Is the sky blue on other planets, like on Earth? What is an atmosphere, and do other planets have one? – Charlie, age 10
G’day Charlie, and thank you so much for your incredibly curious question.
Before I get too excited talking about the atmospheres of other planets, first we have to talk about what an atmosphere actually is.
The atmosphere is normally the outermost layer of a planet. On rocky worlds like Earth it is usually the lightest and thinnest layer.
The thing that makes an atmosphere an atmosphere is what it’s made of. It’s not made up of big lumps of rocks or huge swirling oceans; it is made up of gases.
What’s in an atmosphere?
Atmospheres can contain a wide variety of gases. Most of Earth’s atmosphere is a gas called nitrogen that doesn’t really react with anything. There’s also a fair bit of oxygen, which is what we need to breathe. There are also two other important gases called argon and carbon dioxide, and tiny amounts of lots of other ones.
The mix of gases is what gives a planet’s atmosphere its colour.
Earth’s atmosphere is made up of gases that tend to bounce blue light in all directions (known as “scattering”) but let most other colours of light straight through. This scattered light is what gives Earth’s atmosphere its blue colour.
Do other planets have blue atmospheres? Some of them sure do!
The atmospheres of the two ice giants in our solar system, Neptune and Uranus, are both beautiful shades of blue.
However, these atmospheres are a different blue than ours. It’s caused by the huge amounts of a gas called methane swirling around.
(Side note: methane is also the main component of farts. That’s right, there’s a layer of farts on Uranus.)
Jupiter and Saturn, however, have completely different-coloured atmospheres.
Ice crystals made of a chemical called ammonia in Saturn’s upper atmosphere make it a pale shade of yellow.
Uranus’ atmosphere also contains some ammonia, which makes the planet a slightly greener shade than the deep blue we see on Neptune.
Jupiter’s atmosphere has distinctive brown and orange bands, thanks to gases that may contain the elements phosphorus and sulfur, and possibly even more complicated chemicals called hydrocarbons.**
In some extreme cases, the entire planet might just be a huge atmosphere with no rocky surface at all. Astronomers and planetary scientists like myself are still trying to work out whether Jupiter and Saturn have rocky surfaces, deep down in their atmosphere, or whether they’re both simply huge balls of gas.
However, there are some planets that have no atmosphere at all! The Sun’s closest and smallest neighbour, Mercury, is one example. Its surface is exposed to the vastness of space.
Beyond our solar system
So far I’ve been talking about the atmospheres of planets in our Solar system. But what about planets in other planetary systems, orbiting other stars?
Well, astronomers have been detecting the atmospheres of these planets (which we call “exoplanets”) for the past 20 years! It wasn’t until last year, however, that astronomers managed to detect the atmosphere of a rocky exoplanet. The planet is called LHS 3844b and it’s so far away that the light takes almost 50 years to reach us!
LHS 3844b weighs twice as much as Earth, and we astronomers thought it would have a pretty thick atmosphere. But, to our surprise, it has little to no atmosphere at all! So it might be more like Mercury than Earth.
We still have a lot to learn about far-off planets and discovering one with an Earth-like atmosphere that’s ripe for life is still many years away.
Maybe, Charlie, you could be the first astronomer to detect an Earth-like atmosphere on another world!
Where 2019 was a disappointing year for meteor showers, with two of the big three (the Quadrantids, Perseids and Geminids) lost mainly to moonlight, 2020 promises to be much better.
The year starts with a bang with the Quadrantids providing a treat for northern hemisphere viewers. The Perseids, in August, provide another highlight for those in the northern hemisphere, while the December Geminids round the year off for observers all around the world.
But the big three aren’t the only meteor showers that will put on a show this year. So when should you look up to see the meteoric highlights of the coming year?
Here’s our pick of the showers to watch. We have the time each shower is forecast to peak, finder charts showing you where best to look, and the theoretical peak rates you could see under ideal observing conditions. This is a number known as the Zenithal Hourly Rate (ZHR).
Because the ZHR is the theoretical maximum rate you could see per hour, it is likely that the rates you observe will be lower.
For any meteor shower, if you want to give yourself the best chance to see a good display, it is worth trying to find a good dark site, as far from light polluted skies as possible. Once you’re outside give yourself plenty of time to adapt to the darkness, at least half an hour. Then just sit back, relax, and enjoy the show.
