As we all know, Mercury is the planet closest to the sun. This world, bigger than the Moon, sits closer to the sun than almost any other object in the solar system. While not quite as hostile an environment as Venus, Mercury's proximity to the burning sun makes it an inhospitable, difficult to reach place. As of yet, it has only been explored by two spacecraft, both American.

Today I'll be going over this interesting planet with a somewhat brief overview.

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The Planet Mercury

Where is Mercury?

Mercury, as the innermost planet, lies closer to the sun than Earth and Venus. Unlike the other large planets, Mercury's orbit around the sun is not really that circular. With an orbital eccentricity of 0.21, Mercury's orbit brings the planet from 0.46 AU at aphelion to just 0.3 AU from the sun at perihelion (remember, the Earth is 1 AU from the sun on average). This results in larger temperature swings on Mercury, as it collects more energy from the sun at perihelion than it does at aphelion.

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The orbit is also somewhat inclined, with Mercury's inclination relative the ecliptic plane (the plane Earth orbits in) being about 7 degrees compared to Earth's ~0 degrees. This means that Mercury actually travels above Earth, relatively speaking, as it passes through its rapid 88 day orbital period (year).

Mercury's Unique Characteristics

Mercury's proximity to the sun means that the sun-facing side of the planet is quite hot. At the equator, the average temperature is about 70 degrees C, but this temperature can rise to upwards of 450 C. While warm, this isn't nearly as hot as hellish Venus, mostly due to Venus' dense atmosphere to trap thermal energy on the surface. Mercury isn't really the hottest planet due to the existence of Venus, and as you approach the north and south pole, the temperatures plummet, down to temperatures that would be considered cold on Earth.

Speaking of atmosphere, Mercury doesn't really have one. Technically, a trace atmosphere of oxygen and sodium gas is present. However, at a surface atmospheric pressure of ~ 1 nPa (0.000000001 Pa, or 0.00000000000001 times the surface pressure on Earth at seal level), this "atmosphere" isn't exactly significant, being even less than the constant, outward pressure provided by the sun's ionized solar wind at Earth.

Another unique aspects of Mercury is its significant planetary magnetic field. This field has a strength about 1% of Earth's magnetic field, but is significant nonetheless. Like Earth's magnetic field, Mercury's magnetic field interacts with the charged electrons/ions in the outbound solar wind. This separates Mercury from the planets Venus and Mars, which do not have significant planetary magnetic fields.

Illustration of Mercury's magnetic field
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This magnetic field, likely produced by an inner liquid iron core, can deflect and move solar wind away from the planet itself due to the wind's low kinetic energy. Awhile back I wrote a post (here) on magnetic radiation shielding, which covers the basics of why this happens. Mercury itself can be approximated as an enormous magnetic dipole, emitting a magnetic field.

The Surface of Mercury

No spacecraft has ever made it to the surface of Mercury intact, so I can't share pictures of what it would look like to stand on Mercury.

Externally, the surface of Mercury looks very similar to that of our own Moon. With no atmosphere or water to weather surfaces, thousands of preserved craters of various age litter the surface. Craters do eventually corrode (due to, among other things, other meteor impacts), but stick around much, much longer than they would on Earth (just look at the lack of craters on Earth compared to the moon!).

The largest crater on Mercury is the Caloris basin. This massive, ancient crater is 1500 kilometers wide - about the distance from San Diego to Portland. Over 3.8 billion years ago, an asteroid that no longer exists (about 100 kilometers wide) smashed into the side of the planet Mercury, producing the enormous Caloris crater. Even after all of this time, the giant crater still scars the surface of Mercury, visible as a ring of mountains surrounding a vast plain.

Caloris Planitia
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Perhaps shockingly, the surface of Mercury also contains a significant amount of water ice, despite its proximity to the sun. Polar craters on Mercury may contain significant ice deposits, perhaps covered in dirt. This could be an interesting resource for future exploration of Mercury, since water ice is essentially naturally occurring rocket fuel, as it can be split using electricity into oxygen and hydrogen, a potent high-Isp propellant when both are in liquid form. Using Mercury ice deposits, a hypothetical robotic lander could create more fuel for itself using the abundant solar energy as a power source (remember, the closer you get to the sun, the better solar panels get, in general). I'm thinking about doing a post on things like this (known as in-situ resource utilization) later if you're interested.

