Mars, otherwise known as the “Red Planet”, is the fourth planet of our Solar System and the second smallest (after Mercury). Named after the Roman God of war, its nickname comes from its reddish appearance, which has to do with the amount of iron oxide prevalent on its surface. Every couple of years, when Mars is at opposition to Earth (i.e. when the planet is closest to us), it is most visible in the night sky.
Because of this, humans have been observing it for millennia, and its appearance in the heavens has played a large role in the mythology and astrological systems of many cultures. And in the modern era, it has been a veritable treasure trove of scientific discoveries, which have informed our understanding of our Solar System and its history.
Size, Mass and Orbit:
Mars has a radius of approximately 3,396 km at its equator, and 3,376 km at its polar regions – which is the equivalent of roughly 0.53 Earths. While it is roughly half the size of Earth, it’s mass – 6.4185 x 10²³ kg – is only 0.151 that of Earth’s. It’s axial tilt is very similar to Earth’s, being inclined 25.19° to its orbital plane (Earth’s axial tilt is just over 23°), which means Mars also experiences seasons.
At its greatest distance from the Sun (aphelion), Mars orbits at a distance of 1.666 AUs, or 249.2 million km. At perihelion, when it is closest to the Sun, it orbits at a distance of 1.3814 AUs, or 206.7 million km. At this distance, Mars takes 686.971 Earth days, the equivalent of 1.88 Earth years, to complete a rotation of the Sun. In Martian days (aka. Sols, which are equal to one day and 40 Earth minutes), a Martian year is 668.5991 Sols.
Composition and Surface Features:
With a mean density of 3.93 g/cm³, Mars is less dense than Earth, and has about 15% of Earth’s volume and 11% of Earth’s mass. The red-orange appearance of the Martian surface is caused by iron oxide, more commonly known as hematite (or rust). The presence of other minerals in the surface dust allow for other common surface colors, including golden, brown, tan, green, and others.
As a terrestrial planet, Mars is rich in minerals containing silicon and oxygen, metals, and other elements that typically make up rocky planets. The soil is slightly alkaline and contains elements such as magnesium, sodium, potassium, and chlorine. Experiments performed on soil samples also show that it has a basic pH of 7.7.
Although liquid water cannot exist on Mars’ surface, owing to its thin atmosphere, large concentrations of ice water exist within the polar ice caps – Planum Boreum and Planum Australe. In addition, a permafrost mantle stretches from the pole to latitudes of about 60°, meaning that water exists beneath much of the Martian surface in the form of ice water. Radar data and soil samples have confirmed the presence of shallow subsurface water at the middle latitudes as well.
Like Earth, Mars is differentiated into a dense metallic core surrounded by a silicate mantle. This core is composed of iron sulfide, and thought to be twice as rich in lighter elements than Earth’s core. The average thickness of the crust is about 50 km (31 mi), with a maximum thickness of 125 km (78 mi). Relative to the sizes of the two planets, Earth’s crust (averaging 40 km or 25 mi) is only one third as thick.
Current models of its interior imply that the core region measures between 1,700 – 1850 kilometers (1,056 – 1150 mi) in radius, consisting primarily of iron and nickel with about 16–17% sulfur. Due to its smaller size and mass, the force of gravity on the surface of Mars is only 37.6% of that on Earth. An object falling on Mars falls at 3.711 m/s², compared to 9.8 m/s² on Earth.
The surface of Mars is dry and dusty, with many similar geological features to Earth. It has mountain ranges and sandy plains, and even some of the largest sand dunes in the Solar System. It also has the largest mountain in the Solar System, the shield volcano Olympus Mons, and the longest, deepest chasm in the Solar System: Valles Marineris.
The surface of Mars has also been pounded by impact craters, many of which date back billions of years. These craters are so well preserved because of the slow rate of erosion that happens on Mars. Hellas Planitia, also called the Hellas impact basin, is the largest crater on Mars. Its circumference is approximately 2,300 kilometers, and it is nine kilometers deep.
Mars also has discernible gullies and channels on its surface, and many scientists believe that liquid water used to flow through them. By comparing them to similar features on Earth, it is believed these were were at least partially formed by water erosion. Some of these channels are quite large, reaching 2,000 kilometers in length and 100 kilometers in width.
Mars’ Moons:
Mars has two small satellites, Phobos and Deimos. These moons were discovered in 1877 by the astronomer Asaph Hall and were named after mythological characters. In keeping with the tradition of deriving names from classical mythology, Phobos and Deimos are the sons of Ares – the Greek god of war that inspired the Roman god Mars. Phobos represents fear while Deimos stands for terror or dread.
Phobos measures about 22 km (14 mi) in diameter, and orbits Mars at a distance of 9234.42 km when it is at periapsis (closest to Mars) and 9517.58 km when it is at apoapsis (farthest). At this distance, Phobos is below synchronous altitude, which means that it takes only 7 hours to orbit Mars and is gradually getting closer to the planet. Scientists estimate that in 10 to 50 million years, Phobos could crash into Mars’ surface or break up into a ring structure around the planet.
