ATLANTIC SKIES: The many mysteries of Mercury, our solar system's smallest planet

The planet Mercury is the smallest member of our solar system’s eight planets.

However, its small size (4,880 kilometres in diameter; slightly larger than the dwarf planet Pluto) belies its many extremes.

The ancient Sumerians are believed to be the first culture to incorporate this planet into their religion, personifying it as the god Nabu, the patron god of writing, scribes, literacy, and wisdom.

The ancient Egyptians associated the planet with Thoth, the god of writing, wisdom, and magic.

The name Mercury, by which we know the planet today, comes from the ancient Romans, who named it for Mercurius, the messenger of the gods, symbolizing commerce, communications, and travel.

To the ancient Greeks, the planet was known as Hermes (often drawn wearing winged sandals), herald of the gods, and protector of travelers, thieves, merchants, and orators.

Mercury’s orbital distance from the sun varies between 46 million and 70 million kms. Added to the speed with which Mercury completes one orbit around the sun, the orbital distances mean that the planet can be visible on either side of the sun as, like Venus, both a “morning star” and an “evening star” at some point in time.

It is believed that the ancient Greeks were the first to recognize that the two “stars” on either side of the sun were the same object.

In modern times, the first recorded individuals to observe Mercury are believed to have been the Italian astronomer Galileo Galilei (1564-1642) and the English astronomer Thomas Harriot (1560-1621). In 1631, the French mathematician and philosopher Pierre Gassendi (1592-1655) used a telescope to watch Mercury transit (cross in front of) the sun’s disk, the first astronomer known to have done so.

Although it sits closest to the sun — at only an average distance of 58 million kms — it is not the hottest planet; that trophy goes to Venus, the second closest planet to the sun, which has an average temperature of approximately 460 degrees Celsius, the result of its extensive cloud coverage.

Due to the planet’s orbital eccentricity (its deviation from a perfect circle), Mercury’s surface temperatures range from approximately -170C at night to 420C during the daytime.

Mercury has no natural satellites or ring system, a surface gravity slightly lower than that of Mars, and a very faint magnetic field (just strong enough to deflect the sun’s solar wind). Its apparent magnitude varies from -2.48 (brighter than Venus) when it is at superior conjunction to +7.25 (below naked-eye visibility) when it is at inferior conjunction.

Observable for only a brief period during either the morning or the evening twilight, Mercury spends most of its time lost in the sun’s glare as seen from Earth.

It does, however, exhibit phases, with the “first quarter” and “last quarter” phases occurring when it is on either side of the sun, a “new” phase when it transits the sun’s disk (inferior conjunction), and a “full” phase when at superior conjunction (passing behind the sun as viewed from Earth).

Mercury completes one full orbit around the sun (a Mercurian year) every 87.969 Earth days, during which time one side of the planet remains in sunlight until, during the next orbit (the next Mercurian year), that side remains in darkness, and the opposite side receives the sunlight.

Mercury makes one full axial rotation (a Mercurian day) every 59 Earth days. Mercury’s orbit is inclined seven degrees (the largest of all eight planets) to the plane of Earth’s orbit (the ecliptic) around the sun. As a result, the transit of Mercury across the disk of the sun only occurs when it crosses the plane (ecliptic) during the period when it comes between the Earth and the sun in May or November, once every seven years on average.

One of the solar system’s four rocky terrestrial planets, Mercury has the second highest density of any of the planets.

Its origin is thought by some planetary scientists to have been the result of planetesimal collisions and rock vaporization during the formation of the early solar system. Another possible origin theory is that Mercury may have formed from the solar nebula before the sun’s energy output stabilized. Research in 2007 suggests that Mercury may have a molten core.

Current planetary model simulations suggest that Mercury, at one point in its early development, may have had a solid silicate (a mixture of oxygen and silicon) crust and mantle of about 420 km in depth overlaying a solid outer core, a deeper liquid core layer, and a solid inner iron core, totaling approximately 2,020 km in diameter, comprising 57 per cent of the planet’s volume and 85 per cent of its radius.

Why does Mercury have so many craters?

Much like that of Earth’s moon, Mercury’s surface is heavily cratered, possibly produced by the intense planetesimal, asteroid, and cometary bombardment of the inner solar system approximately 3.9 billion years ago.

The largest crater on Mercury, and one of the largest in the solar system, is the Caloris Planitia (aka Caloris Basin), a huge impact crater with a diameter of 1,550 kms.

