In late October, the European Space Agency's BepiColombo mission headed for Mercury, the least explored planet in the solar system. The abnormal structure of this celestial body gave rise to many hypotheses about the origin. Glaciers hidden in craters give hope for the discovery of traces of life. What mysteries of Mercury scientists hope to reveal.
Forgotten planet
When in 1975 the first spacecraft Mariner 10 sent to Mercury transmitted images to Earth, scientists saw the familiar "lunar" surface, dotted with craters. Because of this, interest in the planet died out for a long time.
Terrestrial astronomy also does not favor Mercury. Due to the proximity of the Sun, it is difficult to examine the details of the surface. The Hubble Orbital Telescope must not be aimed at it - sunlight can damage the optics.
Bypassed by Mercury and direct observation. Only two probes were launched to it, to Mars - several dozen. The last expedition ended in 2015 with the fall of the Messenger spacecraft on the planet's surface after two years of work in its orbit.
Through maneuvers - to Mercury
Promotional video:
There is no technology on Earth to send an apparatus to this planet directly - it will inevitably fall into a gravitational funnel created by the gravitational force of the Sun. To avoid this, you need to correct the trajectory and slow down due to gravitational maneuvers - approaching the planets. Because of this, the journey to Mercury takes several years. For comparison: to Mars - several months.
The Bepi Colombo mission will conduct the first gravity assist near Earth in April 2020. Then - two maneuvers near Venus and six at Mercury. Seven years later, in December 2025, the probe will take its calculated position in the planet's orbit, where it will operate for about a year.
"Bepi Colombo" consists of two devices developed by European and Japanese scientists. They carry with them a variety of equipment for remotely studying the planet. Three spectrometers were created at the Space Research Institute of the Russian Academy of Sciences - MGNS, PHEBUS and MSASI. They will obtain data on the composition of the planet's surface, its gas envelope, and the existence of the ionosphere.
A drop of iron inside
Mercury has been studied for centuries and even before the advent of modern astronomy, its parameters were calculated quite accurately. However, it was not possible to explain the anomalous motion of the planet around the Sun from the point of view of classical mechanics. Only at the beginning of the 20th century was this done using the theory of relativity, taking into account the distortion of space-time near the star.
The movement of Mercury served as proof of the hypothesis of the expansion of the solar system due to the fact that the star is losing matter. This is evidenced by the analysis of the data of the Messenger mission.
The fact that Mercury is different from the Moon, astronomers suspected even after the passage of "Mariner 10" past it. Studying the deviation of the trajectory of the apparatus in the planet's gravitational field, scientists concluded that its high density. The noticeable magnetic field was also embarrassing. Mars and Venus do not have it.
These facts indicated that there was a lot of iron inside Mercury, probably liquid. The photographs of the surface, on the contrary, spoke of some light substances such as silicates. There are no iron oxides like there are on Earth.
The question arose: why, in four billion years, the metal core of a small planet, more reminiscent of someone's satellite, did not solidify?
Analysis of the Messenger data showed that there is an increased sulfur content on the surface of Mercury. Perhaps this element is present in the core and prevents it from solidifying. It is assumed that the liquid is only the outer layer of the core, about 90 kilometers, but inside it is solid. It is separated from the Mercurian crust by four hundred kilometers of silicate minerals that form a solid crystalline mantle.
The entire iron core occupies 83 percent of the planet's radius. Scientists agree that this is the reason for the 3: 2 spin-orbital resonance that has no analogues in the solar system - for two revolutions around the sun, the planet turns around its axis three times.
The passage of Mercury across the solar disk on May 9, 2016.
Where does the ice come from?
Mercury is actively bombarded by meteorites. In the absence of atmosphere, winds and rain, the relief remains intact. The largest crater - Caloris - with a diameter of 1,300 kilometers, was formed about three and a half billion years ago and is still clearly visible.
The blow that formed Caloris was so powerful that it left marks on the opposite side of the planet. Molten magma flooded vast areas.
Despite the craters, the planet's landscape is fairly flat. It is formed mainly by the erupted lavas, which speaks of the turbulent geological youth of Mercury. Lava forms a thin silicate crust, which bursts due to the drying out of the planet, and cracks appear on the surface hundreds of kilometers long - scarps.
The tilt of the planet's axis of rotation is such that the interior of the craters in the north polar region is never illuminated by the sun. In the images, these areas look unusually bright, which gives reason to scientists to suspect the presence of ice there.
If it is water ice, then comets could carry it. There is a version that this is primary water, which remained from the time of the formation of planets from the proto-cloud of the solar system. But why hasn't it evaporated so far?
Scientists are still inclined to the version that ice is associated with evaporation from the interior of the planet. The regolith layer on top does not allow rapid drying (sublimation) of ice.
Caloris Crater, or the Sea of Heat, - one of the largest shock landforms on the planet.
Sodium clouds
If Mercury once had a full-fledged atmosphere, then the Sun killed it long ago. Without it, the planet is subject to sharp temperature changes: from minus 190 degrees Celsius to plus 430.
Mercury is surrounded by a very rarefied gas shell - an exosphere of elements knocked out from the surface by solar showers and meteorites. These are atoms of helium, oxygen, hydrogen, aluminum, magnesium, iron, light elements.
Sodium atoms from time to time form clouds in the exosphere, living for several days. Meteorite strikes cannot explain their nature. Then sodium clouds would be observed with equal probability over the entire surface, but this is not the case.
For example, a peak in sodium concentration was found in July 2008 with the THEMIS telescope in the Canary Islands. Emissions occurred in mid-latitudes only in the southern and northern hemisphere.
According to one version, sodium atoms are knocked out of the surface by a proton wind. It is possible that it accumulates on the night side of the planet, creating a kind of reservoir. At dawn, sodium is released and rises.
Blow, another blow
There are dozens of hypotheses about the origin of Mercury. It is still impossible to reduce their number due to lack of information. According to one version, proto-Mercury, which at the beginning of its existence was twice the size of the current planet, collided with a smaller body. Computer simulations show that an iron core could have formed as a result of the impact. The catastrophe led to the release of thermal energy, the separation of the planet's mantle, the evaporation of volatile and light elements. Alternatively, in a collision, proto-Mercury could be a small body, and a large one was proto-Venus.
According to another assumption, the Sun was initially so hot that it vaporized the mantle of young Mercury, leaving only an iron core.
The most confirmed is the hypothesis that the proto-cloud of gas and dust, in which the rudiments of the planets of the solar system matured, turned out to be heterogeneous. For unknown reasons, the part of the substance close to the Sun was enriched with iron, and thus Mercury was formed. A similar mechanism is indicated by information about exoplanets of the "super-earth" type.
Both Bepi Colombo vehicles are orbiting. Earthlings do not yet have the technology that can deliver a rover to Mercury and land on its surface. Nevertheless, scientists are confident that the mission will shed light on many of the planet's mysteries and the evolution of the solar system.
Two apparatus “ Bepi Colombo ” approaching Mercury.
Tatiana Pichugina