The Moon

The moon is the only other body in the universe that has a human footprint (well, as far as we know!). It has had 24 human visitors; 12 moonwalkers; and over a hundred robotic visits. Our moon is also the only known moon in the universe that doesn’t have a name. It is simply called “the moon” in the same way you might call your cat “cat”. We name all the other moons we have discovered. We have also not been kind to our only satellite.

Moon from the Galileo Spacecraft
NASA’s Galileo spacecraft took this image of Earth’s moon on 7th December 1992 on its way to explore the Jupiter system (from 1995-1997). The distinct bright ray crater at the bottom of the image is the Tycho impact basin. (JPL/Nasa photo journal).

We see it almost every night, so I guess familiarity breeds contempt. To moon about is to behave in a listless, aimless sort of manner. And a person who is mad is sometimes called a lunatic (because the Romans gave it the name Luna). The Greeks called it Selene, which has a resonance with the word “serenity”. There is so much about our moon that evokes peace and romance it is perhaps a pity it isn’t called Serenity.

The moon is about 50 times smaller than the earth. It makes one complete orbit every 27.32 days. It is in a synchronous rotation so it keeps one side permanently facing the earth. Its distance from Earth ranges from 356 400 kilometres (perigee) to 406 700 kilometres (apogee), as it responds to the gravitational pull of the sun. Its diameter is 3 476 kilometres, equatorial circumference 10 917 kilometres.

From the surface of the moon the sky looks black because there are no air molecules to spread light. The surface is a grey, brown almost monochromatic in colour. And with no atmosphere the solar radiation is lethal. The surface temperature has an extreme range from -248°C to 123°C. Gravity is one-sixth that of the earth.

The moon influences tides on Earth. So does the Sun, but to a much lesser extent being 400 times further away than the moon. It is estimated the Sun has one-third of the moon’s influence. The moon also stabilises our tilt, otherwise we would get radical, erratic tilting. Without the moon our ocean ecology would also be disrupted. At a quarter the size of the earth the moon is relatively very large—to the extent some astronomers play with the idea of referring to the Earth-Moon as a binary system. Certainly, the interaction between the two is both significant and very important to our planet.

Formation theory

So how did the moon form? A number of theories have been put forward over the years, variously discounted or pending.

In his co-accretion theory Édouard Roche (1820-1883) proposed that the earth and the moon grew out of the same materials at the same time. But since the moon has a lower iron content than Earth this idea has been rejected by mainstream debate.

George Darwin (1845-1912), a leading expert on tides, believed a portion of molten mass must have spun off Earth, settling at 8 000 kilometres distant and orbiting five to six times a day. He also said the moon was moving away from the planet. It took 75 years to confirm (on 20th July 1969) that, in fact, the moon is receding at the rate of 3.8 centimetres a year. But the molten mass idea does not seem to have stood the test of time.

A third theory by TJJ See (1866-1962) in 1909 is the “capture theory” that the moon was orbiting the Sun until it was captured by the earth. However, the moon comprises basaltic rocks (as in Hawaii) and breccias formed by asteroid impact, and there is a lack on the moon of the expected volatile elements (water and potassium).

More promisingly is the “Giant Impact Theory”, originally suggested in 1946 by Reginald Daly (1871-1957). In this theory the earth was hit by a large planet from which it obtained its iron content. The impact caused Earth to start rotating and slinging vast amounts of debris out into a chaotic orbit. These materials, as much as 10% of the earth, quickly accreted to form the moon, settling into an orbit at a distance of just 24 000 kilometres.

At the time the earth’s day was just six hours. But the moon was so close that it slowed the revolution of the planet. The moon also churned up the seas, mixing up water and minerals. Indeed, without the moon life on Earth may never have got started. Meanwhile, in the absence of an atmosphere, the moon has been collecting even more minerals and various materials from continual meteor impact.

By 1974 the Giant Impact Theory had become the received wisdom. And indeed, research into a more detailed composition of the moon may have finally settled the argument. In October 2023 scientists reported their findings from Atom Probe Tomography analysis of zircon crystals collected during a 1972 Nasa lunar mission.

They were able to pinpoint the massive celestial collision to occurring 4.46 billion years ago.

The moon transits Earth
Moon transit of Earth – Nasa DSCOVR observatory 16th July 2015 – Deep Space Climate Observatory sitting at L1 orbit, which is the neutral gravity point between the earth and the sun approximately 1.6 million kilometres from Earth (and 148 million kilometres from the sun). It launched 11th February 2015.

The light areas of the moon are known as the highlands. The dark areas, which are called maria (Latin for seas), are impact craters that were filled with lava between 4.2 billion and 1.2 billion years ago. Mare Orientale, for example, has a diameter of 900 kilometres. The moon is cratered almost beyond recognition. With no protection from an atmosphere it is a “sitting duck”. Dark circular regions are from large meteorite collisions. Some of the resulting craters are as much as 1 125 kilometres in diameter. The moon’s surface is covered in rubble, charcoal grey in colour, and powdery dust and rocky debris. This is called lunar regolith and was caused by the impact of high velocity micro meteors. Beneath this is a layer of fractured bedrock called “megaregolith”. Geological activity has long since ceased on the moon and it is a lifeless, dusty and dead world. Here is a good observation from one of the Apollo crew: “The moon looks grey, like plaster of Paris; a lonely expanse of nothing; rubble and dust; magnificent.”

The lunar crust is made of calcium-rich granite-like rock. It is about 48 kilometres thick on the near side and 74 kilometres thick on the far side. Severe, long term meteoritic bombardment has cracked the crust down to as deep as 25 kilometres. Below that it is “rock solid”, where the mantle is rich in silicate materials but poor in metals such as iron. The lower mantle lies about 1 000 kilometres below the crust where the materials become partially molten.

Interesting craters

Tycho Crater is one of the most perfect walled craters on the moon with a central mountain peak towering three kilometres above a rough infilled inner region. It is 85 kilometres in diameter. The highland soil is made up of calcium-aluminium silicates. The maria material (crater floor) is made up of iron-magnesium silicate.

Mare Crisium has a remarkably flat floor, varying barely 90 metres in height across its plain. This arose from a lava flow that flooded the maria. Because of its low viscosity the lava solidified into a tranquil pond like formation.

Rupes Altai is by far the longest cliff on the moon. It is about 420 kilometres long, and up to 1.8 kilometres high in places. Altai was created by a severe impact where an existing mountain range blocked the seismic shock and buckled up into forming a cliff.

Impact science

Debris was ejected from the Cabeus Crater and into the sunlight after the LCROSS impact on 9th October 2009. LCROSS was Nasa’s Lunar Crater Observation and Sensing satellite. When its job was done, as a final gesture for research purposes, it was sent on a crash course. From this and other LCROSS data Nasa says there are oases of water-rich soil that could sustain astronauts on the moon. In about a tonne of material you are talking 40-45 litres of water that you could extract. The impacts kicked up large amounts of rock and dust, revealing a suite of fascinating chemical compounds and far more water than anyone had imagined. About 155 kilogrammes of water vapour and water-ice were blown out of the crater. Research analysis suggests some areas of lunar regolith, or soil, must contain as much as 5% by weight of water-ice. And it is in the form of water-ice grains: “such water-ice is very much a friendly resource to work with. You don’t have to warm it very much; you just have to bring it up to room temperature to pull it out of the dirt real easy”.

Cabeus Crater is a depression so deep and dark that the odds of disturbing ice were thought to be very good. Another spacecraft, Nasa’s Lunar Reconnaissance Orbiter (LRO), was passing close by and was able to study the plume of material ejected into the sunlight more than 15 kilometres above the rim of Cabeus. The suite of instruments deployed determined that as much as 20% of this dust plume was made up of volatile compounds, including methane, ammonia, hydrogen gas, carbon dioxide and carbon monoxide. In addition, the instruments saw relatively large amounts of some metals, such as sodium, and mercury. There was even a signature of silver, but this was tiny. Scientists say the water and mix of volatiles could be remnants of comet or asteroid impacts through the eons, but they reckon a number of quite complex chemical and physical processes are also working to cycle and migrate these substances around the moon.

Further discoveries

At the moon’s southern pole the shadowed parts of craters have been found to be among the coldest places in the Solar System. The water-ice is not uniformly distributed across the southern pole. Rather, it is held in pockets. Some of these oases are, like in Cabeus Crater, found in the shadows where LRO’s Diviner instrument sensed temperatures down to as low as -244°C. Under such conditions, ices will stay fixed for billions of years. But the research indicates there is probably water-ice even in areas which receive some sunlight throughout the year, provided it is buried in the soil. Newly discovered regions area called “lunar permafrost areas” and are very extensive.

There is even glass on the moon! Soil collected by astronauts contains tiny balls of glass, where rocks identical to those on Earth’s crust have been heated to extreme temperatures.

Moons in the solar system

There are 205 moons orbiting the planets in the solar system. Those of dwarf planets (Pluto for example, has five moons of its own) are not included in this list. The moon orbiting Earth is the largest in relation to its planet. Titan is twice the size of our moon. Between them Jupiter and Saturn combined have about 160 moons and there may be more waiting to be discovered. The moons of the solar system have a very wide range of environments. Some moons are heated by gravity. Take Io, for example, which is heated by a process called “Tidal Friction”. Europa is a “planet of ice” – a moon colder than the earth’s arctic (which itself is -128°C at its coldest).

Moons of the solar system
Blue Moon

The term, “blue moon” typically appears in the phrase “once in a blue moon”, meaning rarely or not often. In reality, a blue moon occurs on average once every 2.7 years. So what is a blue moon? Well, in a year there are 12 full moons—one per month. But sometimes there is a bonus full moon when two occur in one month. A full moon occurs roughly every 29.5 days. So if there is a full month, say, on the first day of the month, then there is a good chance there will be another one by the end of the same month—but never in February!

Blue moons then by definition will occur at the end of a given month as follows: October 2020; August 2023; May 2026 and December 2028, and so on.  This is the modern definition. Though why it is called “blue” is unfathomable. For the record, and to make things complicated, the original definition of a blue moon was a fourth one occurring in a season. Usually there are three full moons in a season. A season is defined as between each solstice and equinox. It is still triggered by the 29.5-day cycle. In some years there are four full moons in a season. This happens every two to three years and it is the third not the fourth moon in this seasonal period that is called a blue moon. Don’t you just prefer the first definition!

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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar

Last updated: Thursday, 26th October 2023