Mars ‘two moons, Phobos and Deimos, have amazed scientists since their discovery in 1877. They are very small: Phobos’ diameter of 22 kilometers is 160 times smaller than that of our moon, and Deimos is even smaller with a diameter of only 12 kilometers. “Our moon is essentially spherical, while the moons on Mars are very irregularly shaped – like potatoes,” says Amirhossein Bagheri, a doctoral student at the Department of Geophysics at ETH Zurich, adding: “Phobos and Deimos look more like asteroids than natural moons.”
This led people to suspect that they might actually be asteroids caught in Mars’ gravitational field. “But it was here that the problems started,” says Bagheri. Captured objects are expected to follow an eccentric orbit around the planet, and that orbit will have a random slope. Contrary to this hypothesis, the orbits of Mars are almost circular and move in the equatorial plane of Mars. So what is the explanation for the current courses in Phobos and Deimos? To solve this dynamic problem, the researchers relied on computer simulations.
Calculation of the past
“The idea was to trace the circuits and their changes back in the past,” says Amir Khan, senior researcher at the Institute of Physics at the University of Zurich and the Institute of Geophysics at the ETH Zurich. As it turned out, the tracks to Phobos and Deimos seemed to have crossed earlier. “This means that the moons were very likely the same place and therefore have the same origin,” says Khan. The scientists concluded that a larger celestial body orbited Mars at the time. This original moon was probably hit by another body and dissolved as a result. “Phobos and Deimos are the rest of this lost moon,” said Bagheri, lead author of the study, which is now published in the journal. Natural astronomy.
These conclusions were easy to follow but required extensive initial work. First, scientists had to refine the existing theory describing the interaction between the moons and Mars. “All celestial bodies exert tidal forces on each other,” Khan explains. These forces lead to a form of energy conversion known as scattering, the scale of which depends on the size of the bodies, their internal composition and not least the distance between them.
Insight into the interior of Mars and its moons
Mars is currently being explored by NASA’s InSight mission with ETH Zurich’s involvement: The electronics for the mission’s seismometer, which records marsquakes and possibly meteorite impacts, were built on ETH. “These footage lets us see inside the red planet,” Khan says, “and this data is used to limit the Mars model in our calculations and the scattering that occurs inside the red planet.”
Images and measurements from other Mars probes have suggested that Phobos and Deimos are made of highly porous material. At less than 2 grams per. Cubic centimeter, its density is much lower than the average density of the earth, which is 5.5 grams per. Cubic centimeters. “There are a lot of cavities inside Phobos that can contain water ice,” Khan suspects, “and this is where the tide causes a lot of energy to spread.”
Using these findings and their refined theory of tidal effects, the researchers ran hundreds of computer simulations to trace the lunar orbit back in time until they reached the intersection – the moment Phobos and Deimos were born. Depending on the simulation, this time is between 1 and 2.7 billion years earlier. “The exact time depends on the physical properties of Phobos and Deimos, ie how porous they are,” says Bagheri. A Japanese probe scheduled for launch in 2025 will explore Phobos and return samples to Earth. Scientists expect that these samples will provide the necessary details about the interior of the Martian moons, enabling more accurate calculations of their origin.
The end of Phobos
Another thing that their calculations show is that the common ancestor of Phobos and Deimos was further away from Mars than Phobos is today. While the smaller Deimos have remained near the place where they were created, tidal forces cause the larger Phobos to approach Mars – and this process is underway, as scientists explain. Their computer simulations also show the future evolution of the lunar orbits. Deimos appears to be moving very slowly away from Mars, just as our moon is slowly retreating from Earth. However, Phobos will collide with Mars in less than 40 million years or be torn apart by gravity as it approaches Mars.
The orbit of Mars’ moon suggests an ancient ring of Mars
Amirhossein Bagheri et al. Dynamic evidence for Phobos and Deimos as remnants of a disturbed common ancestor, Natural astronomy (2021). DOI: 10.1038 / s41550-021-01306-2
Citation: Mars moons have a common ancestor (2021, February 23) retrieved February 23, 2021 from https://phys.org/news/2021-02-martian-moons-common-ancestor.html
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