Patrick M Shober, Curtin UniversityIf asked where meteorites come from, you might reply “from comets”. But according to our new research, which tracked hundreds of fireballs on their journey through the Australian skies, you would be wrong.
In fact, it is very likely that all meteorites — space rocks that make it all the way to Earth — come not from icy comets but from rocky asteroids. Our new study found that even those meteorites with trajectories that look like they arrived from much farther afield are in fact from asteroids that simply got knocked into strange orbits.
We searched through six years’ worth of records from the Desert Fireball Network, which scans the Australian outback for flaming meteors streaking through the sky. None of what we found came from comets.
That means that of the tens of thousands of meteorites in collections around the world, likely none are from comets, leaving a significant gap in our understanding of the Solar System.
When the Solar System formed, more than 4.5 billion years ago, a disc of dust and debris was swirling around the Sun.
Over time, this material clumped together, forming larger and larger bodies — some so large they swept up everything else in their orbit, and became planets.
Yet some debris avoided this fate and is still floating around today. Scientists traditionally classify these objects into two groups: comets and asteroids.
Asteroids are rockier and drier, because they were formed in the inner Solar System. Comets, meanwhile, formed further out, where ices such as frozen water, methane or carbon dioxide can remain stable — giving them a “dirty snowball” composition.
The best way to understand the origin and evolution of our Solar System is to study these objects. Many space missions have been sent to comets and asteroids over the past few decades. But these are expensive, and only two (Hayabusa and Hayabusa2) have successfully brought back samples.
Another way to study this material is to sit and wait for it to come to us. If a piece of debris happens to cross paths with Earth, and is large and robust enough to survive hitting our atmosphere, it will land as a meteorite.
Most of what we know about the Solar System’s history comes from these curious space rocks. However, unlike space mission samples, we don’t know exactly where they originated.
Meteorites have been curiosities for centuries, yet it was not until the early 19th century that they were identified as extraterrestrial. They were speculated to come from lunar volcanoes, or even from other star systems.
Today, we know all meteorites come from small bodies in our Solar System. But the big question that remains is: are they all from asteroids, or do some come from comets?
But might some of them have come not from asteroids, but from comets that originated in the outer reaches of the Solar System? What would such meteorites be like, and how would we find them?
Fortunately, we can actively look for meteorites, rather than hoping to stumble across one lying on the ground. When a space rock is falling through the atmosphere (at this stage, it’s known as a meteor), it begins to heat up and glow — hence why meteors are nicknamed “shooting stars”.
Larger meteors (at least tens of centimetres across) glow brightly enough to be termed “fireballs”. And by training cameras on the sky to spot them, we can track and recover any resulting meteorites.
The network’s data has resulted in the recovery of six meteorites in Australia, and two more internationally. What’s more, by tracking a fireball’s flight through the atmosphere, we can not only project its path forwards to find where it landed, but also backwards to find out what orbit it was on before it got here.
Our research, published in The Planetary Science Journal, scoured every fireball tracked by the DFN between 2014 and 2020, in search of possible cometary meteorites. In total, there were 50 fireballs that came from comet-like orbits not associated with a meteor shower.
Unexpectedly, despite the fact that just under 4% of the larger debris was from comet-like orbits, none of the material featured the hallmark “dirty snowball” chemical composition of true cometary material.
We concluded that debris from comets breaks up and disintegrates before it even gets close to becoming a meteorite. In turn, this means cometary meteorites are not represented among the tens of thousands of objects in the world’s meteorite collections.
The next question is: if all meteorites are asteroidal, how did some of them end up in such weird, comet-like orbits?
For this to be possible, debris from the main asteroid belt must have been knocked from its original orbit by a collision, close gravitational encounter, or some other mechanism.
Meteorites have given us our most profound insights into the formation and evolution of our solar system. However, it is now clear that these samples represent only part of the whole picture. It is definitely an argument for a sample-return mission to a comet. It’s also testament to the knowledge we can gain from tracking fireballs and the meteorites they sometimes leave behind.
In addition to the year’s other reliable performers we’ve included one wild card: the Aurigids, in late August. Most years, the Aurigids are a very, very minor shower, but they just might put on a show this year.
So here is our pick of the meteoric highlights for 2021.
For each meteor shower, we give you a finder chart showing the radiant (where the meteors appear to come from in the sky) and where best to look in the sky, the full period of activity and the forecast peak. Most meteor showers typically only yield their best rates for about a day around maximum, so the peak night is definitely the best to observe.
The Zenithal Hourly Rate ZHR is the maximum number of meteors you would expect to see under perfect observing conditions. The actual number you will see will likely be lower.
Most meteor showers can only really be observed from either the northern [N] or southern [S] hemisphere, but a few are visible from both [N/S].
