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Mars is the fourth solar system planet from the Sun and the second smallest. Characteristics:
It has two small moons, Phobos and Deimos both smaller than 25 km in diameter. There has been interest in whether Mars might harbor life, and the fact that it could be within the Sun's habitable zone makes the idea plausible. But the average surface temperature is on the order of -70 centigrade, well below water's triple point making surface liquid water unlikely, and water in Martian ground is generally locked in permafrost, making a location suitable for life unlikely. There is evidence that Mars's surface once had water, such as apparent shorelines and river beds, and it is generally accepted that Mars once had considerable water, though that may not have lasted very long compared to the age of the solar system. To have had water, Mars necessarily once had a considerably higher temperature, presumably requiring a greenhouse effect, which in turn would require a thicker atmosphere in the past, including sufficient greenhouse gas(es). Mars's axial tilt cycles over a wider range than Earth's, largely due to its lack of such influence as the Moon has on Earth's. This results in Mars's more pronounced climate changes over the long term.
Far more space missions have been sent to Mars than to any other planet, more than 40, beginning in the 1960s with the NASA Mariner and Viking programs and the Soviet Mars program; subsequent missions include orbiters (e.g., NASA's Mars Global Surveyor (MGS) and Mars Reconnaissance Orbiter (MRO)), landers (e.g., NASA's Phoenix), rovers (e.g., NASA's Spirit, Opportunity, and Curiosity), and flybys of other missions for gravity assists. From the orbiting and landing probes, much of Mars' geography and geology have been mapped and investigated. Formations include the huge volcano, Olympus Mons, taller than any mountain on Earth, and the Medusae Fossae Formation (MFF), a large region of flat lowlands that appears to be a lava deposit. Recently-launched missions include three using the 2020 Mars launch window and arriving in February 2021: NASA's Mars 2020, China's Tianwen-1, and the United Arab Emirates's Hope Probe. Future missions and proposals also include missions by the EU, Japan, and other countries, including the Rosalind Franklin rover mission by the European Space Agency (ESA), which was originally a collaboration with Russia.
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