Astronomers are keenly interested in red dwarfs and the planets that orbit them. Up to 85% of the stars in the Milky Way could be red dwarfs, and 40% of them might host Earth-like exoplanets in their habitable zones, according to some research.
But there are some problems with their potential habitability. One of those problems is tidal locking.
The study of extra-solar planets has revealed discoveries that have confounded expectations and boggled the mind! Whether it’s Super-Earths that become diamond planets, multiple rocky planets orbiting closely together, or “Hot Jupiters” with traces of gaseous metal in their atmospheres, there’s been no shortage of planets out there for which there is no comparison here in the Solar System.
In this respect, WASP-12b is in good company. This Hot-Jupiter, located in a star system 1400 light years from Earth in the direction of the Auriga constellation, was recently studied by a team of astronomers using the Hubble Space Telescope. Due to the particular nature of its atmosphere, which absorbs the vast majority of light it receives instead of reflecting it, this planet appeared pitch black when observed by the Hubble team.
Like all Hot Jupiters, WASP-12b is similar in mass to Jupiter (1.35 to 1.43 Jupiter masses) and orbits very close to its star. At a distance of just 3.4 million km (2.115 million mi), or 0.0229 AU, it takes a little over a day to complete a single orbit. Because of its proximity, one side of the planet is constantly facing towards it’s sun – i.e. it is tidally locked with its star.
Because of its orbit, temperatures on the day side of the planet are estimated to reach as high as 2811 K (2538 °C; 4600 °F). It is because of these extreme temperatures that most molecules are unable to survive on the day side of the planet, so clouds cannot form to reflect light back into space. As a result, most incoming light penetrates deep into the planet’s atmosphere, where it is absorbed by hydrogen atoms and converted into heat energy.
This was what Bell and his team noticed as they observed the planet passing behind its star (aka. an optical eclipse). Using the STIS, they monitored the system for any dips in starlight, which would indicate how much reflected light was being given off by the planet. However, their observations did not detect reflected light, which indicated that the sun-facing side was absorbing most of the light it was receiving.
As Bell explained in a NASA press statement, this was quite the unusual find: “We did not expect to find such a dark exoplanet,” he said. “Most hot Jupiters reflect about 40 percent of starlight.” However, observations conducted of the night side of the planet show that things are quite different there. On this side, temperatures are about 1366 K (1093 °C; 2000 °F) cooler, which allows water vapor and clouds to form.
Back in 2013, scientists working with the HST detected traces of water vapor in the atmosphere (and possible traces of clouds as well) while studying the day/night boundary. As Bell indicated, this new research just goes to show just how diverse this type of gas giant can be:
“This new Hubble research further demonstrates the vast diversity among the strange population of hot Jupiters. You can have planets like WASP-12b that are 4,600 degrees Fahrenheit and some that are 2,200 degrees Fahrenheit, and they’re both called hot Jupiters. Past observations of hot Jupiters indicate that the temperature difference between the day and night sides of the planet increases with hotter day sides. This previous research suggests that more heat is being pumped into the day side of the planet, but the processes, such as winds, that carry the heat to the night side of the planet don’t keep up the pace.”
This is just the latest find in a slew that has confounded scientists expectations about exoplanets. The more we come to learn about the nature and diversity of these distant worlds, the more tantalizing they seem and the more appealing the prospect of exploring them directly someday becomes!
Studies of low-mass, ultra-cool and ultra-dim red dwarf stars have turned up a wealth of extra-solar planets lately. These include the discoveries of a rocky planet orbiting the closest star to the Solar System (Proxima b) and a seven-planet system just 40 light years away (TRAPPIST-1). In the past few years, astronomers have also detected candidates orbiting the stars Gliese 581, Innes Star, Kepler 42, Gliese 832, Gliese 667, Gliese 3293, and others.
The majority of these planets have been terrestrial (i.e. rocky) in nature, and many were found to orbit within their star’s habitable zone (aka. “goldilocks zone”). However the question whether or not these planets are tidally-locked, where one face is constantly facing towards their star has been an ongoing one. And according to a new study from the University of Washington, tidally-locked planets may be more common than previously thought.
The study – which is available online under the title “Tidal Locking of Habitable Exoplanets” – was led by Rory Barnes, an assistant professor of astronomy and astrobiology at the University of Washington. Also a theorist with the Virtual Planetary Laboratory, his research is focused on the formation and evolution of planets that orbit in and around the “habitable zones” of low-mass stars.
For modern astronomers, tidal-locking is a well-understood phenomena. It occurs as a result of their being no net transfer of angular momentum between an astronomical body and the body it orbits. In other words, the orbiting body’s orbital period matches its rotational period, ensuring that the same side of this body is always facing towards the planet or star it orbits.
Consider Earth’s only satellite – the Moon. In addition to taking 27.32 days to orbit Earth, the Moon also takes 27.32 days to rotate once on its axis. This is why the Moon always presents the same “face” towards Earth, while the side that faces away is known as the “dark side”. Astronomers believe this became the case after a Mars-sized object (Theia) collided with Earth some 4.5 billion years ago.
