The surface of Mercury, in short, resembles the moon. Vast plains and many craters suggest that geological activity on the planet ceased billions of years ago.
Surface character
The surface of Mercury (photo further in the article), taken by the Mariner-10 and Messenger probes, looked like a moonlight. The planet is largely dotted with craters of various sizes. The smallest of the visible ones in the most detailed photographs of the Mariner are measured several hundred meters in diameter. The space between the large craters is relatively flat and consists of plains. It looks like the surface of the moon, but takes up much more space. Such areas surround the most noticeable shock structure of Mercury, formed as a result of the collision - the basin of the Plain of Heat (Caloris Planitia). When meeting with Mariner-10, only half of it was illuminated, and it was completely opened by the Messenger during its first flight past the planet in January 2008.
Craters
The most common relief structures on the planet are craters. They largely cover the surface of Mercury. The planet (photos are given below) at first glance looks like the moon, but upon closer examination, they reveal interesting differences.
Gravity on Mercury is more than double that of the moon, partly due to the high density of its huge core, consisting of iron and sulfur. Greater gravity tends to keep the substance ejected from the crater near the collision site. Compared to the moon, it fell at a distance of only 65% of the moon. This may be one of the factors that contributed to the emergence on the planet of secondary craters formed under the influence of discarded material, in contrast to the primary ones that arose directly in a collision with an asteroid or comet. Higher gravity means that the complex shapes and designs characteristic of large craters - central peaks, steep slopes and a flat base - are observed on smaller craters on Mercury (minimum diameter of about 10 km) than on the Moon (about 19 km). Structures smaller than these sizes have simple cup-like outlines. Mercury craters are different from Martian craters, although the two planets have comparable gravity. Fresh craters on the first, as a rule, are deeper than proportional formations on the second. This may be due to a low content of volatile substances in the crust of Mercury or higher impact velocities (since the speed of an object in solar orbit increases as it approaches the Sun).
Craters more than 100 km in diameter begin to approach the oval shape characteristic of such large formations. These structures - polycyclic basins - are 300 km or more in size and are the result of the most powerful collisions. Several dozen of them were found on the photographed part of the planet. Messenger images and laser altimetry have contributed greatly to understanding these residual scars from the early asteroid bombardments of Mercury.
Heat Plain
This shock structure extends for 1550 km. When it was first discovered by Mariner-10, it was believed that its size was much smaller. The internal space of the object is smooth plains, covered with folded and broken concentric circles. The largest ranges extend several hundred kilometers in length, about 3 km in width and less than 300 meters in height. More than 200 fractures, comparable in size to the edges, come from the center of the plain; many of them are hollows limited by furrows (grabens). Where grabens intersect with ridges, they usually pass through them, which indicates their later formation.
Surface types
The Heat Plain is surrounded by two types of terrain - its edge and the relief formed by the discarded rock. The edge is a ring of irregular mountain blocks reaching 3 km in height, which are the highest mountains found on the planet, with relatively steep slopes towards the center. The second much smaller ring is 100-150 km from the first. Beyond the outer slopes there is a zone of linear radial ridges and valleys, partially filled with plains, some of which are dotted with numerous mounds and hills several hundred meters. The origin of the formations that make up the wide rings around the Heat basin is controversial. Some plains on the Moon were formed mainly as a result of the interaction of emissions with an existing surface topography, and this may also be true for Mercury. But the results of the “Messenger” suggest that volcanic activity played a significant role in their formation. There are not only few craters there, compared to the Zhary basin, which indicates a long period of formation of the plains, but they have other features more clearly associated with volcanism than could be seen in the images obtained by Mariner-10. The decisive evidence of volcanism was obtained using images of the Messenger, showing the vents of volcanoes, many of which are located along the outer edge of the Zhara plain.
Raditladi Crater
Caloris is one of the youngest large polycyclic plains, at least in the investigated part of Mercury. It probably formed at the same time as the last giant structure on the moon - about 3.9 billion years ago. Images of the Messenger revealed yet another, much smaller impact crater with a visible inner ring, which could have formed much later, called the Raditladi Basin.
Strange antipode
On the other side of the planet, exactly 180 ° opposite the Zhary plain, there is a stretch of strangely distorted terrain. Scientists interpret this fact, talking about their simultaneous formation by focusing seismic waves from events that affected the antipodal surface of Mercury. The hilly and dotted terrain is a vast zone of hills representing hilly polygons 5-10 km wide and up to 1.5 km high. The craters that existed before were turned into hills and cracks by seismic processes, as a result of which this relief was formed. For some of them, the bottom was flat, but then its shape changed, which indicates their later filling.
