Settle The Stars- Mercury: Turning Slowly, Moving Fast
Alexander Winn takes us to Mercury to discuss how… habitable it is.. or (spoiler alert) it isn’t.
Transcript:
Hey folks, this is Alexander Winn.
In this episode we’ll be visiting Mercury, the fascinating small rocky world zipping around the Sun. This will be the first in a series of episodes exploring the solar system and all the unique histories, challenges, and opportunities it includes, one planet at a time.
Mercury has captured the attention and imagination of stargazers for millennia, and today we’ll take a closer look at the mysteries and discoveries that continue to shape our understanding of this little planet.
Based on all the information accumulated, we can sketch out our likely observations as we make our own imaginary visit to the planet.
Planning the trip would have been slightly different from visiting other planets in the Solar System. A route to Mercury must take into account its unique orbit, which is the most eccentric of all the planets. That means that if you were to look at the orbit from above, you would notice it doesn’t follow a circular path around the Sun. The actual orbital path looks more like an oval, with the Sun slightly off-center from the middle. The result is that the distance between the Sun and Mercury can vary quite a bit, from 46 million kilometers to 70 million kilometers at opposite ends.
As we approach and enter Mercury’s orbit in preparation for landing, we would find the surface to look a lot like that of the Moon – heavily cratered and grey. One crater is particularly striking, due to its massive size. It’s called the Caloris Planitia and could comfortably fit the state of Texas. This crater was caused by an impact so large that the terrain on the opposite side of the planet has also been altered – possibly by shockwaves traveling around the planet and converging on the other side.
There would be mountains and ridges as expected, but upon closer inspection we would notice what look like strange ripples and wrinkles in the landscape. These are features first spotted by probes, and evidence that the planet is actually shrinking – albeit very slowly. The radius of Mercury has decreased by about one to seven kilometers over time as the inside of the planet has cooled, leaving the outer crust to bunch up into these strange shapes.
In the past Mercury had a lively volcanic history. As we peer out of our imaginary spacecraft we can still see some evidence of volcanic activity in the form of cooled lava flows in some locations, but now that the inside of the planet is cooler the volcanoes have stopped.
The inside of the planet has not cooled completely, though – surprisingly our ship’s magnetometer can tell us that right away. Surrounding the planet is a strong and stable magnetic field. This field is much like that on Earth in that it is dipolar and aligned with the rotational axis of the planet, but much weaker – only about 1.1% as strong as our own. These magnetic fields have only been observed on planets with a liquid-hot, iron-rich core, which generates the field with what’s called a dynamo effect.
As we make our preparations for landing, one more striking feature becomes obvious to us: the planet is rotating very slowly. One day on Mercury lasts 58 Earth days, and when you account for the quick time it takes to revolve around the Sun, the small planet completes three of its days every two Mercurian years.
Scientists have some of these observations – the slow rotation, the impact crater, the eccentric orbit, and strange terrain – linking them to a single impact event that has forever changed the planet. There is evidence that the entire planet was once tidally locked in its orbit around the Sun, in the same way the Moon is tidally locked around the Earth. The Moon orbits the Earth and rotates on its axis in an extremely stable arrangement in such a way that only one side of the Moon ever faces the Earth. Similarly, for possibly billions of years Mercury only showed one face to the Sun.
Scientists believe that an object at least 62 miles in diameter smashed into the planet, forming the massive crater and causing the slow rotation we observe today.
Once on the ground, our bodies begin to adjust once again to the presence of gravity. We only weigh a bit more than 1/3 what we do on Earth, but our muscles, skeletons and sense of balance welcome the familiar feeling of solid ground underfoot.
Our imaginations and intuitions are mostly correct as we begin exploring Mercury. It’s the closest planet to the Sun, so it must get pretty hot, right? Absolutely, to the tune of 800 degrees Fahrenheit. That’s over six times hotter than the highest temperature recorded on Earth, in Death Valley California – so double-check your spacesuit coolant. And while you’re at it, check the heating mechanism too, because what is less obvious is that due to its extremely thin atmosphere, almost all this heat dissipates as the planet slowly rotates out of the sunlight. The result is a long, cold night where temperatures can reach as low as -136 degrees Fahrenheit. That’s the largest surface temperature discrepancy anywhere in the Solar System, and just more evidence that the climate on Mercury can be very… well, mercurial.
And not that you need reminding, but better keep that helmet on. The atmosphere on Mercury is barely there and very transient, being replenished by evaporation and radioactive decay of elements on the surface of the planet just as quickly as its being blown away by the solar wind. Certainly nowhere near breathable, let alone fly the space kite we brought.
If it isn’t obvious yet, Mercury is not very hospitable to human visitors. It is much more comfortably observed from afar as we’ve enjoyed doing so far.
[History of Human Exploration]
Mercury is visible to the naked eye, so it’s been on the mind of astronomers on Earth for an awfully long time. The first thing that probably stood out to early stargazers is that against a backdrop of stars that march across the sky in more or less the same formation, Mercury is one of only a handful of lights that follows a different path. We know now that these independent bodies are in fact planets, but back then they were special stars – and deserved special names to designate their higher status.
