NASA currently has two rovers (Curiosity and Perseverance), one lander (InSight), and one helicopter (Ingenuity) exploring the surface of Mars. Perseverance rover – the largest, most advanced rover NASA has sent to another world – touched down on Mars on Feb. 18, 2021, after a 203-day journey traversing 293 million miles (472 million kilometers). The Ingenuity helicopter rode to Mars attached to the belly of Perseverance.

Perseverance is one of three spacecraft that arrived at Mars in 2021. The Hope orbiter from the United Arab Emirates arrived on Feb. 9, 2021. China’s Tianwen-1 mission arrived on Feb. 10, 2021, and includes an orbiter, a lander, and a rover. Europa and India also have spacecraft studying Mars from orbit. In May 2021, China became the second nation to ever land successfully on Mars when its Zhurong Mars rover touched down. An international fleet of eight orbiters is studying the Red Planet from above including three NASA orbiters: 2001 Mars Odyssey, Mars Reconnaissance Orbiter, and MAVEN.

These robotic explorers have found lots of evidence that Mars was much wetter and warmer, with a thicker atmosphere, billions of years ago. Mars is popularly known as the Red Planet because of its distinct rusty color. The planet's color can even be seen with the naked eye from Earth, appearing in the night sky as though it were a red star. The theory is false according to current science. Mars is red because of the way it was formed billions of years ago back when the solar system was young.

The surface of Mars is covered in iron oxide particles. Iron oxide is the same compound that gives rust its red color. Mars has so much iron oxide on its surface because the planet is smaller and has weaker gravity than Earth does. When the planets were forming around four billion years ago, their surfaces would have been made of hellish oceans of molten rock and metals—including naturally occurring iron oxide.

Earth's larger size and stronger gravity meant this molten rock was under higher pressure in its early days, resulting in higher temperatures. This turned the iron oxide into liquid, and caused it to sink down to the planet's core, scientists believe.

This explanation dates back to a 2004 study by David Rubie and colleagues at the University of Bayreuth, Germany. John Murray, a planetary scientist at the Open University in Milton Keynes, UK, told the journal Nature at the time: "I do not know of any other explanation for Mars's rustiness.

The second reason for Mars' redness is its atmosphere. Mars' atmosphere has been extensively analyzed by NASA's Curiosity rover which landed on the Red Planet in 2012. Scientists found that Mars' atmosphere at the surface consists of 95 percent carbon dioxide, 2.6 percent nitrogen, 1.9 percent argon, 0.16 percent oxygen, and 0.06 percent carbon monoxide. Mars' atmosphere appears red because so much of the planet's iron oxide dust gets blown around in enormous storms. These dust storms occur every year, and some are so big they cover areas the size of continents and can last for weeks at a time. Some, which are more rare, circle the entire planet and have caused NASA rovers to stop working. Another reason Mars' atmosphere looks red is more complicated, and involves the way sunlight reflects off of the planet.

Mars' atmosphere is thinner than Earth's, which is one reason why humans wouldn't be able to survive there without a spacesuit. Due to this thin atmosphere, sunlight that is reflected away from Mars appears red due to a phenomenon known as Rayleigh scattering—or the lack of it.

Rayleigh scattering occurs when light hits particles that are smaller in size than the wavelength of that light, such as gas particles in the Earth's atmosphere. This process tends to scatter blue light and is the same reason the Earth's sky looks blue in the daytime

On Mars there is less gas for the sunlight to interact with, so Rayleigh scattering does not occur as much. Conversely, on Mars there is a process called Mie scattering, in which sunlight hits particles that are roughly the same size as the wavelength of that light—such as iron oxide particles. This process tends to deflect blue light less than red light, according to an explanation posted to StackExchange and cited to Eriita Jones of the University of South Australia

Like Earth, Mars has differentiated into a dense metallic core overlaid by less dense materials.Current models of its interior imply a core consisting primarily of iron and nickel with about 16–17% sulfur.This iron(II) sulfide core is thought to be twice as rich in lighter elements as Earth's.The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but it appears to be dormant. Besides silicon and oxygen, the most abundant elements in the Martian crust are iron, magnesium, aluminium, calcium, and potassium. The average thickness of the planet's crust is about 50 kilometres (31 mi), with a maximum thickness of 125 kilometres (78 mi). Meanwhile, Earth's crust averages 40 kilometres (25 mi) in thickness.

