lakes that evaporated to form sulfate-rich sands. The sands were reworked by water and wind, solidified into rock, and soaked by groundwater. Opportunity continues to operate until February 13th, 2019. In 2015, Opportunity broke the record for extraterrestrial travel, rolling greater than the distance of a 26-mile (42-kilometer) marathon.

Launched in August 2005, the Mars Reconnaissance Orbiter reached Mars on March 10th, 2006. The mission contained cameras far exceeding previous missions for exploring the martian surface from orbit to provide detailed views of its geology and structure, as well as help identify obstacles that could jeopardize the safety of future landers and rovers. The orbiter has a range of instruments to find subsurface water, identify surface minerals, and study how dust and water are transported in the martian atmosphere. The spacecraft provides important information for selecting landing sites for future missions and is used by several other spacecraft as the first link in a communication bridge back to Earth.

Mars Phoenix was the first mission chosen as part of NASA's Scout program (an initiative for smaller, lower-cost spacecraft). The Phoenix lander launched on August 4th, 2008, and landed at Vastitas Borealis, the arctic plains of Mars, on May 25th, 2008. The mission used a lander originally intended for use by the canceled 2001 Mars Surveyor lander and carried a suite of instruments based on those that flew on the Mars Polar Lander that launched in 1999 and was lost on arrival. Phoenix landed further north than any previous mission and looked for water on Mars in the polar regions. The mission ended on November 2nd, 2008; during its three months of operations, it dug into an ice-rich layer near the surface, checking oil and ice samples for evidence about whether the site could be hospitable to life.

Depiction of MSL landing system known as the "sky crane".

NASA's MSL delivered the Curiosity rover to Mars using an entirely new landing system. Launched on November 26th, 2011, the rover touched down in Gale Crater on Mars at 10:32 pm PDT on August 5th, 2012 (1:32 am EDT on August 6th, 2012). Curiosity remains operational. The landing system utilized a parachute to slow descent before a rocket system allowed the mission to hover above the surface while a tether lowered the rover to the surface. The rover touched down on its wheels, the tether was cut, and the landing system flew away to crash land a safe distance from the rover.

The landing site (named Bradbury landing) is near the foot of Aeolis Mons ("Mount Sharp"), a layered mountain in Gale crater. Over one hundred scientists participated in open workshops to select the landing site, and Gale crater was chosen out of more than thirty locations considered. The process used data from the Mars Reconnaissance Orbiter as well as earlier orbiters and took advantage of the greater precision offered by the new landing system. MSL's target landing area was only 12 miles long (20 km), roughly a five times improvement on earlier missions. The greater landing precision of MSL made it possible to land at the site close to the crater wall and Mount Sharp.

Curiosity is roughly twice as long (10 feet or 3 m) and five times as heavy as the previous Mars Exploration Rovers. It inherited many design features from Spirit and Opportunity, including six-wheel drive, a rocker-bogie suspension system, and cameras mounted on a mast so the mission team on Earth can select target locations and routes. The Curiosity rover also carries equipment to gather and process samples of rock and soil, distributing them around the onboard instruments. The drive system was engineered to roll over obstacles up to 25 inches (65 cm) and travel up to 660 feet (220 m) per day on martian terrain. The mission is powered by the multi-mission radioisotope thermoelectric generator (MMRTG) from the US Department of Energy. The generator produced 110 watts of electrical power at launch.

Selfie taken by the Curiosity rover on October 11th, 2019 (Sol 2553 of its mission) in a location known as "Glen Etive."

Curiosity's science payload includes the following:

• CheMin, an X-ray diffraction and fluorescence instrument to identify and quantify minerals in rocks and soils as well as measure bulk composition.
• Mars Hand Lens Imager, mounted on the arm to take extreme close-up images of rocks and soils.
• Alpha Particle X-ray Spectrometer, to measure the relative abundances of elements in rocks and soils.
• Mast Camera, to image the rover's surroundings in high-resolution stereo and color.
• ChemCam, which uses laser pulses to vaporize thin layers of material from rocks or soil targets up to 23 feet away (7 m). The instrument includes both a spectrometer to identify atoms excited by the laser and a telescope to capture detailed images of the illuminated area.
• Radiation Assessment Detector, to characterize the radiation environment at the martian surface.
• Mars Descent Imager, to provide a high-definition video used during the landing. Now used for surface imaging as the rover explores.
• Rover Environmental Monitoring Station, to measure atmospheric pressure, temperature, humidity, winds, and UV radiation levels. The instrument was provided by Spain's Ministry of Education and Science.
• Dynamic Albedo of Neutron, designed to measure the subsurface hydrogen (up to 3 feet or 1 m) that may indicate the presence of water in minerals. The instrument was provided by Russia's Federal Space Agency.

Curiosity's goal was to answer: "Did Mars ever have the right environmental conditions to support small life forms called microbes?" Early in its mission, it found chemical and mineral evidence of past habitable environments. Key findings from MSL include:

• Evidence of persistent liquid water in Mars's past. Shortly after landing Curiosity found smooth, rounded pebbles. Upon arriving at Mount Sharp, the rover found 1,000 feet of rock that formed originally as mud at the bottom of a lake.
• Ancient Mars had the right chemistry to support living microbes with sulfur, nitrogen, oxygen, phosphorus, and carbon found in powder samples drilled from the "Sheepbed" mudstone at Yellowknife Bay.
• Organic carbon present in martian rocks.
• There is a seasonally varying background level of atmospheric methane present in Mars's atmosphere.
• Characterization of the radiation environment on the surface of the planet, that could serious health risks for future human missions.
• The presence of heavier hydrogen, carbon, and argon isotopes indicating Mars has lost much of its original atmosphere.

The second mission chosen for NASA's Mars Scout program, the MAVEN orbiter was designed to obtain measurements of the martian atmosphere and understand the planet's history of climate change. The mission launched on November 18th, 2013, and entered martian orbit on September 22nd, 2014.

Depiction of the Mars Atmospheric and Volatile EvolutioN (MAVEN) orbiter.

MAVEN is trying to understand how Mars's atmospheric gases are being lost to space to infer what happened in the past. It is the first mission to make direct measurements of the martian atmosphere using eight science instruments. The spacecraft can alter its altitude to cover the entire upper atmosphere, dropping to an altitude of only 80 miles above the martian surface. The orbiter also provides communication relay support for NASA's landers and rovers on the surface.

Designed by the Indian Space Research Organisation (ISRO), MOM was India's first interplanetary space mission. The orbiter was designed to observe surface features, morphology, mineralogy, and Mars's atmosphere (specifically to search for methane). The mission launched on November 5th, 2013, entering martian orbit on September 24th, 2014, days after MAVEN. The MOM spacecraft is orbiting Mars with a highly elliptical path that has a periapsis of roughly 300 km and an apoapsis of roughly 71,000 km. Its inclination in relation to Mars's equatorial plane is 150 degrees, and the spacecraft takes roughly 73hrs for a single orbit. The mission is still operational and contains the following instruments:

• Mars Colour Camera (MCC)
• Thermal Infrared Imaging Spectrometer (TIS)
• Methane Sensor for Mars (MSM)
• Lyman Alpha Photometer (LAP)
• Mars Exospheric Neutral Composition Analyser (MENCA)

ESA's ExoMars program contains a series of missions to understand if life ever existed on Mars. The first mission launched on March 14th, 2016, was the Trace Gas Orbiter (TGO), a partnership between ESA and Roscosmos (Russia's Federal Space Agency) and the Schiaparelli Landing Demonstration Module. Schiaparelli was released from the orbiter on October 16th, 2016, and entered the martian atmosphere at 14:42 UTC on October 19th, 2016. However, ESA's mission team lost contact shortly before the expected touchdown. On the same day (October 19th), orbit insertion of the TGO occurred.

Still in operation, the TGO studies the martian atmosphere for the presence of methane and other gases potentially present in small concentrations. From its 400 km altitude orbit, the TGO's instruments monitor seasonal changes in the atmosphere’s composition and temperature to create and refine detailed atmospheric models of Mars. Its instruments are also mapping the subsurface hydrogen to a depth of a meter.