Aside from throwing up debris that would eventually form the Moon, the impact is believed to have struck Earth at such an angle that it gave our planet an initial rotation period of 12 hours. In the past, researchers have used this 12-hour estimation of Earth’s rotation as a model for exoplanet behavior. However, prior to Barnes’ study, no systematic examinations had ever been conducted.
Looking to address this, Barnes chose to address the long-held assumption that only smaller, dimmer stars could host orbiting planets that were tidally locked. He also considered other possibilities, which included slower or faster initial rotation periods as well as variations in planet size and the eccentricity of their orbits. What he found was that previous studies had been rather limited and only made allowances for one outcome.
“Planetary formation models, however, suggest the initial rotation of a planet could be much larger than several hours, perhaps even several weeks. And so when you explore that range, what you find is that there’s a possibility for a lot more exoplanets to be tidally locked. For example, if Earth formed with no Moon and with an initial ‘day’ that was four days long, one model predicts Earth would be tidally locked to the sun by now.”
From this, he found that potentially-habitable planets that orbit very late M-type (red dwarf) stars are likely to attain highly-circular orbits about 1 billion years after their formation. Furthermore, he found that for the majority, their orbits would be synchronized with their rotation – aka. they would be tidally-locked with their star. These findings could have significant implications for the study of exoplanets formation and evolution, not to mention habitability.
In the past, tidally-locked planets were thought to have extremes climates, thus eliminating any possibility of life. As an example, the planet Mercury experiences a 3:2 spin-orbit resonance, meaning it rotates three times on its axis for every two orbits it completes of the Sun. Because of this, a single day on Mercury lasts as long as 176 Earth days, and temperature range from 100 (-173 °C; -279 °F) to 700 K (427 °C; 800 °F) between the day side and the night side.
For a tidally-locked planets that orbit close to their stars, it was believed this situation would be even worse. However, astronomers have since come to speculate that the presence of an atmosphere around these planets could redistribute temperature across their surfaces. Unlike Mercury, which has no atmosphere and experiences no wind, these planets could maintain temperatures that would be supportive to life.
In any case, this study is one of many that is putting constraints on recent exoplanet discoveries. This is especially important given that the detection and study of extra-solar planets is still in its infancy, and limited to largely indirect methods. In other words, astronomers make estimates of a planet’s size, composition and whether or not it has an atmosphere based on transits and the influence these planets have on their stars.
In the coming years, next-generations missions like the James Web Space Telescope and the Transiting Exoplanet Survey Satellites (TESS) are expected to improve this situation drastically. In addition to conducting more detailed observations on existing discoveries, they are also expected to uncover a wealth of more planets. If Barnes’ study is correct, the majority of those found will be tidally-locked, but that need not mean they are uninhabitable.
The extra-solar planet known as Proxima b has occupied a special place in the public mind ever since its existence was announced in August of 2016. As the closest exoplanet to our Solar System, its discovery has raised questions about the possibility of exploring it in the not-too-distant future. And even more tantalizing are the questions relating to its potential habitability.
Despite numerous studies that have attempted to indicate whether the planet could be suitable for life as we know it, nothing definitive has been produced. Fortunately, a team of astrophysics from the University of Exeter – with the help of meteorology experts from the UK’s Met Office – have taken the first tentative steps towards determining if Proxima b has a habitable climate.
According to their study, which appeared recently in the journal Astronomy & Astrophysics, the team conducted a series of simulations using the state-of-the-art Met Office Unified Model (UM). This numerical model has been used for decades to study Earth’s atmosphere, with applications ranging from weather prediction to the effects of climate change.
With this model, the team simulated what the climate of Proxima b would be like if it had a similar atmospheric composition to Earth. They also conducted simulations on what the planet would be like it if had a much simpler atmosphere – one composed of nitrogen with trace amounts of carbon dioxide. Last, but not least, they made allowances for variations in the planet’s orbit.
For instance, given the planet’s distance from its sun – 0.05 AU (7.5 million km; 4.66 million mi) – there have been questions about the planet’s orbital characteristics. On the one hand, it could be tidally-locked, where one face is constantly facing towards Proxima Centauri. On the other, the planet could be in a 3:2 orbital resonance with its sun, where it rotates three times on its axis for every two orbits (much like Mercury experiences with our Sun).
In either case, this would result in one side of the planet being exposed to quite a bit of radiation. Given the nature of M-type red dwarf stars, which are highly variable and unstable compared to other types of stars, the sun-facing side would be periodically irradiated. Also, in both orbital scenarios, the planet would be subject to significant variations in temperature that would make it difficult for liquid water to exist.
For example, on a tidally-locked planet, the main atmospheric gases on the night-facing side would be likely to freeze, which would leave the daylight zone exposed and dry. And on a planet with a 3:2 orbital resonance, a single solar day would most likely last a very long time (a solar day on Mercury lasts 176 Earth days), causing one side to become too hot and dry the other side too cold and dry.
By taking all this into account, the team’s simulations allowed for some crucial comparisons with previous studies, but also allowed the team to reach beyond them. As Dr. Ian Boutle, an Honorary University Fellow at the University of Exeter and the lead author of the paper, explained in a University press release:
“Our research team looked at a number of different scenarios for the planet’s likely orbital configuration using a set of simulations. As well as examining how the climate would behave if the planet was ‘tidally-locked’ (where one day is the same length as one year), we also looked at how an orbit similar to Mercury, which rotates three times on its axis for every two orbits around the sun (a 3:2 resonance), would affect the environment.”
In the end, the results were quite favorable, as the team found that Proxima b would have a remarkably stable climate with either atmosphere and in either orbital configuration. Essentially, the UM software simulations showed that when both atmospheres and both the tidally-locked and 3:2 resonance configurations were accounted for, there would still be regions on the planet where water was able to exist in liquid form.
Naturally, the 3:2 resonance example resulted in more substantial areas of the planet falling within this temperature range. They also found that an eccentric orbit, where the distance between the planet and Proxima Centauri varied to a significant degree over the course of a single orbital period, would lead to a further increase in potential habitability.
As Dr James Manners, another Honorary University Fellow and one of the co-authors on the paper, said:
“One of the main features that distinguishes this planet from Earth is that the light from its star is mostly in the near infra-red. These frequencies of light interact much more strongly with water vapor and carbon dioxide in the atmosphere which affects the climate that emerges in our model.”
Of course, much more work needs be done before we can truly understand whether this planet is capable of supporting life as we know it. Beyond feeding the hopes of those who would like to see it colonized someday, studies into Proxima b’s conditions are also of extreme importance in determining whether or not indigenous life exists there right now.
But in the meantime, studies such as this are extremely helpful when it comes to anticipating what kinds of environments we might find on distant planets. Dr Nathan Mayne – the scientific lead on exoplanet modelling at the University of Exeter and a co-author on the paper – also indicated that climate studies of this kind could have applications for scientists here at home.
“With the project we have at Exeter we are trying to not only understand the somewhat bewildering diversity of exoplanets being discovered, but also exploit this to hopefully improve our understanding of how our own climate has and will evolve,” he said. What’s more, it helps to illustrate how conditions here on Earth can be used to predict what may exist in extra-solar environments.
While that might sound a bit Earth-centric, it is entirely reasonable to assume that planets in other star systems are subject to processes and mechanics similar to what we’ve seen on the Solar planets. And this is something we are invariably forced to do when it comes to searching for habitable planets and life beyond our Solar System. Until we can go there directly, we will be forced to measure what we don’t know by what we do.
The ESO’s recent announcement that they have discovered an exoplanet candidate orbiting Proxima Centauri – thus confirming weeks of speculation – has certainly been exciting news! Not only is this latest find the closest extra-solar planet to our own Solar System, but the ESO has also indicated that it is rocky, similar in size and mass to Earth, and orbits within the star’s habitable zone.
However, in the midst of this news, there has been some controversy regarding certain labels. For instance, when a planet like Proxima b is described as “Earth-like”, “habitable”, and/or “terrestrial“, there are naturally some questions as to what this really means. For each term, there are particular implications, which in turn beg for clarification.
For starters, to call a planet “Earth-like” generally means that it is similar in composition to Earth. This is where the term “terrestrial” really comes into play, as it refers to a rocky planet that is composed primarily of silicate rock and metals which are differentiated between a metal core and a silicate mantle and crust.
What this does not mean, at least not automatically, is that the planet is habitable in the way Earth is. Simply being terrestrial in nature is not an indication that the planet has a suitable atmosphere or a warm enough climate to support the existence of liquid water or microbial life on its surface.
What’s more, Earth-like generally implies that a planet will be similar in mass and size to Earth. But this is not the same as composition, as many exoplanets that have been discovered have been labeled as “Earth-sized” or “Super-Earths” – i.e. planets with around 10 times the mass of Earth – based solely on their mass.
This term also distinguishes an exoplanet candidate from those that are 15 to 17 masses (which are often referred to as “Neptune-sized”) and those that have masses similar to, or many times greater than that of Jupiter (i.e. Super-Jupiters). In all these cases, size and mass are the qualifiers, not composition.
Ergo, finding a planet that is greater in size and mass than Earth, but significantly less than that of a gas giant, does not mean it is terrestrial. In fact, some scientists have recommended that the term “mini-Neptune” be used to describe planets that are more massive than Earth, but not necessarily composed of silicate minerals and metals.
And estimates of size and mass are not exactly metrics for determining whether or not a planet is “habitable”. This term is especially sticky when it comes to exoplanets. When scientists attach this word to extra-solar planets like Proxima b, Gliese 667 Cc, Kepler-452b, they are generally referring to the fact that the planet exists within its parent star’s “habitable zone” (aka. Goldilocks zone).
This term describes the region around a star where a planet will experience average surface temperatures that allow for liquid water to exist on its surface. For those planets that orbit too close to their star, they will experience intense heat that transforms surface water into hydrogen and oxygen – the former escaping into space, the latter combining with carbon to form CO².
This is what scientists believe happened to Venus, where thick clouds of CO² and water vapor triggered a runaway greenhouse effect. This turned Venus from a world that once had oceans into the hellish environment we know today, where temperatures are hot enough to melt lead, atmospheric density if off the charts, and sulfuric acid rains from its thick clouds.