The plains
A plain is a relatively flat or smoothly wavy surface of Mercury, Venus, Earth and Mars, which is found everywhere on these planets. It is a "canvas" on which the landscape developed. The plains are evidence of the destruction of the rough terrain and the creation of a smoothed space.
There are at least three ways of “polishing”, due to which, probably, the surface of Mercury was leveled.
One way - raising the temperature - reduces the strength of the crust and its ability to maintain a high relief. For millions of years, the mountains are “drowning”, the bottom of the craters rises and the surface of Mercury is leveled.
The second method involves moving rocks toward lower terrain under the influence of gravity. Over time, the breed accumulates in the lowlands and fills higher levels as its volume increases. thus, lava flows from the bowels of the planet behave.
The third method consists in hitting rock fragments on the surface of Mercury from above, which ultimately leads to the alignment of the rough relief. Examples of this mechanism are rock emissions from crater formation and volcanic ash.
Volcanic activity
Some evidence suggesting a hypothesis about the effect of volcanic activity on the formation of many plains surrounding the Zhary basin has already been given. Other relatively young plains on Mercury, especially noticeable in regions illuminated at a slight angle during the first flyby of the Messenger, demonstrate the characteristic features of volcanism. For example, several old craters were filled to the brim with lava flows, similar to the same formations on the Moon and Mars. However, the widespread plains on Mercury are more difficult to evaluate. Since they are older, it is obvious that volcanoes and other volcanic formations could have eroded or destroyed otherwise, making them difficult to explain. Understanding these old plains is important because they are likely involved in the extinction of most of the craters 10–30 km in diameter compared to the Moon.
Escarp
The most important forms of relief of Mercury, which allow you to get an idea of the internal structure of the planet, are hundreds of serrated ledges. The length of these rocks varies from tens to more than thousands of kilometers, and the height - from 100 m to 3 km. When viewed from above, their edges appear rounded or jagged. It is clear that this is the result of cracking, when part of the soil rose and lay on the adjacent terrain. On Earth, such structures are limited in volume and arise with local horizontal compression in the earth's crust. But the entire investigated surface of Mercury is covered with scarp, which implies that the planet’s crust has decreased in the past. From the number and geometry of the Scarp, it follows that the planet decreased in diameter by 3 km.
In addition, shrinkage must have continued until relatively recent time in geological history, as some escarpes changed the shape of well-preserved (and, therefore, relatively young) impact craters. The deceleration of the initially high planet rotation speed by tidal forces produced compression in the equatorial latitudes of Mercury. Scarps globally distributed, however, suggest a different explanation: later cooling of the mantle, possibly in combination with the hardening of part of the once completely molten core, led to compression of the core and deformation of the cold crust. The reduction in the size of Mercury during cooling of its mantle should have led to more longitudinal structures than can be seen, which indicates the incompleteness of the compression process.
Mercury surface: what is it made of?
Scientists tried to find out the composition of the planet by examining the sunlight reflected from different parts of it. One of the differences between Mercury and the Moon, in addition to the fact that the first is slightly darker, is that its surface brightness spectrum is smaller. For example, the seas of the Earth’s satellite - smooth spaces visible to the naked eye as large dark spots - are much darker than the highlands dotted with craters, and the plains of Mercury are just a little darker. Color differences on the planet are less pronounced, although the pictures of the “Messenger” made using a set of color filters showed small very colorful areas associated with the vents of volcanoes. These features, as well as the relatively inexpressive visible and near infrared spectrum of reflected sunlight, suggest that the surface of Mercury consists of darker-colored silicate minerals that are not rich in iron and titanium, in comparison with lunar seas. In particular, there may be a low content of iron oxides (FeO) in the rocks of the planet, and this leads to the assumption that it was formed under much more reducing conditions (i.e., with a lack of oxygen) than other representatives of the earth group.
Remote sensing issues
It is very difficult to determine the composition of the planet by remote sensing of sunlight and the spectrum of thermal radiation, which reflects the surface of Mercury. The planet is very hot, which changes the optical properties of the particles of minerals and complicates the direct interpretation. However, the Messenger was equipped with several instruments that were not on board the Mariner-10, which measured the chemical and mineral composition directly. These instruments required a long observation period, while the ship remained near Mercury, therefore, there were no concrete results after the first three short flights. Only during the orbital mission of the “Messenger” did enough new information about the composition of the surface of the planet appear.