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Expanded-color image of Mercury’s 52-km Degas crater, showing an abundance of dark material (NASA)

Today brings interesting color themed news from outer space. The Messenger space craft (which was destroyed when it was deliberately crashed into Mercury in spring of 2015) spotted numerous mysterious dark spots on Mercury. Indeed the Messenger spacecraft probably now is a dark spot on Mercury. Apparently the small dense planet has a dark layer close beneath the surface. Asteroid impacts, volcanoes, space probe collisions, and other events which disturb the surface of the planet reveal this extremely dark black/gray layer.

Scientists have been analyzing the data from Messenger and it now seems that this black layer which is the color and texture of pencil lead is actually composed of…graphite, the same material as pencil lead! Apparently when Mercury formed (which featured strange geological processes unseen anywhere else in the solar system) a planet-sized ocean of lava covered the entire world. As Mercury cooled the heavier elements of this lava field crystallized and sank leaving the buoyant pure carbon at the top. This dark layer has been subsequently covered with ejecta, dust, and fragments, but any disruption shows the crystallized carbon is still there.

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Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

There is a lot more data from Messenger left to analyze. I wonder what other surprises the closest planet to the sun still holds.

Artist's conception of MESSENGER above Mercury (NASA)

Artist’s conception of MESSENGER above Mercury (NASA)

On Thursday, humankind is deliberately crashing a spaceship into another planet! We could easily be the evil aliens in someone else’s space drama. Well, at least we could be, if there were any remote chance that Mercury, the intended target of our bombardment, were a possible haven for life.  And bombardment is not really the right word: what is actually scheduled is the seemly & rational conclusion to NASA’s MESSENGER mission, a highly successful exploration of the solar system’s mysterious innermost world.  The mission has been ongoing for more than a decade (a decade of our Earth time—or nearly 40 Mercury years).

A portrait of Mercury from MESSENGER

A portrait of Mercury from MESSENGER

The 485-kilogram (1,069 pound) MESSENGER spacecraft was launched from Cape Canaveral in August 2004. The space probe has an awkward and contrived government acronym, which is why I keep talking about it in all caps—I’m not shouting (although planetary exploration does make me very excited). The craft took some amazing pictures of Venus (a planet which always calls to me) on its way to Mercury.  Then MESSENGER flew by the small planet multiple times before entering orbit on March 18, 2011 (the first human spacecraft to do so).  Since then MESSENGER has extensively scanned and mapped the surface of Mercury—a planet which is surprisingly elusive to astronomers because of its proximity to the sun.  The mission revealed some surprising results which are leading to big new questions.

False Color Maps of Mercury (NASA)

False Color Maps of Mercury (NASA)

Mercury has a small diameter—it is actually smaller in area than some of the moons of Saturn and Jupiter—but it has substantial mass because much of it is made of heavy metals.  The face of the small world is thought to be ancient: scientists speculated that its bland pitted face might date back to the formation of the solar system, but it seems that Mercury does harbor secrets.

The mission featured a big surprise.  Messenger found surface water in the form of ice frozen inside the polar craters of Mercury.  This was not really a shock—astronomers have suspected that ice was present due to radio-telescope readings.  What was surprising was that the ice was coated with tarlike black goo. My poor roommate (who is always wandering the house pointing at films, stains, and accretions in horror) would not be surprised by a black coating on anything, however scientists were taken aback because Mercury was not thought to have any “volatile” compounds.  According to the current models of planetary formation, elements like chlorine, sulfur, potassium and sodium should have boiled away during the cataclysmic high-temperature formation of Mercury…yet there they are, like the scum in my kitchen. The scientific data from MESSENGER is likely to force a rethink of planetary formation (although frankly, considering all of the weird exoplanets that are being discovered, scientists probably need to refine their theories about planetary accretion anyway). The mission also measured subtle planetary flux which should give us a better sense of Mercury’s composition and internal workings.