Mars is seismically active, with InSight recording over 450 marsquakes and related events in 2019. In 2021 it was reported that based on eleven low-frequency Marsquakes detected by the InSight lander the core of Mars is indeed liquid and has a radius of about 1830±40 km and a temperature around 1900–2000 K. The Martian core radius is more than half the radius of Mars and about half the size of the Earth's core. This is somewhat larger than models predicted, suggesting that the core contains some amount of lighter elements like oxygen and hydrogen in addition to the iron–nickel alloy and about 15% of sulfur.

The core of Mars is overlain by the rocky mantle, which, however, does not seem to have a layer analogous to the Earth's lower mantle. The martial mantle appears to be solid down to the depth of about 500 km, where the low-velocity zone (partially melted asthenosphere) begins. Below the asthenosphere the velocity of seismic waves starts to grow again and at the depth of about 1050 km there lies the boundary of the transition zone. At the surface of Mars there lies a crust with the average thickness of about 24–72 km.

Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is less than 1% that of Earth's,except at the lowest elevations for short periods.The two polar ice caps appear to be made largely of water. The volume of water ice in the south polar ice cap, if melted, would be enough to cover the entire surface of the planet with a depth of 11 metres (36 ft). A permafrost mantle stretches from the pole to latitudes of about 60°. Large quantities of ice are thought to be trapped within the thick cryosphere of Mars. Radar data from Mars Express and the Mars Reconnaissance Orbiter (MRO) show large quantities of ice at both poles (July 2005)and at middle latitudes (November 2008).The Phoenix lander directly sampled water ice in shallow Martian soil on 31 July 2008.

Photomicrograph by Opportunity showing a gray hematite concretion, nicknamed "blueberries", indicative of the past existence of liquid water.

Landforms visible on Mars strongly suggest that liquid water has existed on the planet's surface. Huge linear swathes of scoured ground, known as outflow channels, cut across the surface in about 25 places. These are thought to be a record of erosion caused by the catastrophic release of water from subsurface aquifers, though some of these structures have been hypothesized to result from the action of glaciers or lava.One of the larger examples, Ma'adim Vallis, is 700 kilometres (430 mi) long, much greater than the Grand Canyon, with a width of 20 kilometres (12 mi) and a depth of 2 kilometres (1.2 mi) in places. It is thought to have been carved by flowing water early in Mars's history.The youngest of these channels are thought to have formed as recently as only a few million years ago. Elsewhere, particularly on the oldest areas of the Martian surface, finer-scale, dendritic networks of valleys are spread across significant proportions of the landscape. Features of these valleys and their distribution strongly imply that they were carved by runoff resulting from precipitation in early Mars history. Subsurface water flow and groundwater sapping may play important subsidiary roles in some networks, but precipitation was probably the root cause of the incision in almost all cases.

Along crater and canyon walls, there are thousands of features that appear similar to terrestrial gullies. The gullies tend to be in the highlands of the Southern Hemisphere and to face the Equator; all are poleward of 30° latitude. A number of authors have suggested that their formation process involves liquid water, probably from melting ice, although others have argued for formation mechanisms involving carbon dioxide frost or the movement of dry dust.No partially degraded gullies have formed by weathering and no superimposed impact craters have been observed, indicating that these are young features, possibly still active.Other geological features, such as deltas and alluvial fans preserved in craters, are further evidence for warmer, wetter conditions at an interval or intervals in earlier Mars history.Such conditions necessarily require the widespread presence of crater lakes across a large proportion of the surface, for which there is independent mineralogical, sedimentological and geomorphological evidence.

A cross-section of underground water ice is exposed at the steep slope that appears bright blue in this enhanced-color view from the MRO.The scene is about 500 meters wide. The scarp drops about 128 meters from the level ground. The ice sheets extend from just below the surface to a depth of 100 meters or more.

Further evidence that liquid water once existed on the surface of Mars comes from the detection of specific minerals such as hematite and goethite, both of which sometimes form in the presence of water. In 2004, Opportunity detected the mineral jarosite. This forms only in the presence of acidic water, which demonstrates that water once existed on Mars.More recent evidence for liquid water comes from the finding of the mineral gypsum on the surface by NASA's Mars rover Opportunity in December 2011.It is estimated that the amount of water in the upper mantle of Mars, represented by hydroxyl ions contained within the minerals of Mars's geology, is equal to or greater than that of Earth at 50–300 parts per million of water, which is enough to cover the entire planet to a depth of 200–1,000 metres (660–3,280 ft).