For planets that orbit beyond a star’s habitable zone, water ice will become frozen solid, and the only liquid water will likely be found in underground reservoirs (this is the case on Mars). As such, finding planets that are just right in terms of average surface temperature is intrinsic to the “low-hanging fruit” approach of searching for life in our Universe.
But of course, just because a planet is warm enough to have water on its surface doesn’t mean that life can thrive on it. As our own Solar System beautifully demonstrates, a planet can have the necessary conditions for life, but still become a sterile environment because it lacks a protective magnetosphere.
This is what scientists believe happened to Mars. Located within our Sun’s Goldilocks zone (albeit on the outer edge of it), Mars is believed to have once had an atmosphere and liquid water on its surface. But today, atmospheric pressure on the surface of Mars is only 1% that of Earth’s, and the surface is dry, cold, and devoid of life.
The reason for this, it has been determined, is because Mars lost its magnetosphere 4.2 Billion years ago. According to NASA’s MAVEN mission, this resulted in Mars’ atmosphere being slowly stripped away over the course of the next 500 million years by solar wind. What little atmosphere it had left was not enough to retain heat, and its surface water evaporated.
By the same token, planets that do not have protective magnetospheres are also subject to an intense level of radiation on their surfaces. On the Martian surface, the average dose of radiation is about 0.67 millisieverts (mSv) per day, which is about a fifth of what people are exposed to here on Earth in the course of a year.
We can expect similar situations on extra-solar planets where a magnetosphere does not exist. Essentially, Earth is fortunate in that it not only orbits in a pretty cushy spot around our Sun, but that its core is differentiated between a solid inner core and a liquid, rotating outer core. This rotation, it is believed, is responsible for creating a dynamo effect that in turn creates Earth’s magnetic field.
However, using our own Solar System again as a model, we find that magnetic fields are not entirely uncommon. While Earth is the only terrestrial planet in our Solar System to have on (all the gas giants have powerful fields), Jupiter’s moon Ganymede also has a magnetosphere of its own.
Similarly, there are orbital parameters to consider. For instance, a planet that is similar in size, mass and composition could still have a very different climate than Earth due to its orbit. For one, it may be tidally-locked with its star, which would mean that one side is permanently facing towards it, and is therefore much warmer.
On the other hand, it may have a slow rotational velocity, and a rapid orbital velocity, which means it only experiences a few rotations per orbit (as is the case with Mercury). Last, but certainly not least, its distance from its respective star could mean it receives far more radiation than Earth does – regardless of whether or not it has a magnetosphere.
This is believed to the be the case with Proxima Centauri b, which orbits its red dwarf star at a distance of 7 million km (4.35 million mi) – only 5% of the Earth’s distance from the Sun. It also orbits Proxima Centauri with an orbital period of 11 days, and either has a synchronous rotation, or a 3:2 orbital resonance (i.e. three rotations for every two orbits).
Because of this, the climate is likely to be very different than Earth’s, with water confined to either its sun-facing side (in the case of a synchronous rotation), or in its tropical zone (in the case of a 3:2 resonance). In addition, the radiation it receives from its red dwarf star would be significantly higher than what we are used to here on Earth.
So what exactly does “Earth-like” mean? The short answer is, it can mean a lot of things. And in this respect, its a pretty dubious term. If Earth-like can mean similarities in mass, size, composition, and can allude to the fact that planet orbits within its star’s habitable zone – but not necessarily all of the above – then its not a very reliable term.
In the end, the only way to keep things clear would be to describe a planet as “Earth-like” if it in fact shows similarities in terms of size, mass and composition, all at the same time. The word “terrestrial” can certainly be substituted in a pinch, but only where the composition of the planet is known with a fair degree of certainty (and not just its size and mass).
And words like “habitable” should probably only be used when chaperoned by words like “potentially”. After all, being within a star’s habitable zone certainly means there’s the potential for life. But it doesn’t not necessarily entail that life could have emerged there, or that humans could live there someday.
And should these words apply to Proxima b? Perhaps, but one should consider the fact that the ESO has announced the detection of a exoplanet using the Radial Velocity method. Until such time as it is confirmed using direct detection methods, its remains a candidate exoplanet (not a confirmed one).
But even these simple measures would likely not be enough to erase all the ambiguity or controversy. When it comes right down to it, planet-hunting – like all aspects of space exploration and science – is a divisive issue. And new findings always have a way of drawing criticism and disagreement from several quarters at once.
And you thought Pluto’s classification confused things! Well, Pluto has got nothing on the exoplanet database! So be prepared for many years of classification debates and controversy!
Look up in the night sky. On a clear night, if you’re lucky, you’ll catch a glimpse of the Moon shining in all it’s glory. As Earth‘s only satellite, the Moon has orbited our planet for over three and a half billion years. There has never been a time when human beings haven’t been able to look up at the sky and see the Moon looking back at them.
As a result, it has played a vital role in the mythological and astrological traditions of every human culture. A number of cultures saw it as a deity while others believed that its movements could help them to predict omens. But it is only in modern times that the true nature and origins of the Moon, not to mention the influence it has on planet Earth, have come to be understood.
