Scientists have been puzzled about a planetary mystery for almost 40 years: why Mercury did not seem to have shrunk as much as it should have. In the mid-1970s, the NASA spacecraft Mariner did two fly-bys, photographing roughly 45% of the planet’s surface. The results showed planetary geologists and geoscientists shrinkage of only one to three kilometers in its four-billion-year history, which everyone across the board felt was too little with what they knew of the planet.
It turns out that how Mercury shrinks and how much molten metal is in its core are quite different than Earth’s. Harvard Professor Naomi Oreskes in the History of Science department says this proves that past scientists are never silly to have thought something we now consider to be incorrect, as anything is plausible until it does not hold up. European geologists in the 19th century believed that Earth was shrinking. They used mountain formations as evidence, and having, at that time, no knowledge of plate tectonics, assumed that as Earth shrank, it forced land up into mountain ridges. They were right…but not about Earth.
The NASA craft Challenger recently completed a three-year stint photographing the first rock from the sun. Mercury has only one plate. That is, its surface is comprised of one continuous layer of rock with no plate division, which could be interpreted as being without plates, although this is not the way it has been categorized. Additionally, its inner core, entirely made of iron, is the bulk of what the planet consists of, with what is described as a thin layer or “veneer” of rock on the planet’s exterior, about 400 km deep. To offer a point of reference, the distance between Earth’s surface and the beginning of its core is estimated at 6,371 kilometers.
Even though Mercury is the closest planet to the sun, in the grand scheme of things—that is, over a long period like 4 billion years—it has actually been in a process of cooling. Scientists say there may be some liquid left at the center of the core, but as the planet has cooled the iron has been solidifying, causing shrinkage, which, much as a raisin does drying in the sun, causes rifts and ridges. Because of the single rock plate, when the core shrinks on the planet and the lack of a multiple-plate system does not allow for movement associated with plate tectonics, the single layer instead crumples in on itself, and when the crumpling creates deep ridges, tall cliffs are simultaneously formed. There are ranges on the planet 1,000 km long and more.
Paul Byrne, a planetary geologist with the Carnegie Institution, says that it would be a “very dramatic” landscape. Unfortunately anyone adventurous would never be able to visit and walk the amazing terrain, as it is a cool 800 degrees Fahrenheit. One wonders what the temperature was in its initial days, back before cooling had begun. Although Man cannot visit the planet’s surface, for we have no material able to withstand such a heat, images show truly incredible vistas. One fantastic site is referred to as Carnegie Rupes. It is a massive cliff that runs right through a gigantic crater, as if it has grown up through it. This implies that cooling had begun and the cliff was already forming when a large asteroid hit, creating the crater, but instead of destroying the forming ridge, the cliff just kept on evolving, being pushed upwards as the rock on either side of it crumpled towards the planet’s cooling center.
And the answer to the mystery? The planet has gotten smaller by up to 14 kilometers in diameter. Mercury’s shrinkage has offered up quite a sight for scientists and the curious alike.
By Julie Mahfood
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