Showers that can only really be seen from either the northern or southern hemisphere are denoted by [N] or [S], whilst those that can be seen from both are marked by [N/S].
You can download a ICS file of this guide to add to your favourite calender.
Active: December 28 – January 12
Maximum: January 4, 8:20am UTC = 8:20am GMT = 3:20am EST = 12:20am PST
ZHR: 120 (variable, can reach ~200)
Parent: It’s complicated… (Comet 96P/Macholz and asteroid 2003 EH1)
The Quadrantids are the first of the big three meteor showers of the year – the three showers that give fabulous displays with ZHRs in excess of 100, year in, year out.
For most of the fortnight over which the Quadrantids are active, rates are low – just a few meteors per hour. In the hours approaching their peak, rates climb rapidly, before falling away just as rapidly once the peak is past. In total, rates exceed a quarter of their maximum value for just eight hours, centred on the peak.
The Quadrantid radiant is circumpolar (never sets) for locations north of 40 degrees north. As a result, the shower can be observed throughout the hours of darkness for most locations in Europe and many in North America.
The radiant rises highest in the sky in the early hours of the morning, so this is when the best rates can be seen. In 2020, the shower’s peak favours observers in the east of North America, though those in northern Europe should see a good display in the hours before dawn on the morning of January 4.
If skies are clear it is definitely worth wrapping up warm and heading out to observe the most elusive of the year’s big three.
Active: April 14 – 30
Maximum: Variable – between April 21, 10:40pm UTC and April 22, 9:40am UTC (April 22 9:40am UTC = 4:40am EST = 1:40am PST)
ZHR: 18 (variable, can reach ~90)
Parent: Comet C/1861 G1 Thatcher
The Lyrids are a shower with a long and storied history – with records reporting their activity tracing back for millennia. Researchers have even suggested the Lyrids may have been active on Earth for more than a million years.
In the distant past, there are reports the Lyrids produced some spectacular displays – meteor storms, with thousands of meteors visible per hour.
The modern Lyrids are usually more sedate, with peak rates rarely exceeding ~18 meteors per hour. But they do sometimes throw up the odd surprise. An outburst of the Lyrids in 1982 yielded rates of ~90 meteors per hour for a short period.
While no such outburst is forecast this year, the peak of the shower will occur just a day before a new Moon, so skies will be dark and viewing conditions ideal.
Although the Lyrids are best seen from the northern hemisphere, their radiant can reach a useful altitude for observers in the northern half of Australia. Keen observers might be tempted to head out in the early hours of the morning to watch.
The radiant rises during the night so the best rates are seen in the early hours of the morning, before dawn. From northern hemisphere sites, reasonable rates can be seen after about 10:30pm, local time -– but for those at southern hemisphere latitudes, the radiant fails to reach a reasonable altitude until well after midnight.
Lyrid meteors tend to be relatively fast and are often bright. Despite the relatively low rates (at least, compared to the big three) they are well worth a watch, especially as conditions this year will be as close to perfect as possible.
Active: April 19 – May 28
Maximum: May 5, 9pm UTC = May 6, 7am AEST (Qld/NSW/ACT/Vic/Tas) = May 6, 4am AWST (WA) = May 6, 6am JST
Parent: Comet 1P/Halley
While not counted as one of the big three, in many ways the Eta Aquariids stand clear of the pack as the best of the rest.
Only really visible to observers in the tropics and the southern hemisphere, the Eta Aquariids are fragments of the most famous of comets –- Halley’s comet. They mark the first (and best) of two passages made by the Earth through the debris laid down by that comet over thousands of years –- with the other being the Orionids, in October.
The radiant only rises a few hours before dawn, even at southern altitudes, and the further north you go, the closer to sunrise the radiant appears. This is what prevents northern hemisphere observers from taking advantage of the Eta Aquariids –- the Sun has risen by the time the radiant is high enough for the shower to put on a decent show.
The meteors are fast and often bright, and the brighter ones have a reputation for leaving behind noticeable smoky trains. The maximum of the shower is broad, with rates remaining above ~30 meteors per hour for the week around the date of the maximum.
It is well worth getting out to observe the Eta Aquariids at around the time their radiant rises. This gives the maximum amount of time to observe the shower before dawn, but in addition, those few meteors you observe when the radiant is sitting just above the horizon can be spectacular.
