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Two moons of Mars discovered at the same time 150 years ago
It is predicted that in 50 million years, Mars will become a planet with rings like Saturn.
The source of the ring’s material is Phobos, the largest of Mars’ two moons. The moon, which is 23 km in diameter and orbits Mars every 8 hours, is currently spiraling down to Mars at a rate of 1.8 m every 100 years. Phobos’ orbit is about 5,800 km above the surface of Mars, which is very close to its parent planet compared to our moon’s 400,000 km.
Phobos orbiting at such a close distance is gradually being drawn to Mars as it is constantly subjected to tidal forces due to the gravity of its parent planet Mars. Thus, after about 50 million years, Phobos is expected to be destroyed and the broken down fragments will become a ring around Mars.
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The eight planets of the solar system are divided into the four rock planets Mercury, Venus, Earth, and Mars, and the four gas planets of Jupiter, Saturn, Uranus, and Neptune. One, Mars has two. Outside Mars is the asteroid belt, a region of asteroids.
Phobos and Deimos are lumpy like potatoes, more like asteroids than moons. Gravity determines the shape of a celestial body, and for a celestial body to be round like a ball, it must be at least 250 km in diameter.
Two small moons orbiting this red planet, Phobos and Deimos, are cosmic rocks that hold many secrets about the formation of the early solar system. The secrets of their birth are not yet known, but it is believed that they flowed from the asteroid belt between Mars and Jupiter and were captured by Mars’ gravity. Or it could be an asteroid originating farther away in our solar system.
The above picture taken by NASA’s Mars Reconnaissance Orbiter (MRO) clearly captures numerous craters covering Phobos, which looks like an asteroid, with a resolution of about 10m.
Phobos and Deimos were discovered by Asaf Hollo, a high school student at the U.S. Naval Observatory, who discovered the two satellites several days apart in August 1877. This is the first record of discovering an extraterrestrial moon in about 250 years since Galileo Galilei, who discovered Jupiter’s four major moons with his own telescope in 1610.
The two moons were named Phobos (fear) and Deimos (defeat), the two sons of Ares, the god of war in Greek mythology.
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Two moons on Mars with different fates
Phobos, like Earth’s moon, has the same rotation period and orbit period, so it always faces the same side with respect to Mars. Phobos, which rotates with an orbital period of 7 hours and 40 minutes, orbits faster than the rotation speed of Mars, so when viewed from the surface of Mars, it rises in the west and sets in the east.
On Mars, Phobos does not look like Earth’s moon. The farther moon, Deimos, looks like a star in the night sky. When it shines brightest at full moon, it resembles Venus seen on Earth.
The most accepted theory is that Deimos was originally in the asteroid belt together with Forbes, but was captured by Mars passing by after jumping out of the belt with the powerful gravitational pull of Jupiter. Deimos has the longest axis toward Mars, so its rotation period and orbit period coincide. Deimos’s surface is gray and very dark, and its average density (less than 2 g/cm3) is low, indicating that Deimos is made of carbonaceous material, suggesting that it may be an asteroid orbiting in space and caught by Mars’ gravitational pull.
Deimos, which is in an outer orbit about 23,000 km from Mars, is gradually moving away from Mars as opposed to Phobos.
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For reference, the Earth’s moon is also moving away at a rate of 3.8 cm every year, so scientists predict that it will separate from the Earth in 1 billion years. The reciprocal theorem is also a law of the universe.
In 2024, the Japan Aerospace Exploration Agency (JAXA) plans to launch the Mars Moons eXploration (MMX) project to visit Mars moons. The MMX is expected to return to Earth in 2029 after landing on Phobos’ surface and taking samples.
Kwang-Sik Lee, Science Columnist [email protected]