This 47 cents postage stamp was issued by America on 31 May 2016 (= Scott Catalogue # 5072). It shows Planet Mars and is part of a set depicting all the planets of the Solar System.
There are four types of planets in the Solar System: terrestrial planets, gas giant planets, ice giant planets, and dwarf planets. The terrestrial planets are Mercury, Venus, Earth, and Mars. The gas giant planets are Jupiter and Saturn. The ice giant planets are Uranus and Neptune. Dwarf planets include named and unnamed objects - for example, Ceres, Pluto, Quaoar, Sedna, Eris, Varuna, Makemake, and many others. Only two dwarf planets have been visited and imaged (Ceres and Pluto).
Mars, the "Red Planet", is the fourth and farthest-out terrestrial planet from the Sun. There have been many Mars missions, both successes and failures. Mars visits occurred from 1965 to the present day and include flybys, orbiters, landers, and rovers. Missions to Mars have been done by the Americans, Russians, Europeans, Indians, Arabs, and Chinese. Notable landers include Viking 1 and Viking 2 from 1976 to 1982. Martian rovers were named Sojourner, Spirit, Opportunity, Curiosity, and Perseverance. The latter two are still active as of 2025.
Mars orbits the Sun at ~1.4 to ~1.7 astronomical units ("AU"). One "AU" is the average distance between the center of the Sun and the center of the Earth. Both Mars' rotation and orbit around the Sun are in a prograde direction (= counterclockwise motion, when viewed above the North Pole of the Sun). Mars' orbit is slightly noncircular - the orbital eccentricity is about 0.09. In contrast, Earth's orbit around the Sun is close to circular.
Of the four terrestrial planets, Mars is the second-smallest, with a average radius of about 3389 kilometers. It is about 53% the size of Planet Earth. Mars' radius is ~7 kilometers larger from the center of the Martian core to the equator and 13 kilometers smaller from the core's center to the poles. Mars being slightly flattened is a result of its rotation - all planets are slightly to noticeably flattened. The density of Mars is about 3.93 grams per cubic centimeter, making it the fourth-highest density world in the Solar System, but it's the lowest-density terrestrial planet.
One day on Mars is similar to Earth, because it rotates relatively quickly. One Earth day is 24 hours, but one Martian day is 24 hours + 37 minutes (= one sidereal day). Sunrise to sunrise on Mars is 24 hours + 39 minutes (= one solar day). One Martian year is 687 days (Earth days) long, which is about 1.88 Earth years.
Like Earth, Mars has seasons due to a tilted axis. The Martian spin axis is tilted about 25.2 degrees from the vertical. Each season on Mars (spring, summer, fall, winter) is almost twice as long as on Earth. Mars' axis is tilted a bit more than Earth's axis.
Mars has two tiny moons, named Phobos (the larger one) and Deimos (the smaller one). They may be gravitationally-captured asteroids, but not everyone agrees with that interpretation. Some researchers conclude that they formed during large impact events on Mars.
Planet Mars has a low albedo - it's a relatively dark object. "Albedo" refers to the reflectivity of a surface. Bright worlds have high albedo and dark worlds have low albedo. Martian albedo is about 25% (using the more intuitive calculation for albedo, which is the percentage of sunlight reflected back into space).
Unlike Earth, Mars has no global magnetic field, although small, umbrella-shaped, localized magnetic fields occur in some areas. Most of these are in the Martian southern hemisphere. They are considered remnants of a global magnetic field that decayed away long ago. Before 4.1 billion years ago, Mars did have a global magnetic field. It died when the liquid core stopped convecting - no more heat currents. Magnetic fields disappear if hot, molten material stagnates.
The geologic history of Mars is poorly known, compared with Earth, but a Martian geologic time scale has been constructed. Based on geology, the time scale units are (from youngest to oldest): Amazonian Period, Hesperian Period, Noachian Period, and Pre-Noachian Period. Those names are derived from geographic regions. Based on mineralogy, the time units are (youngest to oldest): Siderikan Era, Theiikian Era, and Phyllocian Era. Those names are based on the dominant mineral group forming at the time: iron oxides or sulfates or phyllosilicates / clays.