Known as Earthgrazers, such meteors enter the atmosphere at a very shallow angle, with the result that they can streak all the way across the sky, from horizon to horizon.
The Eta Aquariids reach their peak in 2020 a couple of days before the full Moon. That the radiant does not rise until a few hours before sunrise works to our advantage this year –- the shower’s radiant will rise at around the same time the Moon sets, so the shower can be observed in Moon-free skies, despite the proximity of the Full Moon.
Active: July 17 – August 24
Maximum: August 12, 1pm – 4pm UTC = 3am – 6am HST = 10pm – August 13, 1am JST + filament passage ~3 hours before the main peak
Parent: Comet 109P/Swift-Tuttle
For northern hemisphere observers, the Perseids are perhaps the famous and reliable shower of the year.
While the Geminids offer higher rates, the Perseids fall during the middle of the northern summer, when families are often holidaying and the weather is warm and pleasant. As a result, the Perseids are the most widely observed of all meteor showers, and never fail to put on a spectacular show.
The parent comet of the Perseid meteor shower, 109P/Swift-Tuttle, was last at perihelion (closest to the Sun) in 1991. As a result, during the 1990s, the Perseids offered enhanced rates –- often displaying multiple peaks through the two or three days around their traditional maximum.
Those individual peaks were the result of the Earth passing through individual trails of material, laid down at past perihelion passages of the comet, which have not yet had time to fully disperse into the background of the shower as a whole.
It is now three decades since the comet’s last perihelion passage, but astronomers predict the Earth could well pass through one of those debris trails this year, at around 10am UTC (midnight Hawaii time, 3am Vancouver time), three hours before the normal forecast maximum for the shower.
As a result, peak rates should last for longer, and potentially reach higher values than would normally be expected from a typical Perseid return.
The radiant rises in the mid-evening from northern latitudes, which means the shower can be observed from around 10pm or 11pm, local time. The later in the night you look, the higher the radiant will be, and so the more meteors will be visible.
Unfortunately, the peak of the Perseids in 2020 falls two days after the last quarter Moon, which means moonlight will begin to interfere with the display in the early hours of the morning. The best views of the shower will likely be seen between ~10pm or 11pm local time and ~2am the following morning.
If you can only observe in the hours before dawn, all is not lost. The Perseids are famed for producing plenty of bright meteors. They are worth observing even when the Moon is above the horizon, particularly on the nights around the forecast peak.
Active: October 2 – November 7
Maximum: October 21
The Orionid meteor shower marks the second occasion the Earth encounters the stream of debris left behind by Halley’s comet each year.
In October, Earth passes farther from the centre of Halley’s debris stream than in May, with the result the observed rates for the Orionids are lower than for the Eta Aquariids. Despite this, the Orionids remain a treat for meteor enthusiasts in the northern autumn and southern spring.
The Orionids peak on October 21 but that maximum is often quite broad with activity hovering close to the peak rates for as much as a week around the maximum.
There is some evidence the peak rates vary over time, with a roughly 12 year periodicity, as a result of perturbations by the giant planet Jupiter (which orbits the Sun once every 12 years).
In the final years of the first decade of the 21st Century, the Orionids were markedly more active than expected, with maximum rates in the range 40-70. If the periodicity is real, then 12 years on from the peak of activity it is possible the Orionids will again put on a better than expected show.
So 2020 might well be an ideal year to look up and watch for fragments of Halley’s Comet vapourising high overhead.
The radiant rises just before local midnight, meaning the meteors are best observed in the early hours of the morning. The radiant reaches its highest altitude in the hours before dawn. The Moon will not interfere this year, setting in the early evening, long before the radiant rises.
Observers watching the Orionids are in for an extra treat. While the Orionids are active, so too are the Northern and Southern Taurid meteor showers. Where the Orionids are fast meteors, Taurids are slow, and often bright and spectacular.
Although the rates of both the Northern and Southern Taurids are lower than those of the Orionids (typically just ~5 per hour), their activity makes observations of the Orionids even more productive and exciting.
Active: December 4 – 17
Maximum: December 14, 12:50am UTC = 11:50am AEDT (NSW/ACT/Vic/Tas) = 8:50am AWST (WA) = 5:50pm EST (evening of December 13)
Parent: Asteroid 3200 Phaethon
The Geminids, which peak in mid-December, are truly a case of saving the best until last. The biggest of the year’s big three, the Geminids have, over the past few decades, been growing ever more active and spectacular, with recent years seeing rates in excess of 150 per hour.
