NewMyths.com Issue #59

Hellfire

You can depend on your eyes when your imagination is out of focus. Mark Twain

Even though it is one of our closest planetary neighbours at 77 million kilometers from Earth, the planet Mercury is also one of the least known. There are several good reasons for this. It is not the easiest planet to observe since one has to turn the telescope towards the intense light of the Sun in order to see it. Complicating matters of observation further is the fact that Mercury never strays more than 27 degrees from the Sun.

With such a shallow presence in the sky, in order to see Mercury, you must observe it during the day, making it difficult or even impossible to see (astronomers do most celestial observations at night for a reason), or shortly before sunset or after sunrise with the Sun hanging low over the horizon. At such a low angle, there is also much more atmospheric turbulence that the Earth-bound observer must look through in order to see the planet. Even with the best land-based telescopes, it is still only just possible to view surface features on Mercury a few hundred kilometres across. In spite of the observation difficulties, however, the planet has been known at least since the time of the Sumerians in the third millennium BC.

The ancient Greeks knew Mercury by two names, Apollo for the morning star and Hermes for the evening star. In spite of the name differentiation, the Greeks did know that they were naming the same object. German astronomer Johann Hieronymous Schroeter (1745-1816) was the first to observe the surface of Mercury and to draw some surface detail. Later observations however indicated that some of that detail was inaccurate including the “sighting” of twenty kilometre high mountains. In the 1880’s, Italian astronomer, Giovanni Schiaparelli, more famous for his “canali” on Mars, mapped Mercury as well and suggested that it had a rotational period of 88 days that was the same as its orbital period. When the rotational period or day of a planet is the same as its orbital period or its year, it is known as orbital lock. This means that the same face of the planet faces its sun forever. Mercury turned out to be different than was first anticipated, though. Radar observations in 1962 showed that the planet rotates three times in two of its years. In other words, there is no orbital lock.

Several early science fiction authors have looked at a Mercury with an orbital lock as it made for good storytelling, a planet staring continually into the bright face of a star. It was also, as with all good science fiction, based on the science that was known at the time of writing. One of the earliest is Ray Cummings’ 1931 Tama, Princess of Mercury followed in 1941 Aerita of the Light Country which describes adventures on a tidally locked Mercury. Leigh Brackett wrote several short stories "The Demons of Darkside", "A World is Born", "Cube from Space", and "Shannach-the Last" in the 1940’s that were also set on a tidally locked Mercury. Some early Isaac Asimov tales also take place on a tidally locked world including Runaround, The Dying Night and Lucky Starr and the Big Sun of Mercury. Arthur C. Clarke in his 1952, Islands in the Sky, describes a creature that lives on the permanent darkside of Mercury. In his 1959 The Sirens of Titan, Kurt Vonnegut Jr. describes animals known as “harmoniums” that live on a tidally locked Mercury. They derive sustenance from the energy of vibrations in the rocks of their cave homes.

American astronomer Percival Lowell (1855-1916), also of Martian Canals fame, found canal-like details on Mercury’s surface. Much like his Martian canals, they were later found to be another illusion, by Greek astronomer, Eugenios Antaniadai (1870-1944). In addition, the observations and the maps drawn by Antaniadai were so good that they provided the best surface detail of Mercury up until the arrival of Mariner 10 in the planet’s vicinity on March 29, 1974.

Mercury has a highly eccentric orbit, 46 million kilometers from the Sun at its closest and 70 million at its furthest, which did not follow the predictions of Newtonian gravity. French mathematician Urbain de Verrier, in 1859, stated that the observations of Mercury could be explained by the presence of another planet inside of Mercury’s orbit. The only way to observe the innermost hypothetical planet named Vulcan was to see it as it transited across the disk of the Sun. Several astronomers even claimed to have seen the planet predicted by de Verrier, this in spite of its non-existence. After the solar eclipse of 1878, however, there were no more reports of the hypothesized inner planet. The resolution to the problem of explaining Mercury’s orbit came with Einstein’s General Theory of General Relativity in 1915. No inner planet Vulcan was required. The theory explained the movements of Mercury perfectly, where Newtonian mechanics could not. In spite of the resolution, in 1971, there was again some flurry of activity stating that a planet inside of Mercury’s orbit was found. Later the observations were explained to be comets inside the orbit of Mercury and not a new planetary body.

When Mariner 10 arrived at Mercury, many initial observations were nothing short of startling, and certainly alien. The craft came as close as 327 kilometers to the planetary surface. True, the surface was found to be very much like that of the Moon, heavily cratered. Much of the likeness ends there though. Mariner 10 revealed cliffs, up to three kilometres high and hundreds of kilometres long crisscrossing the surface, a weak magnetic field constantly being challenged from the constant blast of solar wind right at its doorstep and an atmosphere about a trillionth as dense as that of the Earth.

The thin atmosphere of Mercury is unlike any atmosphere of the other terrestrial planets in our solar system which includes Venus, Earth and Mars in that it is only fleeting. With the solar wind at its entrance, the atmosphere is constantly being blasted away and subsequently replenished by the solar wind. An interesting sidebar to the Mariner 10 mission is that it had discovered evidence in the form of ultraviolet anomalies that Mercury had a moon; however, further examination found that the source of the light was instead the star 31 Crateris.

More answers to the questions that arose from the Mariner mission have come from the MErcury Surface, Space ENvironment, Geochemistry, and Ranging (MESSENGER) mission to Mercury. The craft was launched by NASA in August of 2004. It arrived in Mercury’s vicinity in January 2008. It made two more flybys of the planet during which time more mapping of the planet was carried out.

On March 18, 2011, MESSENGER was inserted into Mercury’s orbit. Some of the earlier data from MESSENGER showed that there was an abundance of magnesium and calcium on the nightside and the magnetic field originally discovered by Mariner 10 is offset from the north of the planet’s centre. In 2012, the craft also found that there was not only water ice but organic compounds in the permanently shadowed craters at the north pole. In February 2013, an accurate three dimensional map of Mercury was created. The mission was completed on April 30, 2015 when the craft was destroyed through a crash-landing on the surface of Mercury after running out of fuel. The data from the craft is still being analyzed and will enormously increase our knowledge of this hot (and cold depending on what side you are on) world.

What MESSENGER leaves unanswered may be rewarded by another future planned mission to Mercury. In October 2018, the European Space Agency and Japan will be launching a joint mission, BepiColombo, to the planet, complete with two probes, one to map the planet and the other to study its magnetosphere. It is due to arrive at the planet in early 2026. No human missions are envisioned at least not in the near future.

However, this fact has not deterred science fiction authors who foresee future expeditions and even the colonization of Mercury. Other authors have envisioned travelling to the planet. Alan Nourse wrote the short story,

"Brightside Crossing," about a group of four men who attempted to cross the planet’s sunlit side. Hugh Walter’s Mission to Mercury, is about a rescue mission to a group of explorers trapped on the cold dark side. Ben Bova’s Mercury, is about the exploration of the planet and how its proximity to the Sun might be an opportunity to harness an enormous energy source.

Some science fiction authors went even further and created colonies on Mercury. Gordon Dickson in Necromancer, describes a training camp on the planet. Rendezvous with Rama by Arthur C. Clarke, saw a rebellious government of metal miners on the planet that threaten to destroy the alien craft. Sundiver by David Brin which is about a mission to explore the Sun utilizes Mercury as its base of operations. In Kim Stanley Robinson’s 2312, the planet is home to the huge city Terminator that is populated by artists and musicians. Stephen Baxter’s Manifold: Space depicts Mercury as the last stronghold for the human race after waves of attacks by extraterrestrial colonizers. Mark Anson’s Below Mercury, is a thriller set on an abandoned mine of Mercury.

One question that remains an unknown and may be answered by the joint European Space Agency and Japanese mission is why is Mercury so dense? Though Earth is more dense, the size of Mercury makes scientists wonder why it has such a high density in spite of its small size.

One current theory is that before the planet was formed, the binding of dense particles was favoured by the drag of thin nebular gas. The result was that Mercury became metal rich with silicates remaining on the surface. Another explanation is that Mercury’s rocky outer layer was burned away by its close proximity to the Sun leaving only a rich metal cinder.

There are also questions about the origin of the plains on Mercury’s surface. Are they due to volcanism or impacts by objects from space? Data from MESSENGER appears to have found the answer with the evidence pointing to a volcanic origin. In fact, it found 51 pyroclastic deposits from shield volcanoes inside of impact craters. Pyroclastic flows and their eventual deposits are the result of highly explosive volcanic events here on Earth; the major eruptions on Earth such as Mount Vesuvius and Krakatoa were both pyroclastic.

Mercury also has several large craters surrounded by concentric rings of hills and valleys. It is speculated that the rings are actually the result of asteroid hits creating not only the actual crater but the resultant ripples in the crust, not unlike waves on water, only to have been frozen in place. The largest crater is called Caloris, which is 1550 kilometers in diameter. Geologists speculate that the crater was formed 3.6 billion years. Within the impact crater there are nine overlapping volcanic vents estimated to be around one billion years old, further evidence of a highly active geological past. What makes this impact even more fascinating is that it was so strong that it not only created the large crater, but also disrupted the surface on the other side of the planet as indicated by the many cracks and faults found there. This area is known as the “Weird Terrain.”

The magnetic field of Mercury suggests that there is a swirling molten iron core much like Earth’s core that is responsible for the magnetic field that envelopes us. Though the magnetic field of Mercury is stable, it is continually confronted with high energy radiation and particles from the Sun. This high-energy bombardment results in the creation of twisted bundles of magnetic fields known as magnetic tornadoes. Some astronomers believe that the creation of these magnetic tornadoes may play a role in the maintenance of Mercury’s tenuous atmosphere. Magnetic tornadoes are created through the interaction of the magnetic field of the solar wind and the magnetic field of Mercury. As the solar wind passes the planet, the joined magnetic fields, sometimes up to 800 kilometers wide, twist upwards into space creating a magnetic vortex.

Then there is the ice that has been discovered on Mercury. How do we explain the ice on a planet so near to the Sun? One must realize that in spite of its proximity to the Sun, that Mercury’s nightside is amongst the coldest places in the solar system with temperatures hovering around -173 degrees Celsius (Pluto’s average temperature is -218 degrees Celsius) as opposed to the daytime temperatures of 427 degrees Celsius. With the day as long as 58 Earth days and 15 hours (compared with its year of 88 Earth Days), temperatures have an opportunity to reach extremes. One theory on the origin of the ice is that it arrived there through impacts with comets and meteorites and froze in place in the perpetual shadows. There is also the possibility that the water ice may be the result of out gassing of water vapour from the planet’s interior. Another idea is that it is not true water ice. In Larry Niven’s The Coldest Place, the reader is taken on a ride thinking that the coldest place is Pluto only to find out it is on the nightside of Mercury; the story was written when Mercury was thought to be tidally locked to the Sun.

As inhospitable as the conditions on the planet appear to be, many writers have envisioned the possibility of life. One of the best is the short story by Stephen Baxter, who wrote Cilia-of-Gold. It is a story about an ancient but alien life form that adapted to the harsh planet after crash landing there. John Varley in Retrograde Summer took on a different bent with the premise that the life on Mercury is biologically engineered.

Mercury will remain a planet of contrasts, one of extreme heat and extreme cold. The mechanisms that create this dichotomy are still unknown but with each flyby or orbit by another spacecraft, we go a long way to answering those questions. With each flyby or orbit though the number of questions continues to grow, which makes the exploration of any new world exciting, even a world as “dull” as Mercury was thought to once be.

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