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.


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.


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.


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."


"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?"


"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.


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.


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.

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6 Amazing Facts About Sally Ride

U.S. National Archives and Records Administration, Public Domain, Wikimedia Commons
U.S. National Archives and Records Administration, Public Domain, Wikimedia Commons

You know Sally Ride as the first American woman to travel into space. But here are six things you might not know about the groundbreaking astronaut, who was born on May 26, 1951.

1. Sally Ride proved there is such thing as a stupid question.

When Sally Ride made her first space flight in 1983, she was both the first American woman and the youngest American to make the journey to the final frontier. Both of those distinctions show just how qualified and devoted Ride was to her career, but they also opened her up to a slew of absurd questions from the media.

Journalist Michael Ryan recounted some of the sillier questions that had been posed to Ride in a June 1983 profile for People. Among the highlights:

Q: “Will the flight affect your reproductive organs?”
A: “There’s no evidence of that.”

Q: “Do you weep when things go wrong on the job?”
A: “How come nobody ever asks (a male fellow astronaut) those questions?"

Forget going into space; Ride’s most impressive achievement might have been maintaining her composure in the face of such offensive questions.

2. Had she taken Billie Jean King's advice, Sally Ride might have been a professional tennis player.

When Ride was growing up near Los Angeles, she played more than a little tennis, and she was seriously good at it. She was a nationally ranked juniors player, and by the time she turned 18 in 1969, she was ranked 18th in the whole country. Tennis legend Billie Jean King personally encouraged Ride to turn pro, but she went to Swarthmore instead before eventually transferring to Stanford to finish her undergrad work, a master’s, and a PhD in physics.

King didn’t forget about the young tennis prodigy she had encouraged, though. In 1984 an interviewer playfully asked the tennis star who she’d take to the moon with her, to which King replied, “Tom Selleck, my family, and Sally Ride to get us all back.”

3. Home economics was not Sally Ride's best subject.

After retiring from space flight, Ride became a vocal advocate for math and science education, particularly for girls. In 2001 she founded Sally Ride Science, a San Diego-based company that creates fun and interesting opportunities for elementary and middle school students to learn about math and science.

Though Ride was an iconic female scientist who earned her doctorate in physics, just like so many other youngsters, she did hit some academic road bumps when she was growing up. In a 2006 interview with USA Today, Ride revealed her weakest subject in school: a seventh-grade home economics class that all girls had to take. As Ride put it, "Can you imagine having to cook and eat tuna casserole at 8 a.m.?"

4. Sally Ride had a strong tie to the Challenger.

Ride’s two space flights were aboard the doomed shuttle Challenger, and she was eight months deep into her training program for a third flight aboard the shuttle when it tragically exploded in 1986. Ride learned of that disaster at the worst possible time: she was on a plane when the pilot announced the news.

Ride later told AARP the Magazine that when she heard the midflight announcement, she got out her NASA badge and went to the cockpit so she could listen to radio reports about the fallen shuttle. The disaster meant that Ride wouldn’t make it back into space, but the personal toll was tough to swallow, too. Four of the lost members of Challenger’s crew had been in Ride’s astronaut training class.

5. Sally Ride had no interest in cashing in on her worldwide fame.

A 2003 profile in The New York Times called Ride one of the most famous women on Earth after her two space flights, and it was hard to argue with that statement. Ride could easily have cashed in on the slew of endorsements, movie deals, and ghostwritten book offers that came her way, but she passed on most opportunities to turn a quick buck.

Ride later made a few forays into publishing and endorsements, though. She wrote or co-wrote more than a half-dozen children’s books on scientific themes, including To Space and Back, and in 2009 she appeared in a print ad for Louis Vuitton. Even appearing in an ad wasn’t an effort to pad her bank account, though; the ad featured an Annie Leibovitz photo of Ride with fellow astronauts Buzz Aldrin and Jim Lovell gazing at the moon and stars. According to a spokesperson, all three astronauts donated a “significant portion” of their modeling fees to Al Gore’s Climate Project.

6. Sally Ride was the first openly LGBTQ astronaut.

Ride passed away on July 23, 2012, at the age of 61, following a long (and very private) battle with pancreatic cancer. While Ride's brief marriage to fellow astronaut Steve Hawley was widely known to the public (they were married from 1982 to 1987), it wasn't until her death that Ride's longtime relationship with Tam O'Shaughnessy—a childhood friend and science writer—was made public. Which meant that even in death, Ride was still changing the world, as she is the world's first openly LGBTQ astronaut.