Size, Mass and Orbit:
With a mean radius of 1737 km and a mass of 7.3477 x 10²² kg, the Moon is 0.273 times the size of Earth and 0.0123 as massive. Its size, relative to Earth, makes it quite large for a satellite – second only to Charon‘s size relative to Pluto. With a mean density of 3.3464 g/cm³, it is 0.606 times as dense as Earth, making it the second densest moon in our Solar System (after Io). Last, it has a surface gravity equivalent to 1.622 m/s2, which is 0.1654 times, or 17%, the Earth standard (g).
The Moon’s orbit has a minor eccentricity of 0.0549, and orbits our planet at a distance of between 356,400-370,400 km at perigee and 404,000-406,700 km at apogee. This gives it an average distance (semi-major axis) of 384,399 km, or 0.00257 AU. The Moon has an orbital period of 27.321582 days (27 d 7 h 43.1 min), and is tidally-locked with our planet, which means the same face is always pointed towards Earth.
Structure and Composition:
Much like Earth, the Moon has a differentiated structure that includes an inner core, an outer core, a mantle, and a crust. It’s core is a solid iron-rich sphere that measures 240 km (150 mi) across, and it surrounded by a outer core that is primarily made of liquid iron and which has a radius of roughly 300 km (190 mi).
Around the core is a partially molten boundary layer with a radius of about 500 km (310 mi). This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon’s formation 4.5 billion years ago. Crystallization of this magma ocean would have created a mantle rich in magnesium and iron nearer to the top, with minerals like olivine, clinopyroxene, and orthopyroxene sinking lower.
The mantle is also composed of igneous rock that is rich in magnesium and iron, and geochemical mapping has indicated that the mantle is more iron rich than Earth’s own mantle. The surrounding crust is estimated to be 50 km (31 mi) thick on average, and is also composed of igneous rock.
The Moon is the second densest satellite in the Solar System after Io. However, the inner core of the Moon is small, at around 20% of its total radius. Its composition is not well constrained, but it is probably a metallic iron alloy with a small amount of sulfur and nickel and analyses of the Moon’s time-variable rotation indicate that it is at least partly molten.
The presence of water has also been confirmed on the Moon, the majority of which is located at the poles in permanently-shadowed craters, and possibly also in reservoirs located beneath the lunar surface. The widely accepted theory is that most of the water was created through the Moon’s interaction of solar wind – where protons collided with oxygen in the lunar dust to create H²O – while the rest was deposited by cometary impacts.
The geology of the Moon (aka. selenology) is quite different from that of Earth. Since the Moon lacks a significant atmosphere, it does not experience weather – hence there is no wind erosion. Similarly, since it lacks liquid water, there is also no erosion caused by flowing water on its surface. Because of its small size and lower gravity, the Moon cooled more rapidly after forming, and does not experience tectonic plate activity.
Instead, the complex geomorphology of the lunar surface is caused by a combination of processes, particularly impact cratering and volcanoes. Together, these forces have created a lunar landscape that is characterized by impact craters, their ejecta, volcanoes, lava flows, highlands, depressions, wrinkle ridges and grabens.
The most distinctive aspect of the Moon is the contrast between its bright and dark zones. The lighter surfaces are known as the “lunar highlands” while the darker plains are called maria (derived from the Latin mare, for “sea”). The highlands are made of igneous rock that is predominately composed of feldspar, but also contains trace amounts of magnesium, iron, pyroxene, ilmenite, magnetite, and olivine.
Mare regions, in contrast, are formed from basalt (i.e. volcanic) rock. The maria regions often coincide with the “lowlands,” but it is important to note that the lowlands (such as within the South Pole-Aitken basin) are not always covered by maria. The highlands are older than the visible maria, and hence are more heavily cratered.
Other features include rilles, which are long, narrow depressions that resemble channels. These generally fall into one of three categories: sinuous rilles, which follow meandering paths; arcuate rilles, which have a smooth curve; and linear rilles, which follow straight paths. These features are often the result of the formation of localized lava tubes that have since cooled and collapsed, and can be traced back to their source (old volcanic vents or lunar domes).
Lunar domes are another feature that is related to volcanic activity. When relatively viscous, possibly silica-rich lava erupts from local vents, it forms shield volcanoes that are referred to as lunar domes. These wide, rounded, circular features have gentle slopes, typically measure 8-12 km in diameter and rise to an elevation of a few hundred meters at their midpoint.
Wrinkle ridges are features created by compressive tectonic forces within the maria. These features represent buckling of the surface and form long ridges across parts of the maria. Grabens are tectonic features that form under extension stresses and which are structurally composed of two normal faults, with a down-dropped block between them. Most grabens are found within the lunar maria near the edges of large impact basins.
Impact craters are the Moon’s most common feature, and are created when a solid body (an asteroid or comet) collides with the surface at a high velocity. The kinetic energy of the impact creates a compression shock wave that creates a depression, followed by a rarefaction wave that propels most of the ejecta out of the crater, and then a rebounds to form a central peak.