For observers in northern Europe, the radiant is above the horizon relatively soon after sunset, meaning that the Geminids can readily be observed from around 8pm onwards.
The further south you travel, the later in the evening the radiant rises. For observers in Australia, the times at which the radiant appears above the horizon can be seen below.
As with all showers, the higher the radiant in the sky, the better the observed rates from the Geminids will be. The longer you watch, the better things will get.
Geminid meteors are of medium speed and often bright so they put on a spectacular show even in those years when moonlight interferes.
In 2020 the Moon will be new around this time so it will be possible to spend the entire night watching the Geminids without any interference from our nearest celestial neighbour.
The radiant reaches its highest at around 2am local time making the hours just after midnight the ideal time to catch the Geminids at their best.
The Geminid peak is relatively broad -– with rates remaining high for at least 24 hours around the forecast maximum. Observers across the globe will be treated to a spectacular display from the shower in 2020.
So find a dark site, wrap up warm, and treat yourself to a night spent watching the year’s most spectacular display of natural fireworks.
How did our galaxy form? How do galaxies evolve over time? Where did the Sun’s lost siblings end up?
Three hours north-east of Parkes lies a remote astronomical research facility, unpolluted by city lights, where researchers are collecting vast amounts of data in an effort to unlock some of the biggest questions about our Universe.
Siding Spring Observatory, or SSO, is one of Australia’s top sites for astronomical research. You’ve probably heard of the Parkes telescope, made famous by the movie The Dish, but SSO is also a key character in Australia’s space research story.
In this episode, astrophysics student and Conversation intern Cameron Furlong goes to SSO to check out the huge Anglo Australian Telescope (AAT), the largest optical telescope in Australia.
And we hear about Huntsman, a new specialised telescope that uses off-the-shelf Canon camera lenses – a bit like those you see sports photographers using at the cricket or the footy – to study very faint regions of space around other galaxies.
Listen in to hear more about some of the most fascinating space research underway in Australia – and how, despite gruelling hours and endless paperwork, astronomers retain their sense of wonder for the night sky.
“For me, it means remembering how small I am in this enormous Universe. I think it’s very easy to forget, when you go about your daily life,” said Richard McDermid, an ARC Future Fellow and astronomer at Macquarie University.
“It’s nice to get back into it to a dark place and having a clear sky. And then you get to remember all the interesting and fascinating things, the size, the grandeur and the peacefulness of being in the dark.”
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Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.
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If tiny concentrations of carbon dioxide can hold enough heat to create a global warming impact on Earth, why is Mars cold? Its atmosphere is 95% carbon dioxide.
The recipe for the temperature of a planet’s surface has four major ingredients: atmospheric composition, atmospheric density, water content (from oceans, rivers and air humidity) and distance from the Sun. There are other ingredients, including seasonal effects or the presence of a magnetosphere, but these work more like adding flavour to a cake.
When we look at Earth, the balance of these ingredients makes our planet habitable. Changes in this balance can result in effects that can be felt on a planetary scale. This is exactly what is happening with the increase of greenhouse gases in the atmosphere of our planet.
Increased concentrations of carbon dioxide, methane, sulphur hexafluoride and other gases in the atmosphere have been raising the temperature of our planet’s surface gradually and will continue to do so for many years to come.
As a consequence, places covered in ice start melting and extreme weather events become more frequent. This poses a growing challenge for us to adapt to this new reality.
Small concentration, big effect
It is surprising to realise how little the concentration of carbon dioxide (CO₂) and other greenhouse gases has to change to cause such a shift in our climate. Since the 1950s, we have raised CO₂ levels in the atmosphere by a fraction of a percent, but this is already causing several changes in our climate.
This is because CO₂ represents a tiny part of Earth’s atmosphere. It is measured in parts per million (ppm) which means that for every carbon dioxide molecule there are a million others. Its concentration is just 0.041%, but even a small percentage change represents a big change in concentration.
We can tell what Earth’s atmosphere and climate were like in the distant past by analysing bubbles of ancient air trapped in ice. During Earth’s ice ages, the concentration of carbon dioxide was around 200ppm. During the warmer interglacial periods, it hovered around 280ppm, but since the 1950s, it has continued to rise relentlessly. By 2013, CO₂ levels surpassed 400ppm for the first time in recorded history.