Mars is nicknamed the "Red Planet" because its surface is mostly covered with reddish-brown dust. The color is from hematite (Fe2O3, iron oxide), which is natural rust. 2025 update: the reddish color is probably from ferrihydrite, which is slightly different from hematite. Ferrihydrite is an iron hydroxy-oxide mineral, Fe10O14(OH)2.
Mars experiences very slow weathering - it has a dry, sandy, dusty, desert-like surface. Temperatures range from warm to very cold: +35 degrees Celsius to -153 degrees Celsius (= ~+95 degrees Fahrenheit to -243 degrees Fahrenheit). Daytime on Mars is obviously warmer and nighttime is quite cold. Average temperatures are about -60 degrees Celsius (= -80 degrees Fahrenheit).
Remarkably, Mars has blue-colored sunsets, unlike the reddish-orange sunsets commonly seen on Earth. Martian atmospheric dust scatters blue light into the area around the Sun - the blue coloration is only apparent near sunrise and sunset.
Mars does have air, but it is very thin. Atmospheric pressure on Mars is about 0.63% that of Earth's (= 1 / 160th of Earth's). Martian air pressure was much higher in the geologic past, but most of Mars' air has been lost to space. Mars continues to leak air into space today, due to the planet's small size and relatively low gravity. About 95.3% of Martian air is carbon dioxide gas (CO2). Other gases: ~2.6% nitrogen gas (N2), ~1.9% argon gas (Ar), plus trace gases. Tiny amounts of methane gas (CH4, a type of petroleum gas) are released from the ground. Most methane release is during Martian summer and fall. On Earth, methane gas release is geologic in origin (from volcanic gases) or biologic / biogenic (from organisms). Is is unknown if Martian methane release is geologic or microbial or both.
The low atmospheric pressure of Mars prevents liquid water from existing at the surface. On a pressure-temperature diagram for water (H2O), the water ice field, liquid water field, and water vapor field meet at one point, called the triple point. Mars is past the triple point, so there's no liquid water. In the distant geologic past, air was thicker and liquid water could exist at the surface. River channels and deltas and ancient shorelines are present throughout Mars.
Because Mars has an atmosphere, it does have clouds and weather, but no rain. At the poles, Mars can have water ice snow (H2O) or dry ice snow (CO2). Mars can experience fog in topographic lows such as valleys or impact craters. Frost can occur on the ground - it condenses from the chilly Martian air at night. Mars does not have rain storms, but it does have dust storms. These can be global in extent and last weeks to months. Winds reach up to ~113 kilometers per hour (~70 miles per hour). Smaller-scale weather phenomena include dust devils - these have even been photographed in action by orbiters. Dust devil tracks are common on Mars.
Like all terrestrial worlds, Mars is differentiated into a core, mantle, and crust. The core is rich in iron (Fe), sulfur (S), and oxygen (O). The Martian core is about half the size of Earth's core. Based on seismic studies from the InSight lander, the Martian core has a radius of about 1810 to 1860 kilometers. The core may have one or two parts and may be liquid or solid or both. Like Earth, the Martian mantle is peridotite-rich, at least in the upper part. Martian crust is mostly basalt, a mafic, extrusive igneous rock ("black lava rock") dominated by plagioclase feldspar and pyroxene. Martian crustal rocks may also include significant basaltic andesite and andesite.
Mars does not have plate tectonics - its lithosphere is not fragmented. As such, it is often given the silly nickname "One Plate Planet". Earth is the only world known to have operating plate tectonics. However, Mars may have had some style of plate tectonics in ancient times, as hinted by magnetic stripes in the Southern Hemisphere. Some consider the linear Mariner Valley to be a transform fault.
