Mercury is the smallest planet in the solar system. Not content with it, it has shrunk since it was formed . Because since he was the smallest, he was by far. Of course this shrinkage has occurred on geological scales , over billions of years, and has been so subtle that we would not have been able to perceive it directly in the few decades that we have been observing it closely and in detail. We have not discovered this shrinkage by comparing past and present values of Mercury’s radius, but by observing the consequences that this process has left on the surface of the planet . Specifically observing the folds that populate its surface .
When it formed, Mercury was much hotter as a result of the countless collisions of rocks and planetesimals that eventually gave it its current size, and as a result of the meteorite bombardment it suffered after its formation. Since then it has been cooling down . First the surface and later the interior of the planet. The same has happened of course with the rest of the planets and rocky bodies of the solar system. The difference between Mercury and Venus, Mars or Earth has been quantitative, rather than qualitative. That is, Mercury being considerably smaller than the other three rocky planets, it has cooled much faster . So much so that folds several hundred kilometers long and up to three kilometers high have formed on its surface, as a result of the compression of the planet’s interior when the surface was already completely solid.
In the same way that the skin of a grape wrinkled in the process of becoming a raisin, Mercury’s surface wrinkled as its interior cooled and contracted . These folds also appear on other structures, such as craters or lava plains, crossing and overlapping them , indicating that they have occurred later . We believe that this process was important until about 3.6 billion years ago , although smaller folds , just a few kilometers long and tens of meters high, with ages of about 50 million years, have also been detected. This suggests that the shrinking process of Mercury’s interior continues to this day.
In all, we estimate that Mercury could have shrunk up to 7 kilometers since it formed. Although this is a lot for human scales, we must remember that the planet has a diameter of almost 5,000 kilometers , so this reduction has accounted for just over 0.1% of its size . This further serves as a contrast to the old theory , from over a century ago, that purported to explain Earth’s tectonics before Wegener proposed its continental drift . At the beginning of the 20th century it was well known that the terrestrial continents had moved in the past and were still moving in the present, so they had not always been in their present position. For example, it was well known since the middle of the 19th century that South America and Africa had been united in the past.
However, it was not fully understood what mechanism could muster the energy necessary to achieve something as dramatic as moving an entire continent. One of the most popular proposals of the time was the idea that, since its formation, the Earth had been heating up and, as a consequence, expanding considerably . This hypothesis therefore proposed that, rather than moving the continents, it was the Earth’s surface itself that had expanded. In its origins, the entire surface would have been dominated by the continents and during the expansion the oceanic surfaces that occupied the “remaining gaps” would have been created. According to this ancient theory, the Earth would have increased in size by hundreds or even thousands of kilometers. As we see from the real case of Mercury, these scales would be impossible (in addition to the fact that what the Earth has done is cool down, not heat up).
Folds similar to those seen on Mercury have been detected on the Moon by NASA’s Lunar Reconnaissance Orbiter. These are smaller , with a mean elevation half that of Mercury. In addition, it is estimated that the Moon’s contraction due to cooling should be considerably less than on the planet , so these folds could have their origin, at least partially, elsewhere, such as geological forces on the ground that occupy these folds . .
This cooling of Mercury’s interior is also responsible for the planet having no appreciable magnetic field today . This same process has taken place on the rest of the rocky planets, being considerably slower on Venus and Earth due to their larger size. However, on Mars , which would be halfway between Mercury and Earth, cooling has made the magnetic field disappear, and its oceans of liquid water.
References:
LS Schleicher et al, 2019, Wrinkle ridges on Mercury and the Moon within and outside of mascons, Icarus, Volume 331, https://doi.org/10.1016/j.icarus.2019.04.013.