Lyrids [N/S; N favoured]
Active: April 14–30
Maximum: April 22, 1pm UTC = 11pm AEST (Qld) = 7am CST = 3am Hawaii time
The Lyrids are one of the meteor showers with the longest and most storied histories, with recorded observations spanning millenia. In the past, they were one of the year’s most active showers, with a history of producing spectacular meteor storms.
Nowadays, the Lyrids are more sedate, putting on a reliable show without matching the year’s stronger showers. They still throw up occasional surprises such as an outburst in excess of 90 meteors per hour in 1982.
This year’s peak Lyrid rates coincide with the first quarter Moon, which will set around midnight, local time, for most locations. The best time to observe will come in the early hours of the morning, after moonset.
For observers in the northern hemisphere, the Lyrid radiant will already be at a useful altitude by the time the Moon is low in the sky, so some brighter meteors might be visible despite the moonlight in the late evening (after around 10:30pm, local time).
Once the Moon sets the sky will darken and make the shower much easier to observe, yielding markedly higher rates.
For observers in the southern hemisphere, the Lyrid radiant reaches a useful altitude in the early hours of the morning, when the Moon will have set. If you’re a keen meteor observer, it could be worth setting your alarm early to get out and watch the show for a few hours before dawn.
Lyrid meteors are fast and often quite bright so can be rewarding to observe, despite the relatively low rates (one every five or ten minutes, or so). Remember, this shower always has the potential to throw up an unexpected surprise.
Eta Aquariids [S]
Active: April 19–May 28
Maximum: May 6, 3am UTC = 1pm AEST (Qld/NSW/ACT/Vic/Tas) = 11am AWST (WA)
The Eta Aquariids are an autumn treat for southern hemisphere observers. While not one of the big three, they stand clear as the best of the rest of the annual showers, yielding a fine display in the two or three hours before dawn.
The Eta Aquariids are fast meteors and are often bright, with smoky trains. They are fragments of the most famous comet, 1P/Halley, which has been laying down debris around its current orbit of the Sun for tens of thousands of years.
Earth passes through that debris twice a year, with the Eta Aquariids the best of the two meteor showers that result. The other is the Orionids, in October.
Where most meteor showers have a relatively short, sharp peak, the Eta Aquariids remain close to their best for a whole week, centred on the maximum. Good rates (ZHR > 30 per hour) should be visible before sunrise on each morning between May 3–10.
The Moon will be a waning crescent when the Eta Aquariids are at their best. Its glare should not interfere badly with the shower, washing out only the faintest members.
Observers who brave the pre-dawn hours to observe the Eta Aquariids will have the chance to lie beneath a spectacular sky. The Milky Way will be high overhead, with Jupiter, Saturn and the Moon high to the east and bright, fast meteors streaking across the sky from an origin near the eastern horizon.
Active: July 17–August 24
Maximum: August 12, 7pm–10pm UTC = 8pm–11pm BST = August 13, 4am–7am JST
The Perseids are the meteoric highlight of the northern summer and the most observed shower of the year. December’s Geminids offer better rates but the timing of the Perseid peak makes them an ideal holiday treat.
The Perseids are debris shed behind by comet 109P/Swift-Tuttle, which is the largest known object (diameter around 26km) whose orbit currently intersects that of Earth.
Perseid meteors are fast, crashing into Earth at a speed of about 216,000km/h, and often bright. While the shower is active, at low levels, for more than a month, the best rates are typically visible for at the three nights centred on the peak.
For observers at European latitudes, the Perseid radiant rises by mid-evening, so the shower can be easily observed from 10pm local time, and remains high all through the night. The later in the night you look, the higher the radiant will be and the more meteors you’re likely to see.
Aurigids [N favoured]
Active: August 28–September 5
Maximum: Potential Outburst on August 31, peaking between 9:15pm–9:40pm UTC = 10:15pm–10:40pm BST = 11:15pm–11:40pm CEST = September 1, 1:15am–1:40am Gulf Standard Time = September 1, 5:15am–5:40am AWST (WA)
Where the other showers are reliable and relatively predictable, offering good rates every year, the Aurigids are an entirely different beast.
In most years, the shower is barely visible. Even at its peak, rates rarely exceed just a couple of meteors seen per hour. But occasionally the Aurigids bring a surprise with short and unexpected outbursts of 30-50 meteors an hour seen in 1935, 1986, 1994 and 2019.
The parent comet of the Aurigids, C/1911 N1 Kiess, moves on an orbit with a period far longer than the parent of any other shower on our list.
It is thought the orbit takes between 1,800 and 2,000 years to complete, although our knowledge of it is very limited as it was only observed for a short period of time.
In late August every year, Earth passes through debris shed by the comet at a previous passage thousands of years into the past. In most years, the dust we encounter is very sparse.
But occasionally we intersect a denser, narrow stream of debris, material laid down at the comet’s previous passage. That dust has not yet had time to disