The story of Mercury: Formation to Anthropocene
The place of Mercury in planetary science and popular culture is paradoxical. On the one hand, being a Moon-like planet battered in craters and likely devoid of life, Mercury does not get a lot of attention. On the other hand, because Mercury is the smallest planet and tends to get ignored, there is also a charm to the first rock from the sun, similar to Pluto (re-defined as a dwarf planet, rest in peace).
Mercury is like Earth’s Moon in many ways. It is comparable in size (radius = 2440 km or 1516 miles for Mercury vs. 1740 km or 1080 miles for the Moon), dominated by the same types of features and terrain (craters, lava plains, etc.), and it is also considered to be lifeless and likely geologically dead. Mercury long ago ceased to be hot enough to have molten rock to feed volcanic eruptions. Its surface has changed little in a billion years like the Moon.
Nonetheless, also like the Moon, Mercury has its surprises. Strangely, Mercury still has a global, albeit residual magnetic field, whereas a larger planet like Mars, does not. Also, Mercury, despite being the planet closest to the sun with temperatures as high as 430 °C (800 °F), may have water ice in permanently shadowed craters in its polar regions.
The planet Mercury has a long history dominated by volcanism, tectonics, and impact craters. As we move into the Anthropocene of the solar system, Mercury may also have significance as a representative of the non-living world. Despite being lifeless, Mercury serves as a record of the geologic history of the solar system and represents just how strange a planet can be despite initially looking fairly featureless. Mercury may also be come a test of the concept exogeoheritage, that non-living geologic systems can have intrinsic value like organisms and ecosystems. Our descendants may deconstruct Mercury to build a Dyson sphere, but should we do that? Is there intrinsic value in worlds even if they are not places where life could exist or where humans could settle? What we do with Mercury may determine how our species relates to the cosmos going forward.
I was talking with a friend at the Lunar and Planetary Science Conference a few years back who was studying the planet Mercury and an attendee of the NASA Mercury Exploration Assessment Group (MExAG). He was talking about how challenging it was getting the public excited about the planet Mercury. According to my friend, the main idea they come up with was hosting a concert by a singer who had a crater on Mercury named in their honor.
As a science communicator, I see no problem bridging science and popular culture, but I don’t think we have to rely on popular singers to make Mercury interesting. We just need to tell Mercury’s story, which begins with the geologic timescale. This because to tell a story, you need to know the chapters.
Mercurian geologic timescale
On Earth, transitions in the history of life, specifically mass extinctions play an important role in defining geologic time periods. For example, the present eon in Earth’s history (the Cenozoic) begins with the extinction of the dinosaurs 65 Ma (million years ago). Since there is no life on Mercury, as far as we know, geologic time periods are defined by volcanic and and major impact events.
The major time periods of the Mercurian geologic timescale are the Pre-Tolstojan, Tolstojan, Calorian, Mansurian, and Kuiperian.
Pre-Tolstojan to Tolstojan (4.5-3.9 Ga)
The earliest period in the geologic history is the Pre-Tolstojan. The Pre-Tolstojan begins with the formation of Mercury about 4.6 Ga (billion years ago) and ends with the formation of the Tolstoj crater basin around ~4.0 Ga. During the Pre-Tolstojan and Tolstojan, the solar system was still filled with protoplanets and planetesimals colliding with each other and the forming planets, including Mercury. As a result, the oldest terrain on Mercury is heavily cratered terrain where the impact craters are separated by rugged, hilly intercrater plains. The intercrater plains likely include debris from these ancient impacts events. Also during this time, Mercury was still hot enough to have volcanic eruptions. Some of the oldest craters are filled with basalt from hardened lava that filled the primordial basins when fissures were opened by catastrophic collisions. Many of the major geologic features characteristic of though not necessarily unique to Mercury, including gain multi-ringed crater basins, were formed during this time.
Calorian (3.9-1.7 Ga)
By the end of the Tolstojan (~3.9 Ga), the rate of impact events on Mercury had slowed down but volcanic eruptions were still frequent. The Tolstojan is followed by the Calorian, which is named after Caloris basin, a giant polar basin on Mercury. The Calorian age begins with formation of Caloris basin. The early Calorian is defined by the formation of extensive smooth plains found both within craters and between craters. In contrast to the rugged intercrater plains, the smooth plains are believed to be from gigantic eruptions where the surface was flooded with lava. In addition to lava eruptions (also called effusive eruptions), Mercury also had explosive (gas-driven) eruptions where the Mercurian crust had pockets of volatiles, like liquid water. In most extreme cases, the escape of volatiles from the upper crust of Mercury led to the surface collapsing resulting in nobby, broken terrain called chaos terrain, analogous to badlands terrain on Earth.
Over the course of the Calorian, Mercury cooled, and as it cooled it shrank. The shrinking of Mercury did two things. It resulted in the formation of lobate scarps, essentially wrinkles in the crust of Mercury, akin to the wrinkles in a raisin from the shrinking of the raisin as it dehydrates. The shrinking of Mercury also created internal pressure that made it harder for volcanic eruptions to occur. This leading to a dropping off of volcanic eruptions in the late Calorian. Volcanic eruptions were much less frequent and mostly connected to impact craters where the impact event opened a fissure.
Mansurian (1.7 Ga-280 Ma)
The beginning of the Mansurian age is marked by the appearance of fresh-looking craters that lack the bright rays associated with youngest craters on the Moon, including Mansur Crater itself, which defines the base of the Mansurian age system. By the Mansurian, the giant lava eruptions that formed the smooth plains had long since ceased, except for occasional impact-related instances of volcanism. The dominant geological process on the surface of Mercury was the relentless battering of its surface by asteroids, comets, and other space rocks. Since Mercury is closer to the sun than Earth, objects collide with its surface with higher velocity and with greater frequency compared to farther out in the solar system. The population of space rocks at the orbit of Mercury is not higher necessarily, but the population is more dense since there is less volume within Mercury’s orbit compared to, say, Earth’s orbit. This also leads to to more frequent impacts. Other than impact events and possibly occasional eruptions of lava to flood new craters, not much happened during the Mansurian on Mercury.
Kuiperian (280 Ma-Present)
The Kuiperian age represents the present epoch of Mercury defined by the first appearance of rayed craters, including Kuiper Crater itself, craters that are both fresh looking and have the characteristic rays of the youngest lunar craters. Mercury had changed little in almost two billion years when something unusual happened. On March 29, 1974 a fragment of Earth flew past Mercury. This fragment of Earth, however, was able to transmit information back to Earth. This was the Mariner 10 spacecraft, the first successful flyby of the planet Mercury. The next major mission to explore Mercury, MESSENGER, arrived in 2011. MESSENGER also became part of Mercury when it crash-landed on Mercury’s surface in 2015 as its end of mission.
Mercury in the Anthropocene and beyond
The fragments of Messenger could be considered the first traces of human activity on Mercury, marking 2015 as the beginning of the Mercurian Anthropocene. The Anthropocene for the uninitiated is a proposed term for the period in Earth’s history where humans become a driving force in a planet’s geological processes.
Since Mercury is both unlikely to have life, though it can’t be ruled out completely, and more difficult to terraform than Mars, Mercury’s future in the Anthropocene as a planet looks grim. In Isaac Asimov’s anthology, I Robot, for example, it is described mainly as mining world. A recent idea is to deconstruct Mercury entirely and make it into a Dyson sphere, a hypothetical structure that an advanced stellar-scale civilization might build to capture the energy from a star by enclosing it entirely or almost entirely with a shell or swarm of artificial structures. Most proposals involve leaving an open sliver at the stellar or solar equator to provide light to any inhabited planets. This would in theory allow humanity to harness the full energy of the sun and use it for things like interstellar travel.
Dyson spheres would be relatively easy to detect because of the waste heat that would need to be radiate into heat, SETI researchers have made attempts to search for Dyson spheres areound nearby stars. So far none have been detected other than the initially tantalizing case of Tabby’s star. Should we build a Dyson sphere if it means deconstructing an entire planet like Mercury?
Recent thinking from the SETI community is that Dyson spheres may not be common since it implies a focus on unlimited expansion that is coming to be seen as unsustainable in the long-term. In short, civilizations that are able to persist over geologic timescales, and hence long enough to be detected, may reach an equilibrium point where they are able to exist within the constraints of their planetary system for an indefinite period time rather than maximize their energy consumption and output like our current civilization. This has been called the sustainability solution to the Fermi Paradox. In this scenario, the most advanced civilizations cease to be detectable since they just look like a planet with a biosphere. A civilization like this is unlikely to build a Dyson sphere, and the fact that Dyson spheres have not been detected around other stars is at least consistent with this conclusion. This leads to the provocative question. What would be the cultural adaptations required to form a mindset that promoted this sort of civilization?
This brings us to the planet Mercury and how it can help shape our perspective on space exploration. A recent concept in space exploration is exogeoheritage. Exogeoheritage is inspired by the terrestrial concept of geoheritage, which sees non-living geologic features as having the same intrinsic value as living ecosystems and and cultural artifacts.
Exogeoheritage implies that non-living and non-technological systems have intrinsic value and that we shouldn’t just see them as raw materials for turning the cosmos into a giant suburb. The planet Mercury may not have life or be ideal for human settlement, but it is a unique world with its own history, processes, and character. We may one day decide to make Mercury into a Dyson sphere (to be clear I am not anti-Dyson sphere per se) and maybe that will be the right choice, but first let us make sure to appreciate Mercury and the other planets for their intrinsic value as alien worlds. After all, as the sustainability solution to the Fermi Paradox implies, our long-term future might not look like living in a Dyson sphere. In might just look like living in a healthy relationship with our home planets.
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