NeoScience Daily · SUN 25 OCT 2026 · Issue 01

Mercury runs hot and cold and doesn't care what you pack

It's got a huge metal core, it whips around the Sun in a hurry and it swings from roasting days to freezing nights, so good luck with that.

Why care about the smallest planet?

Mercury is the smallest planet in the solar system, it has no moons, no rings and basically no air, nobody is planning to live there, and when it comes up in conversation it's usually because someone's blaming it for their ex texting them. So why should you and I give a damn about a burnt little ball of iron that most people on Earth have never knowingly seen?

For most of human history that was a fair question, and Mercury would agree with you if it cared, which it doesn't. It sits about 36 million miles from the Sun on average, closer than anything else we call a planet, and it's been getting cooked on one side and frozen on the other for four and a half billion years without asking for anyone's opinion. Stand on it at noon and it's about 810 °F, hot enough to melt lead. Wait for nightfall and it's about -290 °F. Packing for this one is going to be a nightmare.

And yet this tiny scorched rock has done more damage to what humans thought they knew than planets a thousand times its size. It caught Isaac Newton out, it handed Albert Einstein his first big win, and it's hiding ice in the shadows, right next to the Sun, which sounds like a typo and isn't. And it's still shrinking, which I'm told happens to all of us eventually.

So that's what this issue is about: why you and I should care about the solar system's most ignored planet, and why we maybe shouldn't. Along the way we'll get into why its days are longer than its years, how it wrinkled like a forgotten apple, why it's harder to reach than Pluto, the two robots that risked getting fried to go look at it, and whether it has anything to offer humanity, or if it's just a very expensive way to sunburn a robot.

But first you need to know where Mercury lives, because that address is the reason for pretty much everything that's wrong with it.

Scars and spacecraft

BIG SCARS Very large impact basins, including Caloris at 960 miles in diameter and Rachmaninoff at 190 miles, were created by asteroid impacts early in the solar system's history. The first picture shows Caloris as MESSENGER saw it.

ORBIT AND IMAGES MESSENGER's Mercury orbit insertion was on 17 March 2011 (EDT). It acquired over 270,000 images and extensive other data sets. Separately, the equatorial topographic relief of Mercury, in agreement with earlier radar results, is at least 3.4 miles. The second picture is an artist's view of that spacecraft at Mercury.

THE FINAL IMPACT The final determination of the impact time is 19:26:01.166 UTC on 30 April 2015. The final estimate of the impact location is at 54.4398° N, 210.1205° E, and 1,520 miles from the center of Mercury.

ENCOUNTERS Mariner 10 crossed the orbit of Mercury on 29 March 1974, at a distance of about 437 miles from the surface. A second encounter, when more photographs were taken, occurred on 21 September 1974, at an altitude of 29,900 miles. The third and last came on 16 March 1975 at 203 miles, with about 300 frames and magnetic field measurements. The third picture shows the Mariner 10 flight spare spacecraft.

VENUS FIRST The flown spacecraft had passed Venus on 05 February 1974, at a closest range of 3,580 miles. The total cost of the mission was roughly $100 million.

The worst seat in the house

Mercury is first in line from the Sun, and its orbit is the most stretched-out of all the planets, so its distance swings from about 29 million miles at its closest to about 43 million miles at its farthest. For comparison we're about 93 million miles out, and that's already close enough to give you a sunburn through a car window. From Mercury's surface at its closest the Sun looks more than three times as wide as it does from here, and the sunlight is as much as eleven times brighter, so SPF 50 is going to come up a little short.

Mercury itself is small, about 3,030 miles across, which is about 40 percent wider than our Moon. It's actually narrower than two moons in the solar system, Jupiter's Ganymede and Saturn's Titan, so yes, this planet has lost a size contest to things that orbit other planets. It is, however, absurdly heavy for its size, because most of what's inside it is iron. Its metal core takes up about 85 percent of the distance from the center to the surface, which is like an avocado that's almost entirely pit. Because of all that iron, Mercury's gravity is about 38 percent of Earth's, the same as Mars, even though Mars is a much bigger planet.

Nobody knows for sure why it ended up that way. One idea is that something huge slammed into young Mercury and blasted most of its rocky outer layers into space, leaving the core behind, while another is that the early Sun's magnetism or heat sorted things so that iron piled up close in. Either way, you're looking at a cannonball with a thin rock coating, which is going to matter later when we talk about what it's good for.

