7 Hot Facts About Mercury

Mercury, the diminutive planet closest to the Sun, was notoriously mysterious due to its difficulty to explore. That changed on March 18, 2011, when the MESSENGER spacecraft from Johns Hopkins' Applied Physics Laboratory achieved orbit around Mercury. The mission spent the next four years transforming scientists' understanding of how Mercury works and what it is made of. Mental Floss spoke to Sean Solomon, the principal investigator of MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), to learn what's most interesting about the first rock from the Sun.

1. MEET MERCURY BY THE NUMBERS.

Mercury is the smallest terrestrial planet of the solar system. Comparatively, Mercury is about midway in size between Earth's moon and the planet Mars. (Mars is a lot smaller than you might think, and our moon a lot larger.) Mercury is 3032 miles in diameter, which is, as the crow flies, just a little less than the distance from Anchorage to Dallas. Its gravity is 38 percent of Earth's, which means if you weigh 150 pounds here, you'd weigh 57 pounds on Mercury (the same as you would on Mars).

One day on Mercury lasts 59 Earth days, and one year lasts 88, which would make figuring out your age a thorny algebra problem. As you might imagine, days on Mercury can get pretty hot—around 800°F. On Earth a brick of coal at that temperature would burst into flames. (This is not a problem on Mercury, as the planet lacks an atmosphere.) Its nights, meanwhile, are a brisk -280°F. This is the widest day-to-night temperature variation of any planet in the solar system, and would make packing for a trip there very difficult indeed.

2. DESPITE BEING CLOSEST TO THE SUN, IT ISN'T THE HOTTEST PLANET.

Logic would suggest that Mercury is the hottest planet, considering its proximity to the giant fusion reactor at the center of our solar system that is 1,400,000,000,000,000,000,000,000,000,000 cubic meters in volume. The hottest planet honor, however, belongs to its neighbor Venus, one planet away, where the average surface temperature is 864°F. On Venus, lead would melt the way an ice cube melts on Earth.

3. MERCURY HAS SURPRISING CHEMISTRY.

Pretty much everything about Mercury should astound the casual observer, but what most surprises the principal investigator of MESSENGER, the first orbiter mission there? "The chemistry—that was the biggest surprise," says Solomon, who is also director of the Lamont-Doherty Earth Observatory at Columbia University. "We still don't have a good physical and chemical model for planet formation, and so the result that Mercury is this iron-rich planet, in which the silicate fraction is not only not depleted in elements easily removed by high temperatures, but is more abundant in some of those elements than Earth." The big takeaway from Mercury's chemical profile, Solomon says, is that "we don't really understand how the planets were assembled."

4. UNDERSTANDING ITS FORMATION WILL HELP US UNDERSTAND THE TERRESTRIAL PLANETS.

"How did we end up with four bodies of rock and metal that are quite different?" asks Solomon. "Venus and Earth are different because of their different atmospheres. The different evolution of the climate, and the feedback between climate and interior, led to very different tectonic evolution."

Mars and Earth are different because Mars is so much smaller than Earth, only 10 percent of Earth's mass, he explains. As for Mars and Venus: "A lot of Mars's atmosphere was stripped away by the solar wind, so it turned into this cold, barren desert world, whereas Venus has this dense CO2 atmosphere. Runaway greenhouse [effect] turned it into a hothouse world." Earth is in between.

Mercury suggests that the process of planet forming depends on more than simply planet size, solar distance, and differences in atmosphere. The original building blocks of planets also varied across the inner solar system in important ways. "The chemistry varied, volatile abundances varied, and some conditions must have helped during planet formation that can't be ascribed to late-stage processes like a collision," Solomon says.

Now that we've performed one comprehensive study of Mercury, scientists can endeavor to explain the diversity of the terrestrial planets. "We now have filled in the last missing piece in describing the four siblings of that process [of planetary formation]. They're all different, and yet the parental processes, if you will, must have been in common, so it's a kind of planetary genome expression," Solomon says. "How the heck can gene expression be so different among these four siblings, given that they all started out at the same time by the same processes, in just slightly different places in the inner solar system?"