In addition to multiple craters, Mercury’s surface is also marked by numerous ridges and escarpments, referred to as “rupes” — steep slopes or cliffs caused by faulting or erosion, which separate two relatively level areas by differences in elevation. In planetary geology, on planets other than Earth, these surface features are generated by thrust faults, causing breaks in the surface material and pushing older rock upwards above younger rock.

Due to Mercury’s axial tilt (0.027 degrees; almost 0), its polar regions receive very little direct sunlight, with many of the polar craters being in perpetual shadow.

Interestingly, radar observations of Mercury’s polar regions suggest that there could potentially be deposits of water ice (water in a frozen solid state) in the permanently shadowed craters at the planet’s poles. Mercury’s very thin atmosphere, referred to as a “trace atmosphere”, is composed primarily of oxygen, sodium, hydrogen, helium and potassium.

To date, Mercury has only been visited by three spacecraft.

In 1973, NASA launched its Mariner 10 robotic space probe to flyby Mercury and Venus. This mission was followed by NASA’s 2011-2015 MESSENGER orbital mission designed to study the chemical composition, geology, and magnetic field of Mercury.

Launched in 2018, the Bepi Columbo orbiter mission, a joint undertaking by the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA,) is expected to arrive at Mercury in November 2026. The joint mission consists of two orbiter craft — the ESA’s Mercury Planetary Orbiter (MPO), and JAXA’s Mercury Magnetosphere Orbiter (MMO) — both of which will study, over the course of one year, and possibly two, Mercury’s magnetosphere, its magnetic field, and both its interior and surface structure.

Though diminutive in size, Mercury has much to teach astronomers and astrophysicists about the origin and formation of our solar system’s inner planets. No doubt, we will hear many exciting and interesting things about Mercury in the coming months.

Mercury (mag. -0.1 in Libra – the Scales) is not observable this coming week, as it is three degrees below the western horizon at dusk.

Venus (mag. -4.1 in Virgo) is likewise not observable this week, as, having recently passed in front of the sun (inferior conjunction) as seen from Earth, it now sits very close to the sun, at only six degrees separation.

Mars (mag. +1.0, in Cancer – the Crab), visible in the dawn sky, rises around 12:45 a.m. ADT, reaching 58 degrees above the southeast horizon before fading from view by about 7 a.m. ADT.

Jupiter (mag. -2.0, in Leo – the Lion), also visible in the dawn sky, rises around 1:30 a.m. ADT, reaching 54 degrees above the southeast horizon before fading from view in the brightening dawn around 7:25 a.m. ADT.

Saturn (mag. +0.3, in Cetus – the Sea Monster) becomes accessible 13 degrees above the eastern horizon by about 6:45 p.m. ADT, before reaching its highest point in the sky 44 degrees above the southern horizon by about 11:30 p.m. ADT, and remaining observable until around 4:30 a.m. ADT, when it sinks below 10 degrees above the western horizon.

Uranus (mag. +5.7, in Taurus – the Bull), currently approaching opposition, becomes accessible 21 degrees above the eastern horizon around 9:40 p.m. ADT, reaching its highest point in the sky 64 degrees above the southern horizon shortly after 3 a.m. ADT, and then becoming lost in the dawn twilight 41 degrees above the western horizon by about 6:30 a.m. ADT.

Neptune (mag. +7.8, in Pisces – the Fish) becomes accessible when it reaches 22 degrees above the southeast horizon around 7:20 p.m. ADT, reaching its highest point in the sky 43 degrees above the southern horizon around 11 p.m. ADT, and remaining observable until around 2:45 a.m. ADT, when it sinks below 21 degrees above the southwest horizon.

The full moon on Oct. 26 is still often referred to as the “Hunter’s Moon,” even though the name was more applicable in olden times to the sport of hunting by the light of the full moon. Nowadays, in many jurisdictions, this sport is generally prohibited by law. The “Hunter’s Moon,” in many almanacs, is considered to be the first full moon which falls directly after the Harvest Moon; as a result, it can occur in either October or November, depending on when the Harvest Moon occurs (this year, Sept. 26, 2026).

Remember to set your clocks back one hour when you go to bed on the evening of Saturday, Oct. 31 (Halloween), as Atlantic Standard Time (AST) starts at 2 a.m. on Nov. 1.

Until next week, clear skies.

Events:
Oct. 26 – Full “Hunter’s Moon” at 1:11 a.m. ADT
Oct, 28 – Moon at perigee (closest approach to Earth) at 3:05 p.m. ADT; 356,500 kms
Nov. 1 – Atlantic Standard Time (AST) begins at 2 a.m.; last quarter moon at 4:28 p.m. AST