Mercury: The Mysterious ‘Jumping Planet.’

The planet closest to our Sun is a battleground of solar winds, freezing nights and scorching days. Scarred by the ancient assault of millions of asteroids, its 4.6-billion-year history is still much a mystery. Hailed as the Roman God of communication, eloquence and poetry (amongst other things,), you could be forgiven for thinking this tiny metal ball as hard and rugged as they come. As quiet as it is sturdy, Mercury is still largely unknown.


Discovery and History

Mercury, as the story goes, was first discovered by Assyrian Astronomers in the 14th century BC. Known as the ‘Jumping Planet’ (for reasons very complicated and discussed later), it would reappear in Babylonian, Greek and then Roman records over the next centuries. Little studied, you may ask why Mercury resists such scrutiny often afforded the other planets.  In the Roman Pantheon, Mercury was a popular God, but seemingly shunned in the depths of space.

Its eccentric history and mystery can be explained by its curious orbit, surface conditions and relationship with the Sun. Although vastly hidden by light, small transits (crossings of the Sun) have allowed various cultures and astronomers small glimpses of its journey. Its disappearing act, sudden reappearance and ‘odd’ orbit earned it a special place in cultural mythology. Its brutal treatment of probes means it doesn’t give up much either.

It was not until 1631, with the predictions of the little known Johannes Kepler, that astronomers Thomas Harriot and Galileo Galilei used the newly invented telescope to peer at its surface. What they saw would raise questions for centuries.

Orbit, composition and character

Mercury circles terrifyingly close to the sun, dancing close then far in an ‘elliptical’ orbit. At its closest, it brushes by a 46 million km (its ‘Aphelion’) and then retreats to around 70 million km (it’s Perihelion.)  It owns the questionable title of ‘most eccentric’ orbit, meaning its distance from the Sun jumps the most when compared to the other planets. For a God of eloquence, such jerky movement seems ironic.

Mercury orbits the Sun at approximately 180,000 km an hour, giving it an orbital year of around 88 days. Curiously, its ‘tidal locked’ rotation means its day is longer than its year, with the planet taking 176 earth days to spin once on its axis.  When viewed from Earth, the planet may seem to permanently face the sun, but given time the rotation becomes clear. Furthermore, its movement relative to earth means it ‘jumps’ back and forth in the sky as we overtake it.

With a radius of 2440km and mass of 3.3 x 1023 Kg3, it is only 1/3 the size and 1/20th the weight of our blue planet. However, it is almost as dense as us, with its heavy and molten iron core taking up to around 70% of its mass and 55% of its size. Scientists theorize that multiple asteroid impacts are to blame for this high composition, blowing the poor planet’s surface away over time. So although it is small, it is heavy and hardened.

Like Earth, the spinning molten core creates a magnetic field. This field is the same as can be seen on a compass, but varies wildly. Observations by probes have shown leaks and bulges very much unlike our own.

Climate and Topography

Mercury, when viewed by either an astronomer or satellite, can be easily compared to the moon. A tapestry of craters, some lie Caloris as large as 1500 km wide, alternate with a smooth polished surface. Wrinkled peaks, often up to a mile high, are a eulogy to the cooling of the planet in its early history.

Due to its closeness to the sun, and ferocity of solar winds, Mercury has little atmosphere. For this reason, it cannot retain heat and its temperature bounces between -173oC at night and 400o­C during the day. Little can survive its harsh surface, with manned missions to the planet likely no more than a barbecue.

Human Contact

Little of Mercury has been explored, with few spacecraft sent is way. Mercury is a lonely planet, browbeaten and largely ignored. Fly-bys by Mariner 10 in the 1970’s mapped around 45% of the planet, with the follow-up Messenger paying much more attention from 1994 until 2015. Future missions are planned, but for the moment, Mercury is a curiosity best viewed from earth. Like any God, most of our conclusions and knowledge is based on sparse fact, conjecture and fantasy.

Dr Ben Janaway MBChB  // @drjanaway

Any opinions above are the author’s alone and may not represent those of his affiliations. Any comment is based on the best available evidence at the time of writing.  All data is based on externally validated studies unless expressed otherwise. Novel data is representative of the sample surveyed. Online recommendation is no substitute for seeing your own doctor and should not be taken as medical advice.

 Dr Ben Janaway is a medical doctor and Editor for the online healthcare and education source ‘Mind and Medicine’.  He writes regularly for and The Canary, amongst and other national news sources.

He is also working on a number of literary pieces.

Contact Dr Janaway at with stories, commissions or for discussion.

Sources and Further Reading

Image courtesy of



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