August 17, 1877: Asaph Hall discovers Phobos
On August 17, 147 years ago, the American astronomer Asaf Hall He discovered the first known moon of our neighboring planet, Mars. Later that year, he discovered a second Martian moon. Today we call the first and larger one Phobos. And the second and smaller one, Deimos.
To date, Phobos and Deimos remain the only known moons of Mars.
Both Phobos and Deimos are potato-shaped. They look more like asteroids than Earth's much larger companion moon. In fact, Mars likely captured these tiny worlds that are now orbiting the Red Planet. Studies have indicated that within millions of years, Phobos will break up and form a ring around Mars. Some astronomers believe that Phobos alternates between being a planetary ring and then re-clumping together to form a moon. Learn more about Phobos ring theories below.
Phobos and Deimos, fear and terror
The names Phobos and Deimos mean fear and terrorrespectively. Their names refer to the mythological horses that pulled the chariot of the Greek god of war Ares. He was the Greek counterpart of the Roman god of war Mars.
Although it is the larger of Mars' two moons, Phobos is tiny. It has a mean diameter of just 22.2 km, but it is seven times more massive than Deimos, which has a mean diameter of 12.4 km. In the case of these moons, we speak in terms of mean diameter because they are both oblong, not spherical. In contrast, Earth's moon is nearly round, as it is large enough that its gravity has pulled it into a spherical shape.
Earth's moon is also much larger (2,159 miles or 3,475 kilometers across). Because both Phobos and Deimos are so small, they have very weak gravity. And that means they don't have enough gravity to make them round.
Mariner 9 was the first to see them nearby.
During the Mariner 9 mission to Mars in 1971 and 1972, scientists were able to observe Phobos up close for the first time. Like asteroids, its oblong surface displays many small craters. But one of them stood out because it was much larger than the rest. Astronomers named it Stickney Crater, after its size. Angeline Stickney: American academic, suffragist, and mathematician, wife of Asaph Hall.
Stickney looks like a giant hole at one end of Phobos. The rocky body that created it was almost big enough to have smashed the moon into pieces. When the impact occurred, Phobos is thought to have barely survived.
Was Phobos ever a ring? Will it be one again?
Phobos has long, shallow grooves. running across its surfaceradiating from Stickney. Many planetary scientists believe these grooves are early signs of possible structural failure on the moon. They say it's possible that within 50 million years, Phobos could break up and form a ring around Mars.
TO study In 2018 it was suggested that rolling rocks created the grooves, spreading across the surface during the impact. Ken RamsleyBrown University planetary science researcher who led the work, explained:
These grooves are a distinctive feature of Phobos, and planetary scientists have been debating how they formed for 40 years. We believe this study is another step toward an explanation.
More theories about the rings
In 2017, a new theory from scientists at Purdue University suggested that Phobos could not only break up and form a ring around the planet, but also suggests that this ring formation happened earlier.
David Mintona teacher, and Andres Hesselbrockstudent, both at Purdue, developed a computer model showing debris ejected into space from an asteroid or other body crashing into Mars. This event, about 4.3 billion years ago, would cause the material to alternate between becoming a planetary ring and re-clumping together to form the moon Phobos.
Daily Owl #Made
One day #Mars will it have a ring around it?
Over the next 20 to 40 million years, Mars' largest moon, Phobos, will be torn apart by gravitational forces, creating a ring that could last up to 100 million years.#NFT #Moon #Space image.twitter.com/IIYHCkgDhf
—Mars Birds? (@mars_birds) July 12, 2022
Deimos played a role
Another studyA study by Purdue scientists and the SETI Institute in June 2020 also concluded that Mars used to have a ring or series of rings. The scientists based that study on an analysis of the orbit of the other Martian moon, Deimos.
Deimos is smaller than Phobos and has an orbit inclined to Mars' equator by about 2 degrees. Meanwhile, Phobos' orbit is not as inclined. The larger moon is tilted to Mars' equator by only about 1 degree. The scientists said in a statement. statement:
These orbital resonances are delicate but predictable… We can say that only an outward-moving moon could have strongly affected Deimos, meaning that Mars must have had a ring pushing the inner moon outward… This moon may have been 20 times more massive than Phobos, and may have been its 'grandparent' that existed just over 3 billion years ago… (It was) followed by two more ring-moon cycles, the last moon being Phobos.
Basically, a moon about 20 times more massive than Phobos may have once existed, and Mars' rings pushed it outward. And at least twice since then, that moon disintegrated and then formed a new ring, before the material came back together to form a new moon. Phobos is now that current moon. Scientists say it will eventually disintegrate to form a new ring, thus continuing the cycle.
Scientists also now know that Phobos is much younger than Deimos (perhaps only 200 million years old), which would fit the moon-and-ring scenario. The Phobos we see today is simply a newer, smaller version of its original form.
Eclipsed Moons
Asaph Hall probably never imagined the idea of Phobos breaking up and forming a ring around Mars. And he couldn't have imagined the video below, which was captured by NASA's Mars rover, either. Curiosity August 1, 2013. This video shows both moons, Phobos and Deimos, as you would see them from the surface of Mars. In these images you can clearly see some of the large craters on Phobos.
This was the first time that images taken from the surface of Mars captured one moon eclipsing another… but it probably won’t be the last.
In summary: On this date in 1877, American astronomer Asaph Hall discovered Phobos, the larger of Mars' two moons. Later that year he discovered the other moon, Deimos.
Leave feedback about this