These craters range in size from tiny pits to the immense South Pole–Aitken Basin, which has a diameter of nearly 2,500 km and a depth of 13 km. In general, the lunar history of impact cratering follows a trend of decreasing crater size with time. In particular, the largest impact basins were formed during the early periods, and these were successively overlaid by smaller craters.
There are estimated to be roughly 300,000 craters wider than 1 km (0.6 mi) on the Moon’s near side alone. Some of these are named for scholars, scientists, artists and explorers. The lack of an atmosphere, weather and recent geological processes mean that many of these craters are well-preserved.
Another feature of the lunar surface is the presence of regolith (aka. Moon dust, lunar soil). Created by billions of years of collisions by asteroids and comets, this fine grain of crystallized dust covers much of the lunar surface. The regolith contains rocks, fragments of minerals from the original bedrock, and glassy particles formed during the impacts.
The chemical composition of the regolith varies according to its location. Whereas the regolith in the highlands is rich in aluminum and silica, the regolith in the maria is rich in iron and magnesium and is silica-poor, as are the basaltic rocks from which it is formed.
Geological studies of the Moon are based on a combination of Earth-based telescope observations, measurements from orbiting spacecraft, lunar samples, and geophysical data. A few locations were sampled directly during the Apollo missions in the late 1960s and early 1970s, which returned approximately 380 kilograms (838 lb) of lunar rock and soil to Earth, as well as several missions of the Soviet Luna programme.
Much like Mercury, the Moon has a tenuous atmosphere (known as an exosphere), which results in severe temperature variations. These range from -153°C to 107°C on average, though temperatures as low as -249°C have been recorded. Measurements from NASA’s LADEE have mission determined the exosphere is mostly made up of helium, neon and argon.
The helium and neon are the result of solar wind while the argon comes from the natural, radioactive decay of potassium in the Moon’s interior. There is also evidence of frozen water existing in permanently shadowed craters, and potentially below the soil itself. The water may have been blown in by the solar wind or deposited by comets.
Several theories have been proposed for the formation of the Moon. These include the fission of the Moon from the Earth’s crust through centrifugal force, the Moon being a preformed object that was captured by Earth’s gravity, and the Earth and Moon co-forming together in the primordial accretion disk. The estimated age of the Moon also ranges from it being formed 4.40-4.45 billion years ago to 4.527 ± 0.010 billion years ago,roughly 30–50 million years after the formation of the Solar System.
The prevailing hypothesis today is that the Earth-Moon system formed as a result of an impact between the newly-formed proto-Earth and a Mars-sized object (named Theia) roughly 4.5 billion years ago. This impact would have blasted material from both objects into orbit, where it eventually accreted to form the Moon.
This has become the most accepted hypothesis for several reasons. For one, such impacts were common in the early Solar System, and computer simulations modelling the impact are consistent with the measurements of the Earth-Moon system’s angular momentum, as well as the small size of the lunar core.
In addition, examinations of various meteorites show that other inner Solar System bodies (such as Mars and Vesta) have very different oxygen and tungsten isotopic compositions to Earth. In contrast, examinations of the lunar rocks brought back by the Apollo missions show that Earth and the Moon have nearly identical isotopic compositions.
This is the most compelling evidence suggesting that the Earth and the Moon have a common origin.
Relationship to Earth:
The Moon makes a complete orbit around Earth with respect to the fixed stars about once every 27.3 days (its sidereal period). However, because Earth is moving in its orbit around the Sun at the same time, it takes slightly longer for the Moon to show the same phase to Earth, which is about 29.5 days (its synodic period). The presence of the Moon in orbit influences conditions here on Earth in a number of ways.
The most immediate and obvious are the ways its gravity pulls on Earth – aka. it’s tidal effects. The result of this is an elevated sea level, which are commonly referred to as ocean tides. Because Earth spins about 27 times faster than the Moon moves around it, the bulges are dragged along with Earth’s surface faster than the Moon moves, rotating around Earth once a day as it spins on its axis.
The ocean tides are magnified by other effects, such as frictional coupling of water to Earth’s rotation through the ocean floors, the inertia of water’s movement, ocean basins that get shallower near land, and oscillations between different ocean basins. The gravitational attraction of the Sun on Earth’s oceans is almost half that of the Moon, and their gravitational interplay is responsible for spring and neap tides.
Gravitational coupling between the Moon and the bulge nearest the Moon acts as a torque on Earth’s rotation, draining angular momentum and rotational kinetic energy from Earth’s spin. In turn, angular momentum is added to the Moon’s orbit, accelerating it, which lifts the Moon into a higher orbit with a longer period.
As a result of this, the distance between Earth and Moon is increasing, and Earth’s spin is slowing down. Measurements from lunar ranging experiments with laser reflectors (which were left behind during the Apollo missions) have found that the Moon’s distance to Earth increases by 38 mm (1.5 in) per year.
This speeding and slowing of Earth and the Moon’s rotation will eventually result in a mutual tidal locking between the Earth and Moon, similar to what Pluto and Charon experience. However, such a scenario is likely to take billions of years, and the Sun is expected to have become a red giant and engulf Earth long before that.