Now here's where it gets strange. Mercury tears around the Sun every 88 of our days, which is why the Romans named it after their speedy messenger god, and it's the fastest planet there is. However, it can barely be bothered to spin, turning once every 59 days. Those two rhythms lock together so that it spins exactly three times for every two trips around the Sun, and the result is that one full day on Mercury, from one sunrise to the next, takes 176 of our days. So a single Mercury day lasts two Mercury years. You would have two birthdays between breakfast and breakfast, and at certain spots on the planet the Sun rises, stops, backs up, and rises again, like it forgot its keys.

That long, slow day is why the temperature gets so ridiculous. Mercury has almost no atmosphere to move heat around, just a wisp of atoms knocked off the surface by the Sun, which scientists call an exosphere. So the ground that's in the Sun bakes for months at about 810 °F, and the ground that's in the dark loses all of it and falls to about -290 °F. That's a swing of about 1,100 degrees, the biggest of any planet. And here's the insult: despite being the closest, Mercury isn't even the hottest. That title goes to Venus, whose thick air holds its heat in, so Mercury can't even win at the one thing you'd think it had locked up.

That same lack of air and lack of tilt also means the crater floors near Mercury's poles have been sitting in the dark for possibly billions of years, and they're some of the coldest places in the solar system. We'll get to what's in them. First, though, you should know how much trouble this little planet has caused us.

A planet built around its core

Mercury is built around a large metallic core, wrapped in an outer shell comparable to Earth's mantle and crust.

Reading the crust One model of the crust infers an average thickness of 22 ± 11 miles. The estimate assumes Airy isostatic compensation of the topography.

A large metallic core Mercury has a large metallic core with a radius of about 1,289 miles. That is about 85% of the planet's radius.

The outer shell The outer shell, comparable to Earth's mantle and crust, is only about 250 miles thick.

The planet that broke Newton

Nobody discovered Mercury. People have been watching it for at least 3,000 years, though it's a pain to spot, because it never strays far from the Sun, so you only ever catch it low in the sky just after sunset or just before sunrise. The ancient Greeks even thought they were looking at two different objects for a while, and they gave the morning one the name Apollo and the evening one the name Hermes, before someone worked out that it was the same planet commuting. A few times a year it also appears to drift backwards across the sky, because it's overtaking us on the inside lane, and that illusion is what people mean by Mercury in retrograde. It has no effect on your ex, who is texting you for other reasons.

For a long time that's about all anyone could say about it, and even Galileo's telescope wasn't good enough to show Mercury as anything but a dot. Then in 1631 a French astronomer named Pierre Gassendi did something new. Johannes Kepler, the same man who'd worked out how orbits work, had predicted that Mercury would cross the face of the Sun on 7 November that year, and although Kepler had died the year before, Gassendi set up a projection in a darkened room and watched for it. He actually saw a tiny black dot crawl across the Sun's image, and it was so much smaller than anyone expected that he took it for a tiny sunspot at first. It was the first time anyone had ever watched a planet pass in front of the Sun, and it showed Kepler's math worked.

Then Mercury got really annoying. By the 1800s astronomers could predict the planets with Isaac Newton's laws of gravity almost perfectly, and in 1846 those laws had just pointed them straight to Neptune, which is about as big a win as science gets. The man behind that win was a French mathematician named Urbain Le Verrier, so when he noticed in 1859 that Mercury's orbit was drifting by a tiny amount Newton's math couldn't explain, he reached for the same trick. The point in Mercury's orbit where it's closest to the Sun was moving forward slightly too fast, by an amount so small you'd barely measure it over a century. Le Verrier figured there had to be another planet, an unseen one, tucking in even closer to the Sun and tugging on Mercury. It even got a name: Vulcan.

Yes, Vulcan was real for a while, at least on paper. A French country doctor claimed he'd seen it cross the Sun, Le Verrier believed him and announced it, and for decades astronomers kept reporting sightings and hunting for it during eclipses. They never found it, because it was never there.

The real answer showed up in 1915, and it came from Albert Einstein. His new theory of gravity, general relativity, says gravity isn't really a pull, it's space and time bending around heavy things, the way a mattress sags under a bowling ball. Mercury lives so deep in the Sun's sag that the difference shows up in its orbit. Einstein ran the numbers and got the missing amount exactly: 43 arcseconds per century, which is a tiny sliver of a degree. He later said he was beside himself with joy for days. So this runt of a planet handed us the first hard evidence that Isaac Newton wasn't quite right about how the universe works, and Vulcan, a planet with a name, a discoverer and decades of sightings, went in the same drawer as the canals on Mars.