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.

On 18 March 2013, NASA reported evidence from instruments on the Curiosity rover of mineral hydration, likely hydrated calcium sulfate, in several rock samples including the broken fragments of "Tintina" rock and "Sutton Inlier" rock as well as in veins and nodules in other rocks like "Knorr" rock and "Wernicke" rock.Analysis using the rover's DAN instrument provided evidence of subsurface water, amounting to as much as 4% water content, down to a depth of 60 centimetres (24 in), during the rover's traverse from the Bradbury Landing site to the Yellowknife Bay area in the Glenelg terrain.In September 2015, NASA announced that they had found conclusive evidence of hydrated brine flows on recurring slope lineae, based on spectrometer readings of the darkened areas of slopes.These observations provided confirmation of earlier hypotheses based on timing of formation and their rate of growth, that these dark streaks resulted from water flowing in the very shallow subsurface.The streaks contain hydrated salts, perchlorates, which have water molecules in their crystal structure.The streaks flow downhill in Martian summer, when the temperature is above −23° Celsius, and freeze at lower temperatures.

Perspective view of Korolev crater shows 1.9 kilometres (1.2 mi) deep water ice. Image taken by ESA's Mars Express.

Researchers suspect that much of the low northern plains of the planet were covered with an ocean hundreds of meters deep, though this remains controversial.In March 2015, scientists stated that such an ocean might have been the size of Earth's Arctic Ocean. This finding was derived from the ratio of water to deuterium in the modern Martian atmosphere compared to that ratio on Earth. The amount of Martian deuterium is eight times the amount that exists on Earth, suggesting that ancient Mars had significantly higher levels of water. Results from the Curiosity rover had previously found a high ratio of deuterium in Gale Crater, though not significantly high enough to suggest the former presence of an ocean. Other scientists caution that these results have not been confirmed, and point out that Martian climate models have not yet shown that the planet was warm enough in the past to support bodies of liquid water.

Near the northern polar cap is the 81.4 kilometres (50.6 mi) wide Korolev Crater, where the Mars Express orbiter found it to be filled with approximately 2,200 cubic kilometres (530 cu mi) of water ice.The crater floor lies about 2 kilometres (1.2 mi) below the rim, and is covered by a 1.8 kilometres (1.1 mi) deep central mound of permanent water ice, up to 60 kilometres (37 mi) in diameter.

It is humankind’s nature to explore our surroundings if it can be done. Fifty years ago, exploring Mars was not one of the things anyone could do. Those who were curious had to be content with fuzzy images of the planet, quivering in the oculars of telescopes. But that is far from the case today. Forty years ago, spacecraft began to be sent to the planets, and since then, the art of space exploration has become increasingly refined and discoveries have multiplied. We now have the capability, in principle, of reaching and exploring any object in the solar system. At the top of the list of targets of exploration is Mars, the most Earth-like, most accessible, most hospitable, and most intriguing of the planets. Two years ago, in October 2000, NASA recognized this by setting the study of Mars apart in a structured Mars Exploration Program. The present document reports on COMPLEX’s study of the program.

COMPLEX has compared the elements of the Mars Exploration Program with the research objectives for Mars that have been stressed by advisory panels, including this one, for more than 23 years. The committee found that correspondence between the two is not perfect. Currently, NASA focuses on the search for life, and its prerequisite, water, as the main drivers for Mars research, and has favored missions and experiments that support these goals. The space agency is not now in a position to ask direct questions about life on Mars, and has not been since the Viking mission in the 1970s, but the missions supported are designed to find the areas most promising for water and life, and to investigate in situ their chemical and petrographic potential for extant or fossil life.

Since NASA operates within budget constraints, this emphasis on one particular scientific objective necessarily comes at the expense of others. COMPLEX considered the question of whether NASA’s priorities are too heavily skewed toward life-related investigations. The committee decided, however, that this is not the case. The emphasis on life is well justified; the life-related investigations that are planned range over so much of Mars science that they will result in broad and comprehensive gains in our knowledge; and the areas most neglected as a consequence of this emphasis (see Chapter 12) will, to some extent, be investigated by projected missions of our international partners.