5. MERCURY IS SHRINKING.

"There are faults all over the surface, and most of those faults involve horizontal shortening," or shrinking. The idea goes all the way back to Mariner 10, a robotic space probe launched by NASA in 1973, says Solomon. "The faults that accommodate horizontal shortening are seen on top of every kind of terrain, and they have a wide range of orientations. The Mariner 10 proposed—and the MESSENGER team confirmed—that contraction has dominated the history of the planet, and is consistent with the planet shrinking over time as the result of interior cooling and contraction of the interior." This tectonic activity has been active over most of the history of the planet, as the planet continues to cool.

But were you to stand on Mercury's surface, you couldn't expect Seti Alpha VI-like cataclysms as the planet suddenly contracts. "Were we to send a seismic experiment to Mercury, we would probably see mercury-quakes not anywhere near the frequency or size of earthquakes, but something more akin to moonquakes," Solomon says.

6. IT HAS WATER ICE.

The orientation of craters found on the poles of Mercury allows for permanently shadowed regions—that is, areas that never receive sunlight, no matter the planet's rotational position or place in its revolution. The conditions in those craters are amenable to stable water ice, on or mere centimeters below the planet's surface. MESSENGER's nuclear spectrometer yielded measurements consistent with water ice on the north pole, and its camera later captured optical-light images of that ice.

7. IT'S HARD TO GET NEAR—BUT WE'RE GOING BACK.

Only two missions have thus far explored Mercury: the Mariner 10 space probe in 1974, and the MESSENGER orbiter in 2011. This is in part because of the tremendous challenges associated with visiting the planet. "Mercury is in a challenging environment," says Solomon. "The Sun is 11 times brighter than it is at Earth. The surface temperature of the day-side is very hot. The night-side temperature, however, is quite cold, so the swings in temperature are large. The radiation environment that close to the Sun is challenging, as we anticipated going into the mission. We were hit directly by streams of energized particles from the Sun."

Mariner 10 performed three fast flybys of Mercury, and scientists spent the next three decades working largely from the close-up science it performed. Mariner's findings and the questions they raised would further contribute to the scientific rationale of an orbiter—what would be the eventual MESSENGER spacecraft.

A Mercury orbiter, of course, is no small order, and placing a spacecraft in orbit around that planet is one of the great achievements of the American space program. You can't just fly to Mercury and enter orbit. A spacecraft would be moving at a velocity far too great for that, as Mercury lacks the atmosphere to allow aerobreaking. Instead, a trajectory had to be calculated in which MESSENGER bounced around the solar system, from Earth, around the Sun and back to Earth; around the Sun and to Venus; around the Sun and back to Venus; and around the Sun four more times, flying closer and closer to Mercury each time, until at last it could enter Mercury's orbit. In essence, MESSENGER borrowed the gravity of other planets to compensate for what Mercury could not provide on a direct flight.

Due to this circuitous route, MESSENGER had to travel 5 billion miles over six-and-a-half years to reach a planet 100 million miles away. Once there, the challenge continued. The spacecraft had to maintain an orientation that kept between its scientific payload and the Sun a giant sunshade, lest the Sun fry the instruments. But extreme heat wasn't the only problem. So was extreme cold. When the spacecraft crossed into Mercury's shadow, an onboard heater had to warm the spacecraft lest the instruments freeze.

Despite the challenges, we're going back. The next mission bound for Mercury will launch in 2018. BepiColombo, a joint mission between the European and Japanese Space Agencies, will place two satellites in orbit around Mercury, where they will study its composition, tenuous atmosphere, and magnetosphere. Like MESSENGER, the spacecraft will require a complex trajectory—and a very long time to reach its target. It will achieve orbit around Mercury in December 2025.

Could an Astronaut Steal a Rocket and Lift Off, Without Mission Control?

iStock
iStock

C Stuart Hardwick:

Not with any rocket that has ever thus far carried a person into orbit from Earth, no. Large rockets are complex, their launch facilities are complex, their trajectories are complex, and the production of their propellants is complex.