It would be easy to assume that Mercury was named after the god of communication and commerce for obvious reasons. It moves so quickly relative to the other stars, and it never moves far from the Sun in the sky, after all. These are qualities we would expect the messenger for a king to have. But it’s actually likely that the god Mercury (and similar gods in other cultures around the world) only exist because of the planet. Without the unique qualities providing inspiration, perhaps there wouldn’t be a messenger god in the pantheon at all.
Whatever the reason the name has stuck, and it’s very appropriate. We say that things are “mercurial” when they change quickly, which perfectly exemplifies the small planet’s quick movement around the Sun and drastic surface temperature changes. But quick as it is, Mercury was spotted and recognized well before the Romans got around to naming it.
The ancient Greeks actually applied two different names to the planet originally. It was called Hermes when the planet was visible in the evening after sunset, and Apollo when visible in the morning before sunrise. Eventually Hermes stuck for both as it became clear the two were the same object, which the Romans adopted as Mercury.
Ancient Babylonians attributed similar divine qualities to the planet, naming it Nabu after the god of writing and divine messages. The Babylonians also attributed both male and female aspects to the planet because of its appearance as both a morning and evening star.
Cultures around the world have watched Mercury closely for centuries, often linking its movements, characteristics and even mythology with that of Venus. Because these two planets are both closer to the Sun than Earth, they follow a different pattern across the sky than the rest. Being closer to the Sun also means they both exhibit phases when viewed from Earth, like our Moon.
Being closer to the Sun from Earth also means that from here we’re able to observe an event called the “transit” of each planet, which is when the planet passes between the Earth and the Sun. Mechanically it’s like a solar eclipse, except that because Mercury is so much farther away from us from the Moon it doesn’t block as much light from the Sun. You wouldn’t even notice it happening if you weren’t looking at the Sun, but these events fascinated early astronomers as they were valuable opportunities to estimate the size and distance of objects within the Solar System with a little help from geometry.
Transits of Mercury are quite frequent given the speed of the small planet’s orbit, but much rarer are events called “occultations”. These occur when multiple planets align perfectly in their orbits with Earth, and astronomers in 1737 were delighted to observe the occultation of Mercury and Venus from the Royal Observatory at Greenwich. The same event won’t happen again until December 3, 2133.
Mercury’s closeness to the Sun and tiny size makes it quite difficult to observe from Earth, so it was only a matter of time before we tried getting right up close for a look. But of course, being near the Sun also presents unique challenges to probes and spacecraft as well. For starters, there is less room for navigational errors. In wider space adjustments are easy to make with small thrusters. But when targeting a small object so close to the Sun, high speeds and proximity to the star mean that one wrong move could send your expensive probe right into the Sun. Of course, radiation and temperatures threaten to fry the delicate instruments and electronics as well.
But as you might expect, those challenges haven’t been enough to dissuade us from taking a shot at a valuable up-close look at Mercury.
In the mid-1970’s, Mariner 10 made the first fly-by of the planet and provided an unprecedented view of the planet’s geological features and discovered the planet-wide magnetic field. The field was a big shock to everyone at home. Mercury’s slow rotation was though to be a sure indicator of a geologically dead planet incapable of generating such a field, but in fact the field is structured and behaves similar to our own and helps shield the planet from some of the bombardment of solar radiation. Mariner 10 shut down after making three close passes to the planet and – barring an unlikely collision with a space rock – could still be out there orbiting the Sun, occasionally passing by Mercury again for a quiet visit.
In 2005, we got our second chance to view Mercury with updated equipment. NASA’s satellite MESSENGER was equipped with an array of instruments to capture more data about the planet over a year-long observation. Scientists were eager to see more of Mercury’s surface and find out more about the geological history, search for water, and puzzle out how it can maintain even a sparse atmosphere. We discovered that there is in fact water on the surface of Mercury, locked in ice deep in craters and crevices away from the sunlight. MESSENGER completed its mission successfully on April 24, 2015 and went out with a bang – leaving a 52-foot crater as it fell to the surface.
[Future Human Exploration]
The future of Mercury is bright – please pardon the pun. In 2018, the European Space Agency and the Japanese Space Agency joined together to launch BepiColombo, a new mission to probe the planet and discover even more. When it reaches Mercury in 2025 it will spend a year studying the magnetic fields and capturing a variety of new infrared, ultraviolet, X-ray and gamma ray images of the surface to answer some of the biggest questions we have about the planet. As is normal for all scientific ventures, hopefully these new observations will lead to even more questions, more missions, and more discoveries in the future.
[Outro]
Building on the contributions and careful observations of astronomers for centuries, we’ve collected a huge amount of information about one of the most mysterious and unusual planets in our Solar System. I hope you enjoyed learning about Mercury, and I look forward to continuing our voyage across the stars in upcoming episodes.
In the meantime, be sure to subscribe if you haven’t already. Settle the Stars is available on pretty much every podcasting platform, and we’re also mirroring our episodes on YouTube at YouTube.com/EdgeworksEntertainment (and be sure to ring that bell so you know when there’s a new episode). And don’t miss the other awesome shows that are part of the Edgeworks Nebula: Slice of Science, the Synthesis, and our upcoming show You Have My Sword, where Krysti Pryde will be analyzing and deep-diving into the world of Lord of the Rings and JRR Tolkien’s Middle-earth.
Thank you all for listening, and as always, happy terraforming.
Settle the Stars is a proud member of the Edgeworks Nebula, a collection of intriguing and informative podcasts from Edgeworks Entertainment.
