This evening the planet Mars will be 38.6 million miles (or 62.1 million kilometers) away from Earth, the two planets closest approach in 15 years. The closest possible encounter between Mars and Earth happens when Earth is furthest away from the Sun and Mars is closest to the Sun. Earth’s previous closest approach with Mars was recorded in 2003 when Mars was 34.6 million miles (55.7 million km) away. The next time Earth and Mars will be that close is in September 2035.
Mars rises shortly after sunset and is recognizable as a bright, red dot in the night sky. Thanks to a good telescope, it is even possible to spot Mons Olympus on the Martian surface, the tallest volcano in our solar system. It isn’t easy to see the mountain itself, but you can sometimes see white clouds of carbon-dioxide wrapped around its summit caldera. The telescopic feature is called Nix Olympica – the Snows of Olympus, named so by Italian astronomer Giovanni Schiaparelli in 1877. Another clue revealing the volcano is the shadow cast by the mountain.
A video tutorial posted by British chemist and Youtuber Phil Mason shows how it is possible to observe Mons Olympus in October 2020 under the right weather conditions and equipment.
In the year 1971, NASA sent the Mariner 9 spacecraft to orbit around the planet Mars. Upon the arrival of the space-probe, the planetary surface was hidden beneath a global dust storm. As the scientists waited for the dust to settle, the peak of a mountain appeared above the dust clouds. It wasn’t just a mountain, but rather the caldera of the tallest volcano in the entire solar system. This volcano, which is twice as large as Mount Everest, is roughly the size as the state of Arizona.
Olympus Mons is the largest of the volcanoes in the Tharsis Montes region. It is a shield volcano 374 miles (or 624 km) in diameter, and 16 miles (25 km) high. A caldera 50 miles (80 km) wide is located at the summit of Olympus Mons.
To compare, the largest active volcano on Earth is Mauna Loa, a shield volcano 6.3 miles (10 km) high, 75 miles (120 km) across, and with a caldera 3 miles (5 km) wide. The volume of Olympus Mons is about 100 times larger than that of Mauna Loa, and 5o times larger than the volume of Earth’s largest volcanic edifice, Pūhāhonu in the Pacific Ocean. The largest caldera on Earth is believed to be the Apolaki structure, a circular depression with a diameter of approximately 93 miles (150 km) discovered in 2019 in the Philippine Sea. The famous caldera of Yellowstone in Wyoming is only about 37 miles (60 km) wide.
Like the Earth, Mars is a differentiated body with a crust, mantle and core. Large blobs of hot material rise upwards in the silicate mantle, driving plate tectonics and feeding inter-plate volcanism over so-called hotspots on Earth, like in case of Hawai’i. Unlike the Earth, Mars is not thought to have experienced significant plate tectonics. But Mars exhibits hotspot-style volcanism that is similar to inter-plate volcanism.
On Earth, a tectonic plate will move over a hotspot, and sooner or later, the volcano feed by the hotspot will be cut off from the magma reservoir. This limits the size a volcano, as it can grow only over a limited time-span ranging from a few 1,000 to 100,000 years. On Mars, the crust remains stationary, and a volcano will be feed by the rising magma blob for millions of years. This style of low-viscosity, effusive volcanism dominates Mars, and has resulted in the development of massive shield volcanoes and individual flows that are over 600 miles (1.000 km) long.