The lunar surface also experiences tides of around 10 cm (4 in) amplitude over 27 days, with two components: a fixed one due to Earth (because they are in synchronous rotation) and a varying component from the Sun. The cumulative stress caused by these tidal forces produces moonquakes. Despite being less common and weaker than earthquakes, moonquakes can last longer (one hour) since there is no water to damp out the vibrations.
Another way the Moon effects life on Earth is through occultation (i.e. eclipses). These only happen when the Sun, the Moon, and Earth are in a straight line, and take one of two forms – a lunar eclipse and a solar eclipse. A lunar eclipse occurs when a full Moon passes behind Earth’s shadow (umbra) relative to the Sun, which causes it to darken and take on a reddish appearance (aka. a “Blood Moon” or “Sanguine Moon”.)
A solar eclipse occurs during a new Moon, when the Moon is between the Sun and Earth. Since they are the same apparent size in the sky, the moon can either partially block the Sun (annular eclipse) or fully block it (total eclipse). In the case of a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye.
Because the Moon’s orbit around Earth is inclined by about 5° to the orbit of Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes.The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the “Saros Cycle“, which is a period of approximately 18 years.
History of Observation:
Human beings have been observing the Moon since prehistoric times, and understanding the Moon’s cycles was one of the earliest developments in astronomy. The earliest examples of this comes from the 5th century BCE, when Babylonian astronomers had recorded the 18-year Satros cycle of lunar eclipses, and Indian astronomers had described the Moon’s monthly elongation.
The ancient Greek philosopher Anaxagoras (ca. 510 – 428 BCE) reasoned that the Sun and Moon were both giant spherical rocks, and the latter reflected the light of the former. In Aristotle’s “On the Heavens“, which he wrote in 350 BCE, the Moon was said to mark the boundary between the spheres of the mutable elements (earth, water, air and fire), and the heavenly stars – an influential philosophy that would dominate for centuries.
In the 2nd century BCE, Seleucus of Seleucia correctly theorized that tides were due to the attraction of the Moon, and that their height depends on the Moon’s position relative to the Sun. In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance. These figures were greatly improved by Ptolemy (90–168 BCE), who’s values of a mean distance of 59 times Earth’s radius and a diameter of 0.292 Earth diameters were close to the correct values (60 and 0.273 respectively).
By the 4th century BCE, the Chinese astronomer Shi Shen gave instructions for predicting solar and lunar eclipses. By the time of the Han Dynasty (206 BCE – 220 CE), astronomers recognized that moonlight was reflected from the Sun, and Jin Fang (78–37 BC) postulated that the Moon was spherical in shape.
In 499 CE, the Indian astronomer Aryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause of the shining of the Moon. The astronomer and physicist Alhazen (965–1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon in all directions.
Shen Kuo (1031–1095) of the Song dynasty created an allegory to explain the waxing and waning phases of the Moon. According to Shen, it was comparable to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent.
During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognized as a sphere, though many believed that it was “perfectly smooth”. In keeping with medieval astronomy, which combined Aristotle’s theories of the universe with Christian dogma, this view would later be challenged as part of the Scientific Revolution (during the 16th and 17th century) where the Moon and other planets would come to be seen as being similar to Earth.
Using a telescope of his own design, Galileo Galilei drew one of the first telescopic drawings of the Moon in 1609, which he included in his book Sidereus Nuncius (“Starry Messenger). From his observations, he noted that the Moon was not smooth, but had mountains and craters. These observations, coupled with observations of moons orbiting Jupiter, helped him to advance the heliocentric model of the universe.
Telescopic mapping of the Moon followed, which led to the lunar features being mapped in detail and named. The names assigned by Italian astronomers Giovannia Battista Riccioli and Francesco Maria Grimaldi are still in use today. The lunar map and book on lunar features created by German astronomers Wilhelm Beer and Johann Heinrich Mädler between 1834 and 1837 were the first accurate trigonometric study of lunar features, and included the heights of more than a thousand mountains.
Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s, when English astronomer Richard Proctor proposed that they were formed by collisions. This view gained support throughout the remainder of the 19th century; and by the early 20th century, led to the development of lunar stratigraphy – part of the growing field of astrogeology.
With the beginning of the Space Age in the mid-20th century, the ability to physically explore the Moon became possible for the first time. And with the onset of the Cold War, both the Soviet and American space programs became locked in an ongoing effort to reach the Moon first. This initially consisted of sending probes on flybys and landers to the surface, and culminated with astronauts making manned missions.
Exploration of the Moon began in earnest with the Soviet Luna program. Beginning in earnest in 1958, the programmed suffered the loss of three unmanned probes. But by 1959, the Soviets managed to successfully dispatch fifteen robotic spacecraft to the Moon and accomplished many firsts in space exploration. This included the first human-made objects to escape Earth’s gravity (Luna 1), the first human-made object to impact the lunar surface (Luna 2), and the first photographs of the far side of the Moon (Luna 3).
Between 1959 and 1979, the program also managed to make the first successful soft landing on the Moon (Luna 9), and the first unmanned vehicle to orbit the Moon (Luna 10) – both in 1966. Rock and soil samples were brought back to Earth by three Luna sample return missions – Luna 16 (1970), Luna 20 (1972), and Luna 24 (1976).
