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.

First-Ever Map of Titan Reveals That Saturn’s Moon Is a Lot Like Earth

NASA/JPL-Caltech/Univ. Arizona/Univ. Idaho
NASA/JPL-Caltech/Univ. Arizona/Univ. Idaho

If there's any life in this solar system outside Earth, we likely won't find it on Mars or even on another planet. Saturn's moon Titan is the place in our celestial neighborhood that's most similar to our own home, and it's where scientists think we have one of the best chances of discovering life. Now, as Nature reports, newly visualized data shows just how much Titan has in common with Earth.

Between 2004 and 2017, the NASA spacecraft Cassini performed more than 100 fly-bys of Saturn's moon. Titan is unique in that it's the only moon in the solar system with clouds and a dense, weather-forming atmosphere. This has made it hard to study from space, but by flying close to the surface, Cassini was able to capture the landscape in an unprecedented level of detail.

Map of Titan.
The first global geologic map of Titan.
NASA/JPL-Caltech/ASU

NASA's new map of Titan, published in the journal Nature Astronomy, reveals a varied world of mountains, valleys, plains, and sandy dunes that starkly contrast with the desolate wastelands we've seen on neighboring planets. It's also home to seas and lakes, making it the only place in the solar system other than Earth with known bodies of liquid. But instead of water, the pools mottling the moon's surface consist of liquid methane.

Even with its Earth-like geology and atmosphere, chances of finding life on Titan are still slim: Temperatures on the surface average around -300°F. If life does exist there, it's likely limited to microbes in the moon's craters and icy volcanoes.

It will be a while before NASA is able to study Titan up close again: NASA's next drone mission to the body is set for 2034. Until then, scientists have plenty of data recorded by Cassini to teach them more about how the moon formed and continues to change.

[h/t Nature]

A Rare Unicorn Meteor Outburst Could Be Visible for Less Than an Hour on Thursday

joegolby/iStock via Getty Images
joegolby/iStock via Getty Images

Your chances of seeing a unicorn this week are slim, but if you look up on Thursday night, you may see something that's almost as extraordinary. As Sky & Telescope reports, the upcoming Alpha Monocerotid meteor shower could produce a meteor outburst, which means there could be multiple shooting stars per second streaming from the unicorn constellation.

What is a unicorn meteor shower?

There's nothing particularly magical about the Alpha Monocerotids. They appear to originate near the star Procyon, which is next to the constellation Monoceros, the Greek name for unicorn.

The shower is known for occasionally packing a dense flurry of activity into a brief viewing window. The meteors appear between November 15 through the 25th of each year, and peak around the 22nd. Several times a century, the shower treats sky gazers to an "outburst" of shooting stars that lasts less than an hour.

Such an outburst is predicted for 2019. According to astronomers Peter Jenniskens and Esko Lyytinen, the Earth is on track to pass through a thick portion of the tail of the unknown comet that provides debris for the shower. The conditions are almost the same as they were in 1995, when the Alpha Monocerotids lit up the sky at a rate of 400 meteors per hour, which is approaching meteor storm levels. For that reason, the scientists are expecting shooting stars to appear in the same numbers this time around.

How to see the meteor outburst

Timing is crucial if you want to catch the Alpha Monocerotids, even more than with regular meteor showers. The outburst is expected to start at 11:15 p.m. EST and last just 15 to 40 minutes. Luckily, the sun will be fully set by then and the crescent moon won't rise until after 2 a.m, creating optimal viewing conditions for the eastern half of the country. The shooting stars are fast—traveling at 40 miles per second—and they come at random. Don't be surprised to wait a minute between meteors during some parts of the outburst and less than a second at others.

[h/t Sky & Telescope]

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