The yellow patches show areas where water ice is believed to exist. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

The yellow patches show areas where water ice is believed to exist. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

All good things must end, however, and MESSENGER has run out of fuel for maneuvering.  Mission controllers have opted for an operatic exit and they are smashing the craft into the planet’s surface at 8,750 miles per hour (nearly four kilometers per second).  This should create an 18 meter (50 foot) wide crater.  Future scientists will have a known fresh disturbance to use as a benchmark for assessing the ancient craters of Mercury.  Perhaps the plume will reveal some interesting secrets as well.

MESSENGER Crashes into Mercury (Wayne Ferrebee, 2015, ink and colored pencil)

MESSENGER Crashes into Mercury (Wayne Ferrebee, 2015, ink and colored pencil)

Unfortunately, it will be a while before we see the results of our destructive acts.  The site of impact is hidden from Earth, and we have no other spacecraft in any proximity to Mercury. A European and Japanese collaboration called BepiColombo is scheduled to launch from Earth in 2017 and arrive at Mercury in 2024.  Perhaps we will have new questions for whatever answers MESSENGER is about to divulge in its unseen but spectacular final act!

Update: Through some grotesque oversight, NASA failed to portray MESSENGER’s final moments through the magic of art. I took the liberty of providing my own interpretation above.  NASA did not return my questions about whether the spacecraft will wail in a plaintive manner as it impacts the surface–so I am forced to assume that it will.  Did I mention that Mercury has no atmosphere?  You should probably ignore that…

An artist's rendition of NASA's MESSENGER spacecraft

Yesterday NASA’s spacecraft MESSENGER entered orbit around Mercury, the least explored of the Solar system’s rocky inner planets.  This is the first time a spacecraft has been in orbit around Mercury and it represents a tremendous engineering achievement. Since gravity becomes more intense the closer one comes to the sun, Messenger had to slingshot back and forth among the inner planets for some time in order to accomplish the tricky feat. The spacecraft had to undertake a 4.9 billion mile (about 7.9-billion kilometer) journey to enter orbit around the closest planet to the sun. Of course that hefty mileage only is equal to 0.00083 light years!

Having survived the grueling trip, the spaceship must now carry out its mission in the blistering bath of solar radiation.  To survive next to the star, Messenger is equipped with a large sun visor which prevents the little craft from frying like a quail egg.

NO! The Messenger spacecraft is not an old lady playing golf!

Messenger will try to determine the planet’s mineralogical composition and learn about its geological history (the surface of Mercury is reckoned to be one of the oldest in the solar system).  The robot probe will fully map Mercury and analyze the planet’s composition.  Like Earth (but unlike Mars and Venus) Mercury has an internal magnetic field.  Additionally, the tiny world is incredibly dense. In order to learn more about the planet’s core Messenger will measure the extent to which the planet wobbles on its rotational axis.  Studying the partially molten interior of Mercury should provide clues about how the planet formed which will help us better understand the creation of all planets (especially in conjunction with the flood of data regarding exoplanets which we are beginning to receive).

Since the craft will be trying to learn the secrets of Mercury’s molten interior, it is worth reflecting on the deity whom the planet is named after.   Although he was worshipped as a messenger, a herald, and a god of commerce, the Greco Roman god Hermes/Mercury was also quietly worshipped as a god of the underworld. The Greeks and Romans regarded him as a psychopomp who guided souls down to Hades with his magical staff. Because (like the somewhat similar African traveling god Eshu) Hermes was able to go anywhere at will he was one of the only entities in the Greco Roman pantheon free to enter and leave the underworld.

Although we are not capable like Mercury of going everywhere at our whim, I think it is a tremendous accomplishment to navigate a robot spacecraft into broiling orbit around the innermost planet.  That we are using the craft to learn the secrets of the fiery underworld of the swift planet seems like a fitting tribute to the god who was slayer of Argus, giver of charms, messenger, schemer, luck bringer, and patron of travelers and wayfarers (even those voyaging to their last end or to places the ancients could never dream of).

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