This rise represents almost a doubling in concentration, and it clear that, in the recipe for Earth’s surface temperature, carbon dioxide and other greenhouse gases are to be used in moderation.
The role of water
Like flour for a cake, water is an important ingredient of the Earth’s surface. Water makes temperature move slowly. That’s why the temperatures in tropical rainforests does not change much, but the Sahara desert is cold at night. Earth is rich in water.
Let’s have a look at our solid planets. Mercury is the closest planet to the Sun, but it has a very thin atmosphere and is not the warmest planet. Venus is very, very hot. Its atmosphere is rich in carbon dioxide (over 96%) and it is very dense.
The atmosphere of Mars is also rich in carbon dioxide (above 96%), but it is extremely thin (1% of Earth’s atmosphere), very dry and located further away from the Sun. This combination makes the planet an incredibly cold place.
The absence of water makes the temperature on Mars change a lot. The Mars exploration rovers (Spirit at Gusev Crater and Opportunity at Meridiani Planun) experienced temperatures ranging from a few degrees Celsius above zero to minus 80℃ at night: every single Martian day, known as sol.
Terraforming or terra fixing
One of the interesting challenges we face while building space payloads, like we do at Griffith University, is to build instruments that can withstand such a wide temperature range.
I love conversations about terraforming. This is the idea that we could fly to a planet with an unbreathable atmosphere and fix it by using some sort of machine to filter nasty gases and release good ones we need to survive, at the correct amount. That is a recurrent theme in many science fiction films, including Aliens, Total Recall and Red Planet.
I hope we can fix our own atmosphere on Earth and reduce our planet’s fever.
With the discovery of 20 more moons orbiting Saturn, the ringed planet has overtaken Jupiter as host to the most moons in the Solar system. Saturn now has 82 known moons, whereas Jupiter has a paltry 79.
Announced at the International Astronomical Union’s Minor Planet Centre by a team of astronomers from the Carnegie Institute for Science led by Scott S. Sheppard, the discovery is the latest advance in the 400-year history of our understanding of the satellites of our neighbouring planets.
As technology has improved, we have observed more and more of these tiny, distant worlds – and we can be reasonably confident there are still plenty waiting to be discovered.
How do we even know Saturn has moons?
Although most planets of the Solar system are visible to the naked eye and have been known to humans since antiquity, it wasn’t until Galileo Galilei turned a telescope on Jupiter in 1610 that we discovered Earth was not alone in having an orbiting companion.
Galileo saw Jupiter’s four largest moons and could make out what we now know are Saturn’s rings. Decades later, with better telescopes, Christian Huygens and Giovanni Domenico Cassini observed Saturn’s moons.
Curious Kids: why does Saturn have rings?
It became clear that the giant planets are surrounded by multitudes of satellites, resembling smaller versions of the Solar system. By the middle of the 19th century, telescopes had improved enough that the first eight moons of Saturn – including Titan, the largest – had been viewed directly.
The introduction of photographic plates, which enabled the detection of fainter objects with long-exposure observations, helped astronomers increase their count of Saturn’s moons to 14.
It was a long journey (literally) to the next big improvement in our view of Saturn’s moons. Many of the smaller moons were not discovered until the Voyager fly-by missions in the 1980s and the more recent 13-year stopover of the Cassini spacecraft in Saturn’s orbit.
Until these closer visits, we knew little about the moons aside from the fact that they existed.
One of Cassini’s goals was to explore Titan, which is the only moon in the Solar system with a thick, smoggy atmosphere. Another was to take a look at Saturn’s other mid-sized moons, including frozen Enceladus, which may hold an ocean of liquid water beneath its icy crust.
Cassini also discovered much smaller moons, so-called “shepherd moons” that interact with Saturn’s rings by carving gaps and wavy patterns as they pass through a rubble of rocks and snowballs.
Bigger telescopes, more moons
These close-up observations from space advanced our understanding of individual moons that stay near to Saturn. Recently, many more moons have been found in orbits much further from the planet.
These more distant moons could only be detected with large optical telescopes such as the Subaru telescope at Mauna Kea in Hawaii. The telescope is equipped with sensitive cameras that can detect some of the faint objects separated by millions of kilometres from Saturn.
To confirm that these objects are indeed associated with Saturn, astronomers have to observe them over days or even months to reconstruct the shape and size of the moon’s orbit.
