5 Cratered Facts About Mercury

An enhanced-color composite image of Mercury's Caloris basin. Lavas appear orange, and blue areas are likely the original basin floor. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington


by Alex Carter

Mercury is the closest planet to the sun, and you would be forgiven for thinking it’s just a boring lump of rock. After all, there’s nothing interesting on it like aliens or places to drink. Still, Mercury is probably a more interesting place than you think...


Although you would be forgiven for thinking Mercury, being the closest planet to the Sun, is one of the hottest places in the solar system, it in fact is subject to enormous temperature fluctuations. Some regions reach 800°F, but without an atmosphere to retain heat, Mercury’s poles and nightside of the planet plunge well below even the coldest temperatures recorded on Earth. With some regions never getting above -279°F, conditions are perfect for ice to form. Mercury’s regolith is home to possibly a trillion tons of ice, which would make it one of the wettest places in the solar system.

Fitting, then, that the ancient Chinese called it the Water Star.


In the 19th century, scientists were smug in the knowledge that they knew everything there was to know. There were of course a couple of curiosities that they could not understand, one being the precession in the orbit of Mercury. That is to say, Mercury goes around the Sun in an ellipse rather than in a circle, and the ellipse changed the direction it was pointing from time to time. It was thought that there must be a new planet between Mercury and the sun that was changing its orbit—the planet Vulcan. But despite trying, the planet could not be observed.

Albert Einstein eventually disproved the theory of Vulcan via the general theory of relativity. Rather than looking for an external cause, Einstein showed that Mercury was doing exactly what it should do, and that gravity was just acting in ways no one had known it could. Mercury is so close to the Sun that not only is it pulled around the Sun, space itself is too. Were it not for Mercury demonstrating this effect, Einstein probably would not have been so readily believed.

Incidentally, the other curiosity was the photoelectric effect, which needed quantum mechanics to explain. That, not relativity, was what Einstein won his Nobel Prize for.


After six years orbiting the planet, NASA’s MESSENGER probe ran out of fuel in 2014 and could no longer correct its course. As its orbit decayed, it got closer to the planet, and much faster. The resulting crash into Mercury a year later occurred at more than 8000 mph and left a crater more than 50 feet wide. That makes it easily the biggest humanmade crater anywhere in the universe … apart from those on Earth, obviously.


Mercury orbits so close to the Sun that the conventional ideas of days and years don’t really make much sense. Mercury’s rotation (remember, rotation on a planet's axis causes its days) and orbit (which causes its years) are linked through gravity more tightly than Earth. In fact, Mercury rotates three times on its axis every two years. This makes each Mercury day—as in sunrise to sunset and back to sunrise—last two of its years. It’s a weird day, too: The Sun rises, goes backwards in the sky to set, rises again, then finally sets a year later. The next year, from the perspective of Mercury, the Sun would appear to move in the opposite direction.


Mercury’s small size means it has no permanent atmosphere, just a thin layer called an exosphere; its gravity is too weak to hold onto any gas in the wake of the strong solar wind. However, it does have a remarkably strong magnetic field, which means it keeps a hold of whatever ions come its way. There are a lot of ions in that part of space.

So while planets such as Earth are nice and oxygen filled, Mercury’s atmosphere contains the kind of things planets normally don’t hang on to, including ions of magnesium, calcium, sodium (you may remember from chemistry class that those are the ones that explode in water), as well as unusual ions of water. Mercury may smell like wet, metallic burps. 

Mysterious 'Hypatia Stone' Is Like Nothing Else in Our Solar System

In 1996, Egyptian geologist Aly Barakat discovered a tiny, one-ounce stone in the eastern Sahara. Ever since, scientists have been trying to figure out where exactly the mysterious pebble originated. As Popular Mechanics reports, it probably wasn't anywhere near Earth. A new study in Geochimica et Cosmochimica Acta finds that the micro-compounds in the rock don't match anything we've ever found in our solar system.