Topographically, Mars has a striking dichotomy (global asymmetry) - the Northern Hemisphere is mostly low elevation and the Southern Hemisphere is mostly high elevation. The Northern Lowlands on Mars are called the Borealis Basin and may have formed during an ancient giant impact event (?). Mars' Northern Lowlands are relatively smooth, lightly cratered, and consist of lava flows and sediments. The Southern Highlands are ancient, topographically rough, and heavily cratered. Elevational relief on Mars is ~28.4 to 29.5 kilometers (= ~18 miles), which is much greater than Earth's or Venus' total relief. The highest point on Mars is Mount Olympus in the Tharsis region. The lowest point is the deepest impact crater in the Hellas Impact Basin.
Important topographic features on Mars include the Tharsis Bulge, a volcanic region, the Hellas Impact Basin, the Argyre Impact Basin, Mariner Valley, and Elysium, another volcanic region.
Like Earth, Mars has a cryosphere. Earth's crysosphere consists of two large continental glaciers, the Greenland Ice Sheet and the Antarctic Ice Sheet, plus numerous smaller alpine glaciers. Mars' cryosphere has no glaciers, but consists of two large polar ice caps, composed of water ice (H2O) and dry ice (CO2). Both polar ice caps are permanent, but they do expand and shrink with the seasons. Oddly, they have spiral troughs, which are better developed in the North Polar Cap. Also strangely, the Martian polar ice caps have explosion holes. In springtime, sunlight shines through the transparent ice. The warming dry ice sublimates from the bottom-up, resulting in geyser-like eruptions of gas and sediments. The Martian cryosphere also includes some ice-filled impact craters - for example, Korolev Crater near the North Polar Cap.
Mars does have shorelines, but no oceans. A large ocean was present in the Borealis Basin / Northern Lowlands long ago, before the Martian atmosphere passed the triple point of water. Rivers emptied into the ocean in ancient times. The river valleys are completely dry now, but they used to have running water. Tributaries, distributaries, and deltas are present on Mars. The most common stream drainage pattern on Mars (& Earth) is dendritic. The most significant river valley on Mars is Mariner Valley ("Valles Marineris") - it is over 4000 kilometers long (over 2500 miles), 2 to 7 kilometers deep, and over 600 kilometers across at its widest. This is much, much larger than the Grand Canyon on Earth. Mariner Valley possibly formed by crustal fracturing during uplift of the nearby Tharsis Bulge.
Martian valley networks are often considered to indicate a "warm and wet" early Mars, but this may not be accurate. Martian stream networks are juvenile in appearance - they do not appear to be well developed. The headwaters of Mariner Valley, the largest stream channel on Mars, is in a volcanic region. Temporary meltwater from volcanism drained and eroded the landscapes.
Every world with a solid surface has mass wasting - "landslides". Active mass wasting has been photographed on Mars by orbiters. Landslide chutes and debris aprons are common on steep surfaces such as impact crater walls and river valley walls. Dark-colored "seasonal streaks" representing small-scale mass wasting events are common on Mars. They may be composed of dry or wet material (probably the former, because Mars is past the triple point of water).
Wind-blown sand dunes ("eolian dunes") occur on several worlds: Venus, Earth, Mars, Io, Titan, and Pluto. Martian examples include transverse dunes, with linear to somewhat linear ridges, and barchan dunes, which are crescent shaped.
Both Mars and Earth have abundant siliciclastic sedimentary rocks. On Mars, sandstones are common. Sandstones are composed of lithified sand grains, which are between 2 and 1 / 16 of a millimeter in size. Cross-bedded sandstones are often seen. Cross-bedding refers to tilted layers between horizontal layers. It forms in a one-directional current by wind or water. Some Martian sandstones have preserved ripple marks, which also indicate presence of ancient currents. Observed examples on Mars are lacustrine symmetrical ripple marks - an ancient lake once filled an impact crater (for example, Gale Crater).