Two pioneering robotic rovers landed on the Moon – Luna 17 (1970) and Luna 21 (1973) – as a part of Soviet Lunokhod program. Running from 1969 to 1977, this program was primarily designed to provide support for the planned Soviet manned moon missions. But with the cancellation of the Soviet manned moon program, they were instead used as remote-controlled robots to photograph and explore the lunar surface.
NASA began launching probes to provide information and support for an eventual Moon landing in the early 60s. This took the form of the Ranger program, which ran from 1961 – 1965 and produced the first close-up pictures of the lunar landscape. It was followed by the Lunar Orbiter program which produced maps of the entire Moon between 1966-67, and the Surveyor program which sent robotic landers to the surface between 1966-68.
In 1969, astronaut Neil Armstrong made history by becoming the first person to walk on the Moon. As the commander of the American mission Apollo 11, he first sett foot on the Moon at 02:56 UTC on 21 July 1969. This represented the culmination of the Apollo program (1969-1972), which sought to send astronauts to the lunar surface to conduct research and be the first human beings to set foot on a celestial body other than Earth.
The Apollo 11 to 17 missions (save forApollo 13, which aborted its planned lunar landing) sent a total of 13 astronauts to the lunar surface and returned 380.05 kilograms (837.87 lb) of lunar rock and soil. Scientific instrument packages were also installed on the lunar surface during all the Apollo landings. Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17 landing sites, some of which are still operational.
After the Moon Race was over, there was a lull in lunar missions. However, by the 1990s, many more countries became involve in space exploration. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its Hiten spacecraft, an orbiter which released the smaller Hagoroma probe.
In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine to lunar orbit to obtain the first near-global topographic map of the Moon and the first global multispectral images of the lunar surface. This was followed in 1998 by the Lunar Prospector mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters.
Since the year 2000, exploration of the moon has intensified, with a growing number of parties becoming involved. The ESA’s SMART-1spacecraft, the second ion-propelled spacecraft ever created, made the first detailed survey of chemical elements on the lunar surface while in orbit from November 15th, 2004, until its lunar impact on September 3rd, 2006.
China has pursued an ambitious program of lunar exploration under their Chang’e program. This began with Chang’e 1, which successfully obtained a full image map of the Moon during its sixteen month orbit (November 5th, 2007 – March 1st, 2009) of the Moon. This was followed in October of 2010 with the Chang’e 2 spacecraft, which mapped the Moon at a higher resolution before performing a flyby of asteroid 4179 Toutatis in December of 2012, then heading off into deep space.
On 14 December 2013, Chang’e 3 improved upon its orbital mission predecessors by landing a lunar lander onto the Moon’s surface, which in turn deployed a lunar rover named Yutu (literally “Jade Rabbit”). In so doing, Chang’e 3 made the first soft lunar landing since Luna 24 in 1976, and the first lunar rover mission since Lunokhod 2 in 1973.
Between October 4th, 2007, and June 10th, 2009, the Japan Aerospace Exploration Agency‘s (JAXA) Kaguya (“Selene”) mission – a lunar orbiter fitted with a high-definition video camera and two small radio-transmitter satellites – obtained lunar geophysics data and took the first high-definition movies from beyond Earth orbit.
The Indian Space Research Organisation (ISRO) first lunar mission, Chandrayaan I, orbited the Moon between November 2008 and August 2009 and created a high resolution chemical, mineralogical and photo-geological map of the lunar surface, as well as confirming the presence of water molecules in lunar soil. A second mission was planned for 2013 in collaboration with Roscosmos, but was cancelled.
NASA has also been busy in the new millennium. In 2009, they co-launched the Lunar Reconnaissance Orbiter (LRO) and the Lunar CRater Observation and Sensing Satellite(LCROSS) impactor. LCROSS completed its mission by making a widely observed impact in the crater Cabeus on October 9th, 2009, while the LRO is currently obtaining precise lunar altimetry and high-resolution imagery.
Upcoming lunar missions include Russia’s Luna-Glob – an unmanned lander with a set of seismometers, and an orbiter based on its failed Martian Fobos-Grunt mission. Privately funded lunar exploration has also been promoted by the Google Lunar X Prize, which was announced on September 13th, 2007, and offers US$20 million to anyone who can land a robotic rover on the Moon and meet other specified criteria.
Under the terms of the Outer Space Treaty, the Moon remains free to all nations to explore for peaceful purposes. As our efforts to explore space continue, plans to create a lunar base and possibly even a permanent settlement may become a reality. Looking to the distant future, it wouldn’t be far fetched at all to imagine native-born humans living on the Moon, perhaps known as Lunarians (though I imagine Lunies will be more popular!)
We have many interesting articles about the Moon here at Universe Today. Below is a list that covers just about everything we know about it today. We hope you find what you are looking for:
Shining like a beacon in Earth’s sky is the Moon. We’ve seen so much of it in our lifetimes that it’s easy to take it for granted; even the human landings on the Moon in the 1960s and 1970s were eventually taken for granted by the public.
Fortunately for science, we haven’t stopped looking at the Moon in the decades after Neil Armstrong took his first step. Here are a few things to consider about Earth’s closest big neighbor.