Let me give you one simple example:

  • Let’s say astro-Sally is the last woman on Earth, and is fully qualified to fly the Saturn-V.
  • Further, let’s say the Rapture (which as I understand it, is some sort of hip-hop induced global catastrophe that liquefies all the people) has left a Saturn-V sitting on the pad, raring to go.
  • Further, let’s grant that, given enough time, astro-Sally can locate sufficient documentation to operate the several dozen controls needed to pump the first stage propellant tanks full of kerosene.
  • Now what? Oxidizer, right? Wrong. First, she has to attend to the batteries, oxygen, hydrogen, and helium pressurant tanks in her spacecraft, otherwise it’s going to be a short, final flight. And she’ll need to fill the hypergolics for the spacecraft propulsion and maneuvering systems. If she screws that up, the rocket will explode with her crawling on it. If she gets a single drop of either of these on her skin or in her lungs, she’ll die.
  • But okay, maybe all the hypergolics were already loaded (not safe, but possible) and assume she manages to get the LOX, H2, and HE tanks ready without going Hindenburg all over the Cape.
  • And…let’s just say Hermione Granger comes back from the Rapture to work that obscure spell, propellantus preparum.
  • All set, right? Well, no. See, before any large rocket can lift off, the water quench system must be in operation. Lift off without it, and the sound pressure generated by the engines will bounce off the pad, cave in the first stage, and cause 36 stories of rocket to go “boom.”
  • So she searches the blockhouse and figures out how to turn on the water quench system, then hops in the director’s Tesla (why not?) and speeds out to the pad, jumps in the lift, starts up the gantry—and the water quench system runs out of water ... Where’d she think that water comes from? Fairies? No, it comes from a water tower—loaded with an ample supply for a couple of launch attempts. Then it must be refilled.

Now imagine how much harder this would all be with the FBI on your tail.

Can a rocket be built that’s simple enough and automated enough to be susceptible to theft? Sure. Have we done so? Nope. The Soyuz is probably the closest—being highly derived from an ICBM designed to be “easy” to launch, but even it’s really not very close.

This post originally appeared on Quora. Click here to view.

The Science Behind Why the Earth Isn't Flat

Earth as captured from near the lunar horizon by the Lunar Reconnaissance Orbiter in 2015.
Earth as captured from near the lunar horizon by the Lunar Reconnaissance Orbiter in 2015.
NASA

On March 24, 2018, flat-earther Mike Hughes set out prove that the Earth is shaped like a Frisbee. The plan: Strap himself to a homemade steam-powered rocket and launch 52 miles into sky above California’s Mojave Desert, where he'd see Earth's shape with his own eyes.

It didn't matter that astronauts like John Glenn and Neil Armstrong had been to space and verified that the Earth is round; Hughes didn't believe them. According to The Washington Post, Hughes thought they were "merely paid actors performing in front of a computer-generated image of a round globe."

The attempt, ultimately, was a flop. He fell back to Earth with minor injuries after reaching 1875 feet—not even as high as the tip of One World Trade Center. For the cost of his rocket stunt ($20,000), Hughes could have easily flown around the world on a commercial airliner at 35,000 feet.

Hughes isn't alone in his misguided belief: Remarkably, thousands of years after the ancient Greeks proved our planet is a sphere, the flat-Earth movement seems to be gaining momentum. "Theories" abound on YouTube, and the flat-Earth Facebook page has some 194,000 followers.

Of course, the Earth isn't flat. It's a sphere. There is zero doubt about this fact in the real, round world. To say the evidence is overwhelming is an understatement.

HOT SPINNING BODIES

Not every celestial body is a sphere, but round objects are common in the universe: In addition to Earth and all other known large planets, stars and bigger moons are also ball-shaped. These objects, and billions of others, have the same shape because of gravity, which pulls everything toward everything else. All of that pulling makes an object as compact as it can be, and nothing is more compact than a sphere. Say, for example, you have a sphere of modeling clay that is exactly 10 inches in diameter. No part of the mass is more than 5 inches from the center. That's not the case with any other shape—some part of the material will be more than 5 inches from the center of the mass. A sphere is the smallest option.

Today the Earth is mostly solid with a liquid outer core, but when the planet was forming, some 4.5 billion years ago, it was very hot and behaved like more like a fluid—and was subject to the squishing effects of gravity.

And yet, the Earth isn't a perfect sphere; it bulges slightly at the equator. "Over a long time-scale, the Earth acts like a highly viscous fluid," says Surendra Adhikari, a geophysicist at the Jet Propulsion Laboratory in Pasadena, California. The Earth has been spinning since it was formed, and "if you have a spinning fluid, it will bulge out due to centrifugal forces." You can see evidence for this at the equator, where the Earth's diameter is 7926 miles—27 miles larger than at the poles (7899 miles). The difference is tiny—just one-third of 1 percent.