Inferred lacustrine mudrocks are also known on Mars. Calling them "shale" is probably not appropriate, because they do not appear to be very fissile. Shale is the most common sedimentary rock on Earth - it is very fine-grained, with sediment sizes less than 1 / 256 of a millimeter. Mars also has conglomerate, a coarse-grained, poorly-sorted sedimentary rock. Conglomerate has rounded to subrounded large grains (pebbles).
Hematite-rich concretions were famously encountered near Fram Crater during a Martian rover mission. Nicknamed "blueberries", the concretions are small and spherical to subspherical. Concretions are post-depositional mineral masses that range in shape from spherical to irregular.
Despite lacking active plate tectonics, Mars does experience quakes. Marsquakes were first detected in the 1970s by the Viking 2 lander. Over 1,300 quakes were recorded by the InSight lander from 2018 to 2022. Notable Marsquakes have magnitudes in the 2s, 3s, and 4s. The largest detected Marsquake so far was a magnitude ~5 event on 4 May 2022. Some quakes have been caused by recent impact events.
Mars has large volcanic regions, many individual volcanoes, and many lava flows. Volcanism on Mars is currently inactive (active volcanism is known on Venus, Earth, and Io). Of the various types of volcanism (silicate volcanism, cryovolcanism, sulfur volcanism, ferrovolcanism), Mars used to have silicate volcanism. The largest volcanic features on Mars include the Northern Lowlands, with widespread lava plains, and Tharsis, with multiple giant, basaltic shield volcanoes. The largest volcano in Tharsis is Mount Olympus ("Olympus Mons"), a basaltic shield volcano similar to Mauna Loa Volcano at Hawaii, but much larger. Other huge Tharsis volcanoes are Mount Alba, Mount Arsia, Mount Pavonis, and Mount Ascraeus. In 2024, another Tharsis volcano, highly dissected by fluvial erosion, was recognized - Noctis Volcano occurs at the head of Mariner Valley. The large Tharsis mountains are hotspot volcanoes, like the Hawaii Hotspot and Yellowstone Hotspot on Earth - large mantle plumes occur below them. The Martian mantle plumes at Tharsis were possibly generated by a large ~antipodal impact.
Mount Olympus is the largest volcano in the entire Solar System and stands at ~24 kilometers high. It was still active in the late Amazonian on the Martian geologic time scale.
Tharsis shield volcanoes experienced effusive eruptions of basalt lava. Explosive volcanism also occurred on Mars - for example, Eden Patera is an inferred caldera complex that had explosive eruptions.
Most Martian volcanic rocks are basalt (mafic), including vesicular basalt with gas bubbles. Columnar-jointed basalt lava flows have been imaged on Mars near Marte Valley. Famous examples of columnar jointing on Earth include Giant's Causeway in Ireland, Devils Tower in Wyoming, and Devils Postpile in California. Columnar jointing forms as lava flows cool and contract.
Martian volcanic lithologies also include andesite, dacite, trachyte, tephrite, and others.
Large to small impact craters are abundant on Mars. Over 400,000 Martian craters are over 1 kilometer-sized. The # 1 largest impact structure is the Hellas Impact Basin, with a diameter of ~2000 kilometers.
Earth is the only world definitely known to harbor life. Mars may have and/or may have had life forms. Very small, bacteria-like structures were found inside Martian igneous rocks (for example, the Allan Hills 84001 Meteorite, a 4.09 billion year old orthopyroxenite). Fossils are expected in sedimentary rocks, but not igneous rocks. However, fossils do occur in igneous rocks on Earth, but very rarely. Martian microfossils have been found in multiple rocks - most are exceedingly small. Many consider the structures to be too small to be bacteria, and that they are mineral formations. On Earth, extremely small nannobateria do exist, despite assertions to the contrary. So, the size argument by itself does not wash. The Martian microfossils are associated with organic matter. Many people reject these structures as fossils, but they may be. Possible evidence for still-extant Martian life is the seasonally-varying release of methane gas (CH4) from the ground.
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Synthesized from many sources.
