Mars

Mars is the fourth planet from the sun and the second-smallest planet in the solar system. It has long been called the "Red Planet," due to the abundant iron oxide on its surface giving it a reddish appearance when seen from Earth. For much of modern history, Mars was considered the planet most likely to host extraterrestrial life because of its Earth-like size (0.107 Earths in mass, 0.151 Earths in volume) and orbit (1.5 astronomical units or 230 million kilometers from the sun, with an orbital period of 1.88 years). Further, Mars has a rotational period of 1.025 days and an axial tilt of 25 degrees, similar to those of Earth, leading to the expectation of Earth-like seasons. Though this hope continues to be held, the focus now is on the possibility of microbial life, perhaps in a subsurface water source.

Despite the apparent lack of life, the Martian surface is very similar to Earth’s, including polar ice caps, valleys, deserts, and mountains (Olympus Mons is the second highest in the solar system). Many features speak of probable celestial impacts, including the Borealis basin that covers most of the northern hemisphere. Others support the theory that the Martian surface once had ample liquid water in the form of rivers, lakes, and even seas.

The core of Mars is believed to be mainly iron, nickel, and sulfur, including a partially fluid amount of iron sulfide. Tectonic and volcanic activity due to the silicate mantle seems to be dormant. The crust of Mars is made up of abundant quantities of silicon, oxygen, iron, magnesium, aluminum, calcium, and potassium. This makes for a rocky surface of basalt, feldspar, and silicon glass, covered with a thick layer of extremely fine iron oxide dust. Relative to the overall size of the planet, the Martian crust is three times thicker than that of Earth.

Atmosphere and Climate

Because Mars lacks a magnetosphere, the solar wind strips atmospheric atoms away from the ionosphere, a process that has been detected by probes and studied directly by the MAVEN (Mars Atmosphere and Volatile EvolutioN) orbiter. The thin atmosphere of Mars is one of the crucial differences between it and the Earth. The atmosphere consists predominantly of carbon dioxide (about 96 percent), followed by argon and nitrogen (almost 2 percent each) and trace amounts of oxygen, carbon monoxide, water vapor, neon, formaldehyde, hydrogen, xenon, methane, and other compounds. The methane in the atmosphere seems to be introduced by plumes in the northern hemisphere. The atmospheric pressure, less than 1 percent that of Earth, is too low for water to remain liquid on the surface except under certain circumstances, a condition believed to have been different in the past based on the presence of water-influenced landforms. The fine surface soil of Mars leads to a high concentration of particulate dust in the Martian air, contributing to the appearance of an arid, dried-out planet.

Mars’s axial tilt gives it a seasonality that is more like Earth’s than any other planet in the solar system, though the thin atmosphere leads to much colder temperatures, which range from the Earth-like 35 degrees Celsius in the summer to –143 degrees Celsius at the polar caps. Dust storms, which become more common the closer Mars’s orbit brings it to the sun, can reach sizes that encompass the whole surface of the planet.

Planetary Rings and Moons

Mars has two moons, Phobos (fourteen miles in diameter) and Deimos (eight miles in diameter), named for the sons of the god Mars. They are small and irregularly shaped, which led to the theory that both are asteroids that were pulled into Mars’s orbit. Their mineral composition supports this idea, which conveniently also explains Phobos’s unusually unstable orbit. Phobos rises in the west and sets in the east, with an orbit (6,000 kilometers from Mars) that is degrading as a result of Mars’s tidal forces. Phobos will eventually either be pulled close enough to Mars’s center of gravity to crash into the surface, or will be broken into smaller pieces that will form a ring around the planet. Its surface is heavily pockmarked with craters and grooves. Another possibility for its formation is that it was formed from material ejected from Mars that accumulated in orbit.

Seen from the planet’s surface, Deimos rises very slowly in the east. Although it has a thirty-hour orbit (about twenty-three thousand kilometers from Mars), it is just outside synchronous orbit, and the planet’s own rotation causes almost three days to pass between its rise and fall.

A popular model predicts the appearance of a dust ring in orbit around Mars between Phobos and Deimos, but this has not yet been confirmed.

History of Observation and Exploration

Mars is easily seen from Earth with the naked eye, though with an apparent magnitude of –3.0 it is not as bright as Venus or Jupiter. The planet was regularly observed by ancient cultures and first observed via telescope in 1610, by Galileo Galilei. When nineteenth-century telescopes became advanced enough to perceive surface details, an optical illusion fooled many into perceiving “canals” on Mars, which fed the belief in life on the Red Planet.