Scientists have known for several years that the fragment, known as the Hypatia stone, was extraterrestrial in origin. But this new study finds that it's even weirder than we thought. Led by University of Johannesburg geologists, the research team performed mineral analyses on the microdiamond-studded rock that showed that it is made of matter that predates the existence of our Sun or any of the planets in the solar system. And, its chemical composition doesn't resemble anything we've found on Earth or in comets or meteorites we have studied.

Lead researcher Jan Kramers told Popular Mechanics that the rock was likely created in the early solar nebula, a giant cloud of homogenous interstellar dust from which the Sun and its planets formed. While some of the basic materials in the pebble are found on Earth—carbon, aluminum, iron, silicon—they exist in wildly different ratios than materials we've seen before. Researchers believe the rock's microscopic diamonds were created by the shock of the impact with Earth's atmosphere or crust.

"When Hypatia was first found to be extraterrestrial, it was a sensation, but these latest results are opening up even bigger questions about its origins," as study co-author Marco Andreoli said in a press release.

The study suggests the early solar nebula may not have been as homogenous as we thought. "If Hypatia itself is not presolar, [some of its chemical] features indicate that the solar nebula wasn't the same kind of dust everywhere—which starts tugging at the generally accepted view of the formation of our solar system," Kramer said.

The researchers plan to further probe the rock's origins, hopefully solving some of the puzzles this study has presented.

[h/t Popular Mechanics]

The Ozone Layer Is Healing, Thanks to an International Ban on Harmful Man-Made Chemicals

The ozone layer is on the mend, thanks to a decrease in human-produced chemicals called chlorofluorocarbons, or CFCs, in the atmosphere. Using data from NASA's Aura satellite, scientists were able to measure the chemical composition of the thinned gas layer above the Antarctic and found about 20 percent less ozone depletion than there was in 2005. They published their findings on January 4 in the journal Geophysical Research Letters.

In 1985, UK scientists published a landmark study in the journal Nature announcing their discovery of an annually recurring hole in the ozone layer above Antarctica. (Each September, as the Southern Hemisphere's winter arrives, the Sun's UV rays trigger a reaction between the ozone and chemical elements from CFCs, chlorine and bromine, which destroys the ozone molecules.) The finding led to the Montreal Protocol in 1987, an international treaty that gradually banned the production and use of CFCs in refrigerants, aerosol sprays, solvents, and air conditioners.

In July 2016, Antarctic researchers published a study in the journal Science reporting that the ozone layer appeared to be healing (although it wasn't projected to completely patch up for decades). They tracked this progress by monitoring the Antarctic ozone hole's area, height, and chemical profile. Still, they didn't know whether this progress could be attributed to the Montreal Protocol's mandate.

NASA itself has used Aura to monitor the hole since the mid-2000s. After analyzing data produced by the Microwave Limb Sounder, a satellite instrument aboard Aura that measures trace gases, the space agency has confirmed that the CFC ban has led to the big decrease in ozone depletion during the Antarctic winter.

By winter, ozone-busting chlorine compounds have converted into hydrochloric acid, a process that occurs after it's destroyed ozone particles and reacts with methane. "By around mid-October, all the chlorine compounds are conveniently converted into one gas, so by measuring hydrochloric acid, we have a good measurement of the total chlorine," researcher Susan Strahan said in a NASA statement. Scientists compared these hydrochloric acid levels with nitrous oxide, which is similar in nature to CFCs but isn't diminishing in the atmosphere.

Their study is billed as "the first to use measurements of the chemical composition inside the ozone hole to confirm that not only is ozone depletion decreasing, but that the decrease is caused by the decline in CFCs," according to NASA. But while these initial results are promising, scientists say that the ozone layer's full recovery is still a long way off.

"As far as the ozone hole being gone, we're looking at 2060 or 2080,” study co-author Anne Douglass said. “And even then there might still be a small hole."


More from mental floss studios