THE SHADOW KNOWS

The ancient Greeks figured out that Earth was a sphere 2300 years ago by observing the planet's curved shadow during a lunar eclipse, when the Earth passes between the Sun and the Moon. Some flat-Earth believers claim the world is shaped like a disk, perhaps with a wall of ice along the outer rim. (Why no one has ever seen this supposed wall, let alone crashed into it, remains unexplained.) Wouldn't a disk-shaped Earth also cast a round shadow? Well, it would depend on the orientation of the disk. If sunlight just happened to hit the disk face-on, it would have a round shadow. But if light hit the disk edge-on, the shadow would be a thin, straight line. And if the light fell at an oblique angle, the shadow would be a football–shaped ellipse. We know the Earth is spinning, so it can't present one side toward the Sun time after time. What we observe during lunar eclipses is that the planet's shadow is always round, so its shape has to be spherical.

The ancient Greeks also knew Earth's size, which they determined using the Earth's shape. In the 2nd century BCE, a thinker named Eratosthenes read that on a certain day, the people of Syene, in southern Egypt, reported seeing the Sun directly overhead at noon. But in Alexandria, in northern Egypt, on that same day at the same time, Eratosthenes had observed the Sun being several degrees away from overhead. If the Earth were flat, that would be impossible: The Sun would have to be the same height in the sky for observers everywhere, at each moment in time. By measuring the size of this angle, and knowing the distance between the two cities, Eratosthenes was able to calculate the Earth's diameter, coming up with a value within about 15 percent of the modern figure.

And when Columbus set sail from Spain in 1492, the question wasn't "Would he fall off the edge of the world?"—educated people knew the Earth was round—but rather, how long a westward voyage from Europe to Asia would take, and whether any new continents might be found along the way. During the Age of Exploration, European sailors noticed that, as they sailed south, "new" constellations came into view—stars that could never be seen from their home latitudes. If the world were flat, the same constellations would be visible from everywhere on the Earth's surface.

Finally, in 1522, Ferdinand Magellan's crew became the first people to circle the globe. Like Columbus, Magellan also set off from Spain, in 1519, heading west—and kept generally going west for the next three years. The expedition wound up back at the starting point (though without Magellan, who was killed during a battle in the Philippines). And speaking of ships and seafaring: One only needs to watch a tall ship sailing away from port to see that its hull disappears before the top of its mast. That happens because the ship is traveling along a curved surface; if the Earth were flat, the ship would just appear smaller and smaller, without any part of it slipping below the horizon.

THE EVIDENCE IS ALL AROUND (AND ALL ROUND)

But you don't need a ship to verify the Earth's shape. When the Sun is rising in, say, Moscow, it's setting in Los Angeles; when it's the middle of the night in New Delhi, the Sun is shining high in the sky in Chicago. These differences occur because the globe is constantly spinning, completing one revolution per day. If the Earth were flat, it would be daytime everywhere at once, followed by nighttime everywhere at once.

You also experience the Earth's roundness every time you take a long-distance flight. Jetliners fly along the shortest path between any two cities. "We use flight paths that are calculated on the basis of the Earth being round," Adhikari says. Imagine a flight from New York to Sydney: It would typically head northwest, toward Alaska, then southwest toward Australia. On the map provided in your airline's in-flight magazine, that might look like a peculiar path. But wrap a piece of string around a globe, and you'll see that it’s the shortest possible route.

"If the Earth were flat," Adhikari says, "the trajectory would be completely different." How different depends on which way the globe is sliced into a flattened map, but if it looked like it does on a Mercator-projection map, it might head east and pass over Africa.

Engineers and architects also take the Earth's curvature into account when building large structures. A good example is the towers that support long suspension bridges such as the Verrazano Narrows bridge in New York City. Its towers are slightly out of parallel with each other, the tops being more than 1.5 inches further apart than their bases. If the Earth were flat, the bottom of the towers would be separated by the exact same distance as the top of the towers; the planet's curvature forces the tops of the towers apart.

And for the last half-century, we've had eyewitness and photographic proof of the Earth's shape. In December 1968, the crew of Apollo 8 left Earth for the Moon. When they looked out of the Command Module windows, they saw a blue-and-white marble suspended against the blackness of space. On Christmas Eve, lunar module pilot William Anders snapped the famous "Earthrise" photograph. It gave us an awe-inspiring perspective of our round planet that was unprecedented in human history—but it wasn't a surprise to anyone.

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