Mars is one of the few celestial bodies on which probes sent from Earth have landed, rather than simply investigating via flyby or from orbit. The flyby mission of the Mariner 4 probe in 1965 disappointed many by finding no signs of water, but later missions complicated the picture: water molecules were found in 2007 by the Mars rover Spirit, and the following year the Phoenix Mars lander discovered frozen ice in the soil.

The Viking probes in the 1970s conducted tests to detect the presence of microbial life, but the results were inconclusive and remained the subject of debate.

Beginning in 2002, there were at least two active missions on Mars at any given time, and in 2012, Mars became home to two rovers (Opportunity and Curiosity) and three orbiters (Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter, or MRO), all sending signals back to Earth. Two more orbiters, the Mars Orbiter Mission probe Mangalyaan and MAVEN (short for "Mars Atmosphere and Volatile Evolution Mission"), were launched in 2013. The InSight (short for "Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport") lander is intended to construct more accurate models of the Martian interior. (Though initially scheduled to launch in 2016, an instrument failure caused the InSight launch to occur in May 2018 instead.) In 2020, NASA launched the Perseverance rover, which landed on Mars in 2021. One of the main objectives the National Aeronautics and Space Administration (NASA) outlined for these missions was to "follow the water" in order to better understand the past and current conditions of Mars and with the assumption that any potential life would be connected to water sources. Data collected from InSight was published by NASA in 2021.

As the variety of rovers and orbiters brought in new data, a more detailed understanding of Mars began to emerge. Most important were the clarification of the amount and distribution of ice on the planet and the discovery that liquid water is still present under certain conditions. Probes variously confirmed the presence of water vapor, dug up material thought to be ice, and found minerals only formed with water. In 2011, researchers analyzing images from the MRO noticed dark streaks on sloping terrain that appeared to change seasonally, leading many to speculate that they were formed by flowing water.

In 2015, additional data from the MRO indicated presence of hydrated salts in the streaks, termed recurring slope linneae (RSL). This provided solid evidence that briny liquid water flows on Mars when temperatures are warm enough, and wicks close enough to the surface to discolor the ground, further raising hopes that life could be found on the planet, whether current or as a fossil presence. However, in 2017, new research by scientists with the US Geological Survey suggested that these supposed water flows may in fact be sand flows, and that while the RSLs may still contain some water, the amount present is much less than was previously hoped—less than 3 percent of the flows, if at all.

In 2018, Italian researchers reported in an article for the journal Science that they believed they had discovered the presence of a subglacial lake approximately one mile below the ice in the Planum Australe region near the planet's south pole. They estimate this reservoir of liquid water, which would be the first and largest known body of stable liquid water found on the planet, to be about 20 kilometers (12 miles) wide and detected it through the use of the Mars Express spacecraft's radar instrument Mars Advanced Radar for Subsurface and Ionosphere Sounding. While this discovery, which involved more than three years of dedicated radar surveying in the specific area, has potentially provided more evidence supporting the argument for life on Mars, further observation efforts were needed to confirm the finding.

Two years later, a paper published in Nature Astronomy revealed that research based upon that 2018 discovery had indeed continued. This time methodically going over radar data from more than one hundred observations conducted by Mars Express over the span of seven years (2012 to 2019), scientists claimed that they had found evidence supporting the first lake's existence under Mars's surface that also indicated that three other, smaller lakes existed near it. While some argued that this additional study gave further credence to the potential for life on the planet as it showed that the occurrence of subsurface bodies of water seemed natural for the planet rather than isolated, others cautioned that more would need to be known about the levels of salt concentration in the bodies of water, as the water could likely only be habitable for life if it were not too salty. At that point, it was hoped that continued Mars Express observations could provide more insight.

Further insight into the core of Mars was provided by researchers in 2023, who, in two separate studies published in the journal Nature, presented evidence that suggested there was a massive ocean of magma beneath the surface of Mars. This evidence contradicted data gathered by NASA in 2021 that suggested the core of Mars was much larger and much less dense than expected for a planet of Mars’s size. Instead, the 2023 studies concluded that Mars’s core was much smaller than the 2021 NASA data suggested and instead obscured by a massive ocean of magma. In 2024, NASA reported that the Perseverance rover may have found evidence that microbial life existed at one point on Mars.

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