Meanwhile, Deimos measures about 12 km (7.5 mi) and orbits the planet at a distance of 23455.5 km (periapsis) and 23470.9 km (apoapsis). It has a longer orbital period, taking 1.26 days to complete a full rotation around the planet. Mars may have additional moons that are smaller than 50- 100 meters (160 to 330 ft) in diameter, and a dust ring is predicted between Phobos and Deimos.
Scientists believe that these two satellites were once asteroids that were captured by the planet’s gravity. The low albedo and the carboncaceous chondrite composition of both moons – which is similar to asteroids – supports this theory, and Phobos’ unstable orbit would seem to suggest a recent capture. However, both moons have circular orbits near the equator, which is unusual for captured bodies.
Another possibility is that the two moons formed from accredit material from Mars early in its history. However, if this were true, their compositions would be similar to Mars itself, rather than similar to asteroids. A third possibility is that a body impacted the Martian surface, who’s material was ejected into space and re-accreted to form the two moons, similar to what is believed to have formed the Earth’s Moon.
Atmosphere and Climate:
Planet Mars has a very thin atmosphere which is composed of 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water. The atmosphere is quite dusty, containing particulates that measure 1.5 micrometers in diameter, which is what gives the Martian sky a tawny color when seen from the surface. Mars’ atmospheric pressure ranges from 0.4 – 0.87 kPa, which is equivalent to about 1% of Earth’s at sea level.
Because of its thin atmosphere, and its greater distance from the Sun, the surface temperature of Mars is much colder than what we experience here on Earth. The planet’s average temperature is -46 °C (-51 °F), with a low of -143 °C (-225.4 °F) during the winter at the poles, and a high of 35 °C (95 °F) during summer and midday at the equator.
The planet also experiences dust storms, which can turn into what resembles small tornadoes. Larger dust storms occur when the dust is blown into the atmosphere and heats up from the Sun. The warmer dust filled air rises and the winds get stronger, creating storms that can measure up to thousands of kilometers in width and last for months at a time. When they get this large, they can actually block most of the surface from view.
Trace amounts of methane have also been detected in the Martian atmosphere, with an estimated concentration of about 30 parts per billion (ppb). It occurs in extended plumes, and the profiles imply that the methane was released from specific regions – the first of which is located between Isidis and Utopia Planitia (30°N 260°W) and the second in Arabia Terra (0°N 310°W).
It is estimated that Mars must produce 270 tonnes of methane per year. Once released into the atmosphere, the methane can only exist for a limited period of time (0.6 – 4 years) before it is destroyed. Its presence despite this short lifetime indicates that an active source of the gas must be present.
Several possible sources have been suggested for the presence of this methane, ranging from volcanic activity, cometary impacts, and the presence of methanogenic microbial life forms beneath the surface. Methane could also be produced by a non-biological process called serpentinization involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.
The Curiosity rover has made several measurements for methane since its deployment to the Martian surface in August of 2012. The first measurements, which were made using its Tunable Laser Spectrometer (TLS), indicated that there were less than 5 ppb at its landing site (Bradbury Landing). A subsequent measurement performed on September 13th detected no discernible traces.
On December 16th, 2014, NASA reported that the Curiosity rover had detected a “tenfold spike”, likely localized, in the amount of methane in the Martian atmosphere. Samples measurements taken between late 2013 and early 2014 showed an increase of 7 ppb; whereas before and after that, readings averaged around one-tenth that level.
Ammonia was also tentatively detected on Mars by the Mars Express satellite, but with a relatively short lifetime. It is not clear what produced it, but volcanic activity has been suggested as a possible source.
Historical Observations:
Earth astronomers have a long history of observing the “Red Planet”, both with the naked eye and with instrumentation. The first recorded mentions of Mars as a wandering object in the night sky were made by Ancient Egyptian astronomers, who by 1534 BCE were familiar with the planet’s “retrograde motion”. In essence, they deduced that the planet, though it appeared to be a bright star, moved differently than the other stars, and that it would occasionally slow down and reverse course before returning to its original course.
By the time of the Neo-Babylonian Empire (626 BCE – 539 BCE), astronomers were making regular records of the position of the planets, systematic observations of their behavior and even arithmetic methods for predicted the positions of the planets. For Mars, this included detailed accounts of its orbital period and its passage through the zodiac.
By classical antiquity, the Greeks were making additional observations on Mars’ behavior that helped them to understand its position in the Solar System. In the 4th century BCE, Aristotle noted that Mars disappeared behind the Moon during an occultation, which indicated it was farther away than the Moon.
Ptolemy, a Greek-Egyptian astronomer of Alexandria (90 CE – ca. 168 CE), constructed a model of the universe in which he attempted to resolve the problems of the orbital motion of Mars and other bodies. In his multi-volume collection Almagest, he proposed that the motions of heavenly bodies were governed by “wheels within wheels”, which attempted to explain retrograde motion. This became the authoritative treatise on Western astronomy for the next fourteen centuries.
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