Several years ago, the American automatic interplanetary station discovered this strange vertically protruding object on Phobos, and it is still unknown what it is.
"When people find out about this, they will ask themselves: who left it there?"
These words were spoken in 2009 by Buzz Aldrin, the second astronaut in history to set foot on the lunar surface.
Aldrin was referring to a strange, regular-shaped object, the so-called monolith, found on the surface of the Martian satellite Phobos.
And he was right: the mysterious monolith has haunted many since then. Many theories have emerged linking this object with alien activity; A music album released this year by Les Claypool and Sean Lennon is even named after him.
So what are we dealing with?
Phobos is one of the two satellites of Mars
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The monolith, as scientists believe, is a large stone block, reaching, according to some estimates, 90 m in height. It is located in the center of a desert plain, which makes it especially visible.
But the monolith on Phobos is not a unique object. There is something similar on the surface of Mars.
The scientific community does not consider such phenomena as evidence of the existence of an advanced extraterrestrial civilization.
The Martian monolith, according to scientists, is almost certainly a stone fragment of relatively regular shape, rolling down from a nearby mountain.
Other unusual objects on Mars, such as the "floating spoon" or the "Egyptian pyramid," may have been caused by natural erosion.
And the famous "face" on the Martian surface will cease to seem creepy if you look closely at it or look at it from a different angle.
But the monolith on Phobos has not yet been studied so thoroughly.
Martian Monolith
Perhaps this is just a piece of rock formed as a result of a meteorite impact - there is evidence of periodic asteroid activity on Phobos.
Another possible explanation is that such debris falls on Phobos from the surface of Mars, which from time to time is bombarded by asteroids.
Some calculations indicate that of every million debris dotting Phobos' surface, about 250 or more are of Martian origin.
However, it is possible that the monolith on Phobos is not related to asteroids. It may be part of the solid rock of the satellite itself, peeking through the layer of rock debris.
This version was presented several years ago by planetary scientists who discussed the possibility of sending an expedition to Phobos.
If the hypothesis is confirmed, perhaps the monolith will give scientists an answer to the question of the origin of the Martian satellite.
In this case, the value of the monolith will increase significantly. According to a number of scientists, Phobos is the most mysterious celestial body in the solar system.
Two small moons orbit around Mars - Phobos and Deimos
Phobos is one of two small satellites orbiting Mars. Another is named Deimos. Scientists do not fully understand how these two objects were formed.
Both satellites are small and irregular in shape; they outwardly look like asteroids, which once in the distant past were captured by the gravitational field of Mars. However, the peculiarities of their orbits refute this hypothesis.
Another explanation is that both moons were formed billions of years ago from the same material as Mars itself. But accurate astronomical measurements show that Phobos' density is significantly lower than that of a typical Martian rock.
The third version remains: a long time ago, Mars experienced a destructive collision with a large protoplanet, as a result of which its two satellites were formed.
A similar hypothesis explains the origin of the moon. But Phobos and Deimos are much smaller than the Moon. As a result of the collision, Mars was supposed to have a much larger satellite.
New answer choices are contained in two scientific papers published in 2016.
Monolith protruding from the surface of Phobos
Both studies support the collision theory. One of the works contains additional calculations indicating the non-asteroid origin of Phobos and Deimos, and the second explains in detail why Mars now has only two small satellites.
According to this version, as a result of the collision, Mars did indeed have a large satellite.
Thanks to its gravitational interaction with Mars, several smaller satellites have formed from the remaining space debris, including Phobos and Deimos.
Gradually, the height of the orbits of the large satellite and all the others - except Phobos and Deimos - decreased until they collapsed and fell on the Martian surface in the form of debris. Thus, Mars has only two of its present moons.
This hypothesis could be confirmed or refuted by the proposed expedition to Phobos, the purpose of which should be to analyze the composition of this celestial body.
In 2015, the US aerospace agency NASA suggested that Phobos is gradually collapsing, as evidenced by the furrows on its surface.
However, there is plenty of time to fly to Phobos: according to scientists, in the next 30-50 million years, it will not be destroyed.