The Rembrandt Basin, another massive Mercury crater, superimposed onto the United States east coast.
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Over time, Mercury's internal has shrank in size as it cools. This caused the production of some interesting land formations up on the surface. One such example is Discovery Rupes, a giant cliff visible in the below picture:

Discovery Rupes
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This cliff is absolutely enormous. From bottom to top, the Discovery Rupes cliff is 2 kilometers high. If this is a straight-down vertical drop, it would be higher than anything on Earth.

This is Mount Thor, on the remote Canadian Baffin Island. It is the tallest vertical cliff on Earth, but the drop is only about 1.3 kilometers. Discovery Rupes, at 2 kilometers, is significantly higher than this assuming the drop is vertical.
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Mercury even has extinct volcanoes and lava flow beds, a remnant of its more violent past. Today, it remains a sterile but fascinating rocky world holding out against the onslaught of the sun.

Verifying General Relativity

Mercury also holds a place of honor as one of the first observations verified by Einstein's theory of general relativity. Mercury's perihelion (the point in an orbit closest to the sun) was observed to precess (essentially rotation of the orbit) ever so slightly faster than it should have if Newton's Law of Gravitation was entirely correct. This was originally thought to be caused by another planet (named Vulcan) closer to the sun than Mercury. This is a pretty reasonable assumption, since any planet that close to the sun would only be able to be observed just after sunset, just before sunrise, and during eclipses due to the massive brightness of the sun.

Adding on a relativistic correction provided by Einstein accounts for this extra precession. Unfortunately I haven't yet studied GR, so I can't really go into any much detail on why that works. The short of it is that Mercury being deep in the sun's large gravity well amplifies GR's effects.

Exploration

Only two spacecraft have visited Mercury: Mariner 10 and MESSENGER, both American/NASA spacecraft.

Reaching Mercury is extremely difficult. It being close to the sun means that a spacecraft must gain a lot of velocity in order to approach or orbit this planet. Because of this, very few spacecraft have visited the planet.

The first spacecraft to arrive was Mariner-10, a NASA spacecraft that managed to fly by Mercury three times before running out of maneuvering propellant. Launched in 1973, Mariner-10 used a Venus flyby to assist in its transfer to Mercury, and took the first close up images of the planet.

Mariner-10
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Almost certainly, Mariner-10 is still out there to this day, orbiting the sun in an elliptic orbit. It almost certainly doesn't function anymore due to being in the hostile space environment for decades, but somewhere above our heads, it's still up there. When you send something into a solar orbit, it stays there for a long time.

The second spacecraft to reach Mercury was NASA's MESSENGER probe (a perfect example of the excessive use of acronyms in the aerospace industry). For four years, MESSENGER orbited the planet Mercury, taking images of all sides of the planet and revolutionizing our knowledge of this world.

MESSENGER at Mercury
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At the end of its mission, MESSENGER impacted the surface of Mercury. This destroyed the spacecraft but gave it the prestigious honor of being the first and only Earth-made object to rest on the surface of Mercury. The debris is still up there somewhere on the surface, along with a new 16-meter crater.

This is the last picture of Mercury that MESSENGER sent back before impact. Shortly after this picture was taken, MESSENGER slammed into the planet going several thousand meters per second, completely destroying itself.
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In the future, an ESA mission known as BepiColomno will visit the planet. Sadly, the lander portion of this mission was cancelled, something I have been sad about since I found out.

Conclusion

Despite being closer to us than most of the other planets, Mercury being as close to the sun as it is makes it very difficult to reach and explore. It's lack of atmosphere makes this even more difficult, since landing spacecraft can't use it to burn off excess velocity and must instead carry additional landing fuel onboard. These challenges mean that nothing has ever landed on Mercury and survived (MESSENGER, of course, landing at several kilometers per second, leaving a moderately sized crater). However, I am confident that this world will be further explored in the not-so-distant future. Hopefully as small spacecraft become more capable and launches become cheaper, missions to this remote world will be more viable.

Let me know if you have any comments, questions, corrections, or thoughts. I'd be happy to discuss any of this with you if you'd like. I hope you can share some of my fascination for other worlds in the solar system.

Thanks for reading!

Sources Utilized:
NASA MESSENGER: End of Mission
NASA Mariner-10
Smithsonian: Mecury's Surface Features
Mercury Wikipedia Entry
NASA Mercury Page
Caloris Basin Wikipedia Entry