Many small moons are fragments of shattered large moons
Such observations revealed a population of moons that are often described as “irregular” moons. They are split into three distinct groups: Inuit, Gallic, and Norse. They all have large, elliptical orbits at an angle to those of moons closer to the planet.
Each group is thought to have formed from a collision or fragmentation of a larger moon. The Norse group consists of some of the most distant moons of Saturn, which orbit in the opposite direction to the rotation of the planet. This suggests they could have formed elsewhere and were later captured by the gravitational force of Saturn.
Of the 20 new moons, 17 belong to the Norse group including the furthest known moon from the planet. Their estimated sizes are of the order of 5km in diameter.
Have we found all the moons now?
Are we likely to find even more moons around Saturn? Absolutely.
Some of the newly discovered moons are very faint and at the limit of detection with currently available instruments. New, bigger telescopes such as Giant Magellan Telescope will allow us to observe even fainter objects.
In the meantime, the 20 new moons need names. Carnegie Science has invited everyone to help.
Even though we can see the Moon shining brightly in the night sky – and sometimes in daylight – it’s hard to put into perspective just how large, and just how distant, our nearest neighbour actually is.
So just how big is the Moon?
That answer isn’t quite as straightforward as you might think. Like Earth, the Moon isn’t a perfect sphere. Instead, it’s slightly squashed (what we call an oblate sphereoid). This means the Moon’s diameter from pole to pole is less than the diameter measured at the equator.
Why the Moon is such a cratered place
But the difference is small, just four kilometres. The equatorial diameter of the Moon is about 3,476km, while the polar diameter is 3,472km.
To see how big that is we need to compare it to something of a similar size, such as Australia.
From coast to coast
The distance from Perth to Brisbane, as the crow flies, is 3,606km. If you put Australia and the Moon side by side, they look to be roughly the same size.
But that’s just one way of looking at things. Although the Moon is about as wide as Australia, it is actually much bigger when you think in terms of surface area. It turns out the surface of the Moon is much larger than that of Australia.
How far is the Moon?
Asking how far away is the Moon is another of those questions whose answer is more complicated than you might expect.
The Moon moves in an elliptical orbit around the Earth, which means its distance from our planet is constantly changing. That distance can vary by up to 50,000km during a single orbit, which is why the size of the Moon in our sky varies slightly from week to week.
The Moon’s orbit is also influenced by every other object in the Solar System. Even when all of that is taken into account, the distance answer is still always changing, because the Moon is gradually receding from the Earth as a result of the tidal interaction between the two.
That last point is something we’ve been able to better study as a result of the Apollo missions. The astronauts who visited the Moon placed an array of mirror reflectors on its surface. Those reflectors are the continual target of lasers from the Earth.
By timing how long it takes for that laser light to travel to the Moon and back, scientists are able to measure the distance to the Moon with incredible precision, and to track the Moon’s recession from Earth. The result? The Moon is receding at a speed of 38mm per year – or just under 4 metres per century.
Drive me to the Moon
Having said all that, the average distance between the Moon and Earth is 384,402km. So let’s put that into context.
If I were to drive from Brisbane to Perth, following the fastest route suggested by Google, I would cover 4,310km on my road trip. That journey, driving across the breadth of our country, would take around 46 hours.
If I wanted to clock up enough kilometres to say that I’d covered the distance between the Earth and the Moon, I’d have to make that trip more than 89 times. It would take five-and-a-half months of driving, non-stop, assuming I didn’t run into any traffic jams on the way.
Fortunately, the Apollo 11 astronauts weren’t restricted to Australian speed limits. The command module Columbia took just three days and four hours to reach lunar orbit following its launch on July 16 1969.
An eclipse coincidence
The equatorial diameter of the Sun is almost 1.4 million kilometres, which is almost exactly 400 times the diameter of the Moon.
That ratio leads to one of astronomy’s most spectacular quirks – because the distance between the Earth and the Sun (149.6 million kilometres) is almost (but not quite) 400 times the distance between the Earth and the Moon.
Explainer: what is a solar eclipse?
The result? The Moon and the Sun appear almost exactly the same size in Earth’s sky. As a result, when the Moon and the Sun line up perfectly, as seen from Earth, something wonderful happens – a total eclipse of the Sun.