Mercury wasn't done embarrassing us, either. For most of the 1900s astronomers were sure it kept one face to the Sun forever, the same way the Moon keeps one face to us, and they had drawn maps to prove it. In 1965 radar measurements from Puerto Rico showed it actually turns every 59 days. So decades of careful maps were just wrong. Everyone had kept catching Mercury at the same point in its rhythm and seeing what they expected to see, and it took an Italian scientist named Giuseppe Colombo to work out the three-to-two spin that explains it.

So this little planet has a track record of making very smart people look silly. To find out what else we'd gotten wrong somebody had to go there, and Mercury had a few things to say about that.

Gassendi and the transit

Pierre Gassendi (1592 to 1655). On November 7, 1631, the French astronomer Pierre Gassendi sighted the first confirmed transit of Mercury through a telescope. Earlier, in 1607, Kepler had thought he saw one by projecting the Sun's image through a pinhole, but he had only detected a sunspot.

How Mercury kills you, and your robot

If you're wondering why a planet this close has barely been visited, it's because getting there is backwards. You'd think falling toward the Sun would be the easy direction, and that's the problem. Everything falling toward the Sun picks up speed, so by the time a spacecraft reaches Mercury it's going so fast it just screams straight past. To actually stop and stay, you have to shed an enormous amount of speed, and Mercury has barely any air to brake against and not much gravity to catch you with. It takes a bigger change in speed to get into orbit around Mercury than it took to fling New Horizons past Pluto, even though Pluto is about 100 times farther out. It's like trying to stop a shopping cart at the bottom of a very long hill with no brakes, and you only get one shot at the parking space.

And once you're there, it's trying to kill your robot in a few different ways. Sunlight at that distance can be up to eleven times stronger than it is out here, and then the planet below is cooking at 810 °F and throwing its own heat back up at you. So you're basically a sandwich in a toaster, with the Sun on one side and the ground on the other. There's no air to spread heat out and no magnetic shield worth much, because although Mercury does have a global magnetic field, it's only about 1 percent as strong as Earth's. So the solar wind, those charged particles pouring off the Sun, often gets slammed right down to the surface.

Now picture yourself there instead of the robot. Take a step outside at noon and your suit has to fight 810 °F ground, which melts lead, so your boots are about to have a worse day than you are. Step into shadow and your suit now has to keep you warm against -290 °F. The air, what there is of it, is a near-perfect vacuum, so you've got no oxygen and no pressure, and without that suit you'd be unconscious in about 15 seconds. Assuming the suit holds, the radiation is next, because with no atmosphere and a weak magnetic field the Sun's particles hit you more or less directly, and during a solar storm you're standing about as close to the blast as anyone can get. And since there's no air to burn up incoming rocks either, the ground has been sandblasted by tiny meteorites for billions of years, which is why the place looks like the Moon's less successful cousin.

The ground under you isn't exactly welcoming, either. It's covered in craters, including one called Caloris, about 963 miles across, from an impact so violent that the shock seems to have gone through the entire planet and scrambled the ground on the opposite side. It's also covered in cliffs, some hundreds of miles long and up to about 1.9 miles high, and those cliffs are the best story on the planet, because they're there since Mercury is shrinking. As that giant iron core has cooled, the whole planet has contracted, and the crust, which can't shrink, has buckled and folded over itself like the skin on an apple that's been left in a drawer. In total the planet's radius has dropped by up to about 4.3 miles, and it's still at it.

So Mercury will bake you, freeze you, irradiate you and suffocate you, all while slowly wrinkling beneath your feet. Naturally, our response was to send a robot with a parasol.

A solar sail to Mercury

HARD TO REACH The host opens by saying Mercury is one of the least explored terrestrial planets because it is one of the hardest places to get to. Reaching it takes as much propellant as reaching Pluto, so people keep looking for cheaper ways to go deep into the solar system.

SUNLIGHT AS THE ENGINE His guest, a professor at Brown University, has proposed the Mercury Scout mission. A gigantic solar sail would carry a new set of instruments to Mercury using sunlight pressure alone, the strongest wind being close to the Sun, where Mercury is.

CLOSE TO THE SUN Asked why Mercury is worth the trip, the guest says it is the closest planet to the Sun. Many exoplanets are found very close to their stars, so Mercury is the best bet in our solar system for understanding how such planets form.

STARVED OF OXYGEN The guest calls Mercury basically an oxygen-starved planet. He says that has given it very different magma compositions and a different internal structure, and that many exoplanets may be similar.

Two robots and a parasol

The first visitor was NASA's Mariner 10, which flew past Mercury three times in 1974 and 1975. To get there it did something nobody had done before, swinging past Venus and using Venus's gravity to bend its path toward Mercury, and every mission since has borrowed that trick. It sent back the first close-up pictures, however thanks to the geometry of its flybys it kept seeing the same side lit up, so it only mapped about 45 percent of the planet. The other half stayed a blank spot on the map for more than 30 years.

That gap didn't get filled until NASA's MESSENGER, which launched in 2004 and took six and a half years to get into orbit, with a route that included one flyby of Earth, two of Venus and three of Mercury, all to bleed off speed. That's a very long way around for a planet that's sometimes closer to us than Mars is. To survive once it got there, MESSENGER carried a big sunshade of ceramic cloth layered over plastic insulation, and it kept that shade pointed at the Sun at all times. Behind it the electronics sat at about room temperature, while the front of the shade was at about 700 °F, so the most advanced machine ever sent to Mercury stayed alive the same way you do at the beach.

MESSENGER orbited from March 2011 until April 2015, far longer than its original one-year plan. It mapped the whole planet, measured its strange off-center magnetic field, found the planet had shrunk more than anyone had thought, and found a surface rich in things like sulfur and potassium, which should have boiled away if Mercury had formed that close to the Sun in its current form. So that's one more mystery about how it got there. And when it finally ran out of fuel, NASA let it crash into the surface at about 8,700 miles per hour, leaving one new crater on a planet that really didn't need another.

The next one gets there in November 2026. BepiColombo, a joint mission from Europe and Japan named after Giuseppe Colombo, launched in 2018 and is carrying two separate orbiters. It needed nine flybys of Earth, Venus and Mercury, and a problem with its thrusters in 2024 pushed its arrival back about a year, to around late 2026. So the trip takes about eight years, which is roughly how long it takes a person to go from kindergarten to puberty, and it's just about as awkward. It's built to finish what MESSENGER started, and it's the only thing humanity has going to Mercury at all, so no pressure.

But the single best thing MESSENGER found wasn't in the map. It was in the dark.

Watching BepiColombo arrive at Mercury

BEPICOLOMBO The broadcast follows the start of BepiColombo's arrival at Mercury. The scientific objectives behind BepiColombo can be viewed through 12 questions, so listen for which question each new announcement touches.

INSTRUMENTS The Mercury Planetary Orbiter carries a payload of 11 instruments. When the presenters name one, notice what it is meant to measure.

STRONG SUNLIGHT The direct solar radiation hitting the spacecraft is about ten times more intense than in Earth's proximity.

The machines that reached Mercury

Spacecraft, from a flyby probe to an orbiter still on its way, are how people have read the closest planet to the Sun.

Mariner 10, a flyby probe Mariner 10 crossed the orbit of Mercury on 29 March 1974, at a distance of about 437 miles from the surface. Its third and last encounter, at an altitude of 203 miles, brought about 300 frames of photography. The total cost of the mission was roughly $100 million.

MESSENGER, an orbiter at Mercury MESSENGER reached Mercury orbit insertion on 17 March 2011. From orbit it acquired over 270,000 images and extensive other data sets.

BepiColombo's Mercury orbiter The Mercury Planetary Orbiter carries a sophisticated payload of 11 instruments. The direct solar radiation hitting the spacecraft is about ten times more intense than in Earth's proximity, so the hardware has to work in a harsh setting.

MESSENGER acquired over 270,000 images of Mercury.

Ice on the hottest doorstep

Here's a sentence that shouldn't be true: there's ice on Mercury. Back in 1991 astronomers bounced radar off Mercury's poles and saw something unusually bright coming back, the kind of bright you get off ice, and nobody was totally sure they believed it. MESSENGER settled it. It found water ice sitting on the floors of craters near the poles, and it even measured the amount, somewhere between 110 billion and 1.1 trillion tons. Some of it sits right out in the open and some is hiding under a dark layer about 3.9 to 7.9 inches thick, probably carbon-rich stuff that came along with the comets and asteroids that delivered it.

The way it works is simple, and I love it. Mercury's axis barely tilts, so at its poles the Sun just rolls around the horizon and never climbs, and the bottoms of deep craters there have never seen it, possibly for billions of years. Nothing warms those floors, and with no air to carry heat over, they stay colder than -270 °F while the crater's rim a few miles away gets roasted. So you've got a place hot enough to melt lead a short walk from a sheet of ice that's probably tens of millions of years old, on the same planet, on the same afternoon. If you left a beer in the wrong crater it would stay cold for a billion years.