Launched on May 5th, 2018, NASA's InSight (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport) mission placed a single geophysical lander on Mars to study the interior of the planet. The mission aims to understand the processes behind the formation of the rocky inner planets of the solar system. The lander touched down on Mars at Elysium Planitia on November 26th, 2018.

Depiction of the InSight lander after it has deployed its instruments on the martian surface.

InSight's science payload includes two instruments:

• Seismic Experiment for Interior Structure (SEIS), provided by the French space agency (CNES) with the participation of the Paris Institute of Earth Physics (IPGP), the Swiss Federal Institute of Technology (ETH), the Max Planck Institute for Solar System Research (MPS), Imperial College London, and the Jet Propulsion Laboratory (JPL).
• Heat Flow and Physical Properties Package (HP3), provided by the German space agency (DLR).

Additionally, the Rotation and Interior Structure Experiment (RISE), led by JPL, is using the spacecraft communication system to provide precise measurements of planetary rotation. The instruments are housed in a spacecraft based on the previous Phoenix Lander design, built by Lockheed Martin Space and providing low-cost, low-risk access to the surface of Mars.

In July 2021, three papers published data from InSight's SEIS instrument. Each paper focused on a different layer of the planet (core, mantle, and core). The papers include measurements of the depth and composition of the planet's layers and shows that Mars's core is molten.

Image of the Hope Probe with some key parameters.

The Emirates Mars Mission is the first interplanetary satellite developed by the United Arab Emirates (UAE). The mission containing the Hope spacecraft launched on July 19th, 2020, aboard a Japanese launch vehicle, entering Mars's orbit on February 9th, 2021. The Hope probe aims to answer key questions about the martian atmosphere, with three objectives:

1. Understand climate dynamics and the global weather map by characterizing the lower atmosphere of Mars.
2. Learn how the weather changes and study the escape of Hydrogen and Oxygen by comparing the lower atmosphere conditions with the upper atmosphere.
3. Measure the structure and variability of hydrogen and oxygen in the upper atmosphere and identify why Mars is losing them into space.

The probe is equipped with three instruments:

• The Emirates Exploration Imager (EXI), a multiband camera capable of 12-megapixel images with a spatial resolution lower than 5 miles
• The Emirates Mars Infrared Spectrometer (EMIRS), designed to measure the dust, ice clouds, water vapor, and temperature profile of the martian atmosphere. Designed in collaboration with Arizona State University.
• The Emirates Mars Ultraviolet Spectrometer (EMUS), to measure changes in the thermosphere, the structure of hydrogen exospheres, and ultraviolet emissions of hydrogen, oxygen, and carbon monoxide

China's first mission to Mars, Tianwen-1, consists of an orbiter and rover named Zhurong. The mission launched on July 23rd, 2020, and entered orbit on February 10th, 2021. On May 14th, 2021, the Zhurong successfully landed on Utopia Planitia, a vast plain where NASA's Viking 2 lander had previously landed in the 1970s. Before Zhurong, only NASA had successfully landed and operated spacecraft on the surface of Mars. Tianwen-1 aims to investigate pockets of water beneath the martian surface and lay the groundwork for a possible sample return mission planned for the end of the 2020s.

The scientific objectives of Tianwen-1 are:

1. To study the characteristics of the martian topography and geological structure.
2. To study the characteristic of the soil on the martian surface and the distribution of water ice.
3. To investigate the substance composition of the martian surface.
4. To study the ionosphere, surface climate, and environmental characteristics of Mars.
5. To study the martian physical fields (electromagnetic, gravitational) and internal structure.

Picture of the Zhurong rover next to its landing platform on Mars, the image was taken from a wireless camera dropped off by the rover.

The mission's scientific payload consists of seven instruments:

• Moderate Resolution Imaging Camera (MoRIC)—a wide field-of-view camera that can image the Mars surface from altitudes of 800 km to 265 km and collect data to study Mars topography, geomorphology, and geological structure.
• High-Resolution Imaging Camera (HiRIC), a TDI-CCD operating in a linear sweep mode to obtain high-resolution optical images of Mars and support studies of topography, geomorphology, and geological structure.
• Mars Orbiter Scientific Investigation Radar (MOSIR), uses electromagnetic pulses emitted by the radar and reflected to the receiver after penetrating the target. The echos provide information on both the Mars surface and subsurface, potentially detailing the depth and spatial distribution of the underground water ice.
• Mars Mineralogical Spectrometer (MMS), to analyze the mineral composition and distribution, study Mars's chemical composition and its evolution, and analyze Mars's resources and distribution.
• Mars Orbiter Magnetometer (MOMAG), fluxgate magnetometer to measure the magnetic field and study the interaction mechanism between the martian ionosphere, the magneto-sheath, and the solar wind.
• Mars Ion and Neutral Particle Analyzer (MINPA), to detect low energy ions and neutral particles in the space plasma environment of Mars and understand the mechanisms behind Mars's atmosphere escaping.
• Mars Energetic Particles Analyzer (MEPA), to obtain measurements of the electron, proton, alpha-particle, and heavy ions environment.

NASA's Mars 2020 mission delivered the Perseverance rover as well as the Ingenuity helicopter to the red planet. Launched on July 30th, 2020, the mission landed on Mars at Jezero Crater on February 18th, 2021. The mission's main task is to seek signs of ancient life and collect samples of rock and soil for possible future return missions to Earth. The Perseverance rover’s body and other major hardware (such as the cruise stage, descent stage, and aeroshell/heat shield) build upon the success of NASA's previous Curiosity rover with many heritage components. As with Curiosity, Perseverance is powered by a multi-mission radioisotope thermoelectric generator (MMRTG) from the US Department of Energy. The car-sized Perseverance rover has almost the same dimensions as Curiosity: 10 feet long (not including the arm), 9 feet wide, and 7 feet tall (about 3 meters long, 2.7 meters wide, and 2.2 meters tall). At 2,260 pounds (1,025 kilograms), Perseverance is roughly 278 pounds (126 kilograms) heavier than Curiosity.

Photo taken by the Perseverance rover in Jezero Crater.

The rover incorporates a drill that can collect core samples of promising rocks and soils, setting them aside in a "cache" on the surface of Mars where a future mission may potentially return the samples to Earth. Perseverance has seven instruments:

• Mastcam-Z, camera system to determine mineralogy of the surface and assist in rover operations.
• SuperCam, providing imaging, chemical composition analysis, and mineralogy from a distance.
• Planetary Instrument for X-ray Lithochemistry (PIXL), an X-ray fluorescence spectrometer and high-resolution imager to map the fine-scale elemental composition of martian surface materials.
• Scanning Habitable Environments with Raman & Luminescence for Organics and Chemicals (SHERLOC), the first UV Raman spectrometer to sly to the surface of Mars and provide fine scale-imaging to map mineralogy and organic compounds.
• The Mars Oxygen In-Situ Resource Utilization Experiment (MOXIE), technology demonstration to investigate the viability of producing oxygen from the martian atmospheric carbon dioxide.
• Mars Environmental Dynamics Analyzer (MEDA), a series of sensors providing measurements of temperature, wind speed/direction, pressure, relative humidity, and dust size/shape.
• The Radar Imager for Mars’s Subsurface Experiment (RIMFAX), a ground-penetrating radar instrument providing centimeter-scale resolution of geologic structures in the subsurface.

The Ingenuity helicopter is designed as a technology demonstration to test the first powered flight on Mars. Once the rover found a suitable "airfield" location, it released Ingenuity to the surface to perform a series of test flights over a 30-martian-day experimental window. After three successful flights, the helicopter completed its technology demonstration. The first flight was on April 19, 2021, where Ingenuity took off, climbed to about 10 feet (3 meters) above the ground, hovered in the air briefly, completed a turn, and then landed. This was the first powered, controlled flight in the thin martian atmosphere and the first flight in any world beyond Earth. The helicopter has completed its technology demonstration phase performing additional experimental flights of incrementally farther distances and greater altitudes. Ingenuity is performing a new operations demonstration phase, exploring how future rovers and aerial explorers could work together.

Depiction of the Ingenuity Mars helicopter.

There are a number of planned missions to Mars. These include:

• ExoMars Rosalind-Franklin Rover—the second part of ESA's ExoMars program in collaboration with Roscosmos had a planned launch in September 2022. However, ESA formally halted launch plans following Russia's invasion of Ukraine. The mission planned to use a Russian launch vehicle, landing platform, radioisotope heating units, and Russian contributions to the science payload. Among the unique features of the rover was the ExoMars drill unit to collect samples up to 2 meters below the surface, where the soil is protected from the high martian radiation environment.
• Tera-hertz Explorer (TEREX)—a joint venture between Japan’s National Institute of Information and Communications Technology (NICT) and the University of Tokyo Intelligent Space Systems Laboratory (ISSL), the TEREX mission will send an orbiter and lander carrying a terahertz sensor to Mars to measure oxygen isotope ratios in the atmosphere. The lander (TEREX-1) is planned for launch in 2022, with the orbiter (TEREX-2) launching in the following 2024 window.
• Mars Orbiter Mission 2 (MOM 2)—the Indian Space Research Organization’s follow-up to MOM, the new mission has so far announced an orbiter as the main component of the mission, with a lander and rover as potential additions. Although there has been no official launch date announcement, it is expected to be around 2024.
• Martian Moons Exploration (MMX)—planned Japanese mission to Phobos in 2025. The mission intends to land on Phobos and collect samples, make observations of Deimos and Mar's during flybys then return the probe to Earth with samples in 2029.
• Mars Ice Mapper—collaboration between NASA, Japanese space agency (JAXA), Canadian space agency, and Italian space agency to send an orbiter to map water ice resources on Mars using two types of radar.
• Mars Sample Return—NASA has plans for a sample return mission to bring martian rocks and soil back to Earth for unprecedented analysis. The long-term goal is currently under development in collaboration with various centers and ESA. The mission is intended to continue the work of the Mars 2020 Perseverance rover, which has been stashing samples of particular interest. The mission's current plan includes a lander arriving at Mars in 2028 near the Perseverance rover in Jezero crater and a separate orbiting spacecraft that would rendezvous with the sample return container in orbit before propelling them back to Earth.

China's space program has plans to return to Mars in 2030 in order to return samples from the Tianwen-1 mission. Russia has stated its intent for a crewed mission to Mars between 2040 and 2040. Private companies such as SpaceX have also shown interest in sending missions to Mars.

Astrobiology combines disciplines (astronomy, biology, geology, physics, etc.) to study the potential for life beyond Earth. Given its extensive exploration compared with other bodies, Mars has become a primary area of interest for astrobiologists.

Liquid water is fundamental to life on Earth and has become an important area of study on Mars. The Pathfinder and MER missions were given the objective to "follow the water." While water is present on Mars, most is ice found in the planet's cold polar regions. Water ice is not found on the surface near the equator as it is not cold enough for exposed water ice to be stable.

Evidence suggests Mars once had significant amounts of flowing water with surface features such as riverbeds, deltas, lake basins, and inland seas. Models suggest the water was lost to the atmosphere or is trapped within minerals in the crust. Missions to Mars have detected evidence of water below the surface of the planet.

Another important factor preventing life on Mars is the high radiation environment on the surface of the planet. Mars lacks the magnetic shield and thick atmosphere that offers protection on Earth. Mars also has no ozone layer in its atmosphere, meaning ultraviolet radiation harmful to organic compounds reaches the surface unhindered. To find signs of life on Mars, future missions are looking to investigate surface regions where erosion from wind-blown sand has recently exposed very ancient material or alternately obtain samples from shielded regions beneath the surface.

The Radiation Assessment Detector on MSL's Curiosity rover has made detailed measurements of the cosmic ray and solar energetic particle environment on the surface of Mars. A paper published in 2013 analyzed the measurements taken during Curiosity's first 300 sols, during which the instrument detected a single spike associated with a solar energetic particle event. Between August 2012 and June 2013, the RAD instrument measured a dose rate due to galactic cosmic rays of 0.67 millisieverts (mSv) per day, roughly a third of the measured daily dose measured inside the spacecraft during transit to Mars (1.8 mSv).