Sadly, such spectacular eclipses will eventually come to an end on Earth. Thanks to the Moon’s recession, it will one day be too distant to perfectly obscure the Sun. But that day will be a long time coming, with most estimates suggesting it will occur in something like 600 million years’ time.
While we’ve dispatched out robot envoys to the icy depths of the Solar System, the Moon remains the only other world on which humanity has walked.
Fifty years after that first adventure, the number of people to have walked on the Moon who are still alive is in sharp decline. Twelve people have had that experience but, as of today, just four remain.
Vast as the Moon is, those 12 moonwalkers barely scratched the surface. Hopefully, in the coming years, we will return, to inspire a whole new generation and to continue humanity’s in-person exploration of our nearest celestial neighbour.
Look up on a clear night and you can see some circular formations on the face of our lunar neighbour. These are impact craters, circular depressions found on planetary surfaces.
About a century ago, they were suspected to exist on Earth but the cosmic origin was often met with suspicion and most geologists believed that craters were of volcanic origin.
Around 1960, the American astrogeologist Gene Shoemaker, one of the founders of planetary science, studied the dynamics of crater formation on Earth and planetary surfaces. He investigated why they – including our Moon – are so cratered.
Images from Apollo
By 1970, there were more than 50 craters discovered on Earth but that work was still considered controversial, until pictures of the lunar surface brought by the Apollo missions confirmed that impact cratering is a common geological process outside Earth.
Unlike Earth’s surface, the lunar surface is covered with craters. This is because Earth is a dynamic planet, and tectonics, volcanism, seismicity, wind and oceans all play against the preservation of impact craters on Earth.
In contrast to Earth, our Moon has been inactive over long geological timescales and has no atmosphere, which has allowed the persistent impact cratering to remain over eons. The lunar cratering record spans its entire bombardment history – from the Moon’s very origins to today.
The big ones
The largest and oldest impact crater in the Solar system is believed to be on the Moon, and it is called the South Pole-Aitken basin, but we cannot see it from Earth because it is on the far side of the Moon. The Moon is tidally locked to Earth’s rotation and the same side always faces toward us.
But this crater, more than 2,000km across, is thought to predate any other large impact bombardment that occurred during lunar evolution. Impact simulations suggested it was formed by a 150-250km asteroid hurtling into the Moon at 15-20km per second!
From Earth, the human eye can observe areas of different shades of grey on the surface of the Moon facing us. The dark areas are called maria, and can be up to more than 1,000km across.
They are volcanic deposits that flooded depressions created by the formation of the large impact basins on the Moon. These volcanic eruptions were active for millions of years after these impacts occurred.
No other large impact event has occurred on the Moon since then. This is a good sign, because it implies there were no very large impacts occurring on Earth either after this time in evolutionary history. (The asteroid that wiped out the dinosaurs on Earth 66 million years ago was only about 10-15km in size and left a crater larger than 150km in size, which was substantial enough to cause a mass extinction.)
As seen from Earth
They are called complex craters because they are not entirely bowl-shaped, but are a bit shallower and include a peak in the centre of the crater as a consequence of the material collapsing into the hole made during impact. Tycho and Copernicus are both 80-100km across but have spectacular central peaks and prominent “ejecta rays” – areas where material was ejected across the lunar surface after an impact.
The formation of these craters excavated underlying material that was brighter than the actual surface. This is because lunar surface is subjected to space weathering, which causes surface rocks to darken.
Still a target for impacts
The Apollo 12, 14, 15, and 16 missions placed several seismic stations on the Moon between 1969 and 1972, creating the first extraterrestrial seismic network (ALSEP). During one year of operations, more than 1,000 seismic events were recorded, of which 10% were associated with meteoroids impacts.
So the Moon is still being hit by objects, albeit mostly tiny ones. But as there is no atmosphere on the Moon, there is no gas to help burn up these rocks from space and stop them smashing into the Moon.
The seismic network was functional until it was switched off in 1977, in preparation for new space missions. No one expected that the next fully operational extraterrestrial seismometer would not be placed on a planetary surface (Mars) until 40 years later.
Nowadays, from Earth, using a small telescope (and armed with a little patience), you can see so-called “impact flashes”, which are small meteorite impacts on the lunar surface that is facing us.
Thanks to the atmosphere on Earth, similar-sized rocks from space cannot make an impact here because they tend to predominantly burn up, but on the Moon they crash into the soil and release its kinetic energy of the impact via bright thermal emission.