That ice also has a bigger story to tell. Water didn't start out on Mercury. Most of it was probably delivered by comets and asteroids crashing in, which is one of the leading ideas for how Earth got a lot of its water too. So those frozen crater floors may be a record of how water gets moved around the solar system, sitting right next to the Sun, in the last place anyone would have looked.

And that's the thing about Mercury: every time we get a good look, it turns out to be weirder than the boring burnt rock we'd written it off as. Which brings us to the question nobody asks about it.

Four small facts about Mercury.

What Mercury actually offers you and me

So strip away the cool stories, and let's be blunt: is Mercury actually good for anything? Well, compared with Mars, Mercury has no fans, no billionaires and no plans. Nobody on Earth has announced a serious plan to send people there, which puts it behind Mars, the Moon, and a few things that exist only in Elon Musk's tweets. That being said, here's what it has that's real.

It has the most sunlight of any planet, up to eleven times more than Earth gets, with no clouds in the way and no weather to worry about. In a far future where humanity is doing serious work off Earth, that's a lot of free power for anyone who can survive the location. People who think about this stuff for fun have even suggested Mercury as the place you'd build big solar power arrays, and Mercury turns so slowly that at the equator the sunrise only creeps along at about 2.2 miles per hour on average, which is walking pace. So a base on wheels could in theory stay in comfortable twilight forever by just never stopping, and you could also, technically, outrun the dawn in a pair of very good shoes.

And all that sunlight happens to land on that cannonball I mentioned. Mercury holds an unusual amount of iron, it may have other useful metals near the surface, and its low gravity makes launching stuff off it much cheaper than launching from Earth. So if humanity ever gets serious about building things in space, a planet that's mostly metal, soaked in free solar power, is at least worth a mention. Again, that's a very big if.

It also has that ice, and of all the places to find it, it's next door to a free power supply. Those craters at the poles sit close to sunlit rims, so you could in principle have ice and solar power within reach of each other, and that's the same recipe that makes Mars worth talking about, since water means drinking water, oxygen and rocket fuel.

All of that is the far future, however. What Mercury offers right now is a test. It's the best natural laboratory we've got for gravity, because anything that's wrong with our theories shows up there first, and that's already how it took Newton down. It's also a sample of what happens to a rocky planet right up against its star, and astronomers have now found huge numbers of small rocky planets orbiting very close to other stars, so if you want to understand those worlds, Mercury's the only one we can actually go and poke.

Now the honest limits. Getting there takes years, and it's expensive even by space standards. There's no atmosphere, the radiation is brutal, a base anywhere but a polar crater has to deal with a 1,100 degree swing, and any plan for mining it depends on a space economy that doesn't exist. So as a home, Mercury's a terrible idea, and as a backup for humanity, it loses to Mars in every single way, and Mars is trying to kill you too.

No Mercury print yet.

NeoPrint makes custom 3D printed objects. This is what the store lists today.

NeoPrint has no Mercury print or kit yet. Its store today lists 7 objects in the Moon Collection, shown as concepts, plus the Fossil Collection and a Make Your Own page. None of them is a Mercury product, so there is nothing to buy for this issue.

Visit the NeoPrint store

So, should you and I care about Mercury?

Mercury doesn't care about us. It'll keep whipping around the Sun every 88 days, wrinkling slowly as its core cools, whether or not anyone ever looks up.

It's a burnt, airless, shrinking lump of iron, and you'd be dead within seconds of showing up. That being said, it's also the planet that told us Newton wasn't the last word, it's what a world turns into when it gets too close to its star, and it's been keeping ice in the dark for tens of millions of years, a short walk from ground that melts lead.

So yeah, give a damn, a small one, about the size of Mercury. It isn't going to save anyone, and it isn't asking to. It just keeps quietly proving us wrong, and it's been doing it for free, which is more than I can say for most of the experts on TV.

Next time, the daily updates take one piece of this and go all the way in. We'll stand in the permanent dark of a polar crater, follow the crash of MESSENGER, and watch the Sun stop and back up at noon over the Caloris basin.

What stuck?

  1. Mercury spins exactly three times for every two trips around the Sun. What does that make a single Mercury day?
  2. Mercury is the closest planet to the Sun, so why is it not the hottest planet?
  3. Why did Le Verrier think there was an unseen planet called Vulcan?
  4. What did Einstein's general relativity say actually explained Mercury's odd orbit?
  5. Falling toward the Sun sounds easy, so why is it so hard to get a spacecraft into orbit around Mercury?
  6. Why can ice survive on Mercury, a planet that melts lead?

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