Mars (planet)

Mars (planet)
Name	Mars
Caption	Global mosaic of Mariner 9, Viking and MRO data
Discovered	Known in antiquity
Aphelion	249,200,000 km
Perihelion	206,700,000 km
Semimajor	227,940,000 km
Eccentricity	0.0934
Period	687 days
Inclination	1.85°
Mean radius	3,389.5 km
Mass	6.4171×10^23 kg
Density	3.93 g/cm^3
Surface temp	−87 °C (mean)
Atmosphere	Carbon dioxide (95.3%)
Moons	Phobos, Deimos

Mars (planet) Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System after Mercury. Its reddish appearance, caused by iron oxide on its surface, made it a prominent object in the skies of ancient cultures such as the Babylonians, Egyptians, and Greeks, inspiring namesakes in the Romans and later scientific study by missions from agencies like NASA, ESA, Roscosmos, ISRO, and CNSA.

Overview

Mars occupies a position between Earth and the Asteroid belt, sharing orbital neighborhood dynamics with bodies cataloged by the Minor Planet Center. Its 687‑day orbital period and 25.19° axial tilt produce seasonal cycles analogous to Earth's seasons described in observations by the Hubble Space Telescope and modeled using data from the Mars Global Surveyor and Mars Odyssey. The planet figures centrally in debates at forums like the International Astronomical Union and in planning by institutions including the Jet Propulsion Laboratory and the European Space Agency for future exploration.

Physical characteristics

Mars's radius and mass give it a surface gravity about 0.38g, measured by landed missions such as Viking 1, Pathfinder, and Curiosity. Its bulk composition—silicate mantle and iron core—has been constrained by seismometry from InSight and by orbital gravimetry from Mars Reconnaissance Orbiter and Mars Express. The planet's thin atmosphere, dominated by carbon dioxide, creates low surface pressure measured by instruments on Phoenix and Perseverance, while magnetometer data from Mars Global Surveyor revealed remnant crustal magnetism distinct from the global dynamo observed at Earth.

Geology and surface features

Mars hosts the largest volcano in the Solar System, Olympus Mons, and a canyon system, Valles Marineris, mapped in detail by MOLA aboard Mars Global Surveyor. Basin structures such as Hellas Planitia and valley networks studied by Viking and Mars Reconnaissance Orbiter indicate a complex erosional and impact history comparable to terrains examined by the Lunar Reconnaissance Orbiter on the Moon. Sedimentary deposits, sulfate minerals, and phyllosilicates detected by CRISM and APXS point to aqueous alteration episodes investigated in field analog programs at Atacama Desert, Antarctic Dry Valleys, and Iceland.

Atmosphere and climate

Mars's atmosphere is thin and cold, composed mainly of carbon dioxide with traces of nitrogen and argon; its climate has been reconstructed using data from Mars Climate Sounder, Thermal Emission Spectrometer, and meteorological stations on Viking and InSight. Seasonal carbon dioxide ice deposition at the poles produces caps observed by MRO and Mars Express, and planet‑encircling dust storms documented since telescopic records by observers like Giovanni Schiaparelli affect surface operations in missions such as Opportunity and Spirit. Isotopic measurements from SAM on Curiosity and from returned meteorite studies inform models connecting Mars's atmospheric loss to interactions with the solar wind measured by MAVEN.

Moons and ring potential

Mars's two small moons, Phobos and Deimos, were discovered by Asaph Hall and characterized by close imaging from Viking and Mars Reconnaissance Orbiter; their irregular shapes and composition link to hypotheses involving captured asteroids or reaccumulated debris from giant impacts studied in simulations by research groups at institutions like Caltech and MIT. Tidal decay of Phobos predicts eventual disruption or surface impact, a process analogous to tidal interactions cataloged in studies of Roche limit phenomena and discussed in planning by agencies including JAXA and ISRO for sample and rendezvous missions. Transient ring material has been proposed and modeled in numerical studies by the ESO and dynamical researchers at University of California, Berkeley.

Exploration and missions

Mars has been the target of flybys, orbiters, landers, and rovers launched by national and international programs: early flybys by Mariner program, orbiters like Mars Reconnaissance Orbiter and Mars Express, landers including Viking and Phoenix, and rovers such as Sojourner, Spirit, Opportunity, Curiosity, and Perseverance. Recent missions include sample‑cache operations coordinated with planned retrieval campaigns by NASA and ESA and orbital communications support from satellites like MRO and Mars Odyssey. Commercial and international initiatives by companies and agencies including SpaceX, CNSA, and Roscosmos are shaping architectures for cargo, crewed transit, and in situ resource utilization concepts tested in analog programs like Mars Desert Research Station.

Potential for life and human exploration

Evidence for past habitable conditions—river deltas, lakebeds, organic molecules detected by SAM and Mastcam‑Z, and recurring slope lineae debates analyzed by HiRISE—drives astrobiology investigations involving teams affiliated with NASA Astrobiology Institute, SETI Institute, and major universities such as Caltech and MIT. Planetary protection policies developed by COSPAR and the Committee on Space Research govern sample return planning with stakeholders including NASA and ESA. Human exploration concepts by NASA's Artemis program partners, private ventures like SpaceX, and national agencies outline architectures for surface habitats, in‑situ resource utilization (water extraction, oxygen production) and radiation protection informed by research at facilities like Johnson Space Center and European Space Agency centers. Ethical, legal, and logistical frameworks for habitation are debated in forums including the United Nations Office for Outer Space Affairs and the International Institute of Space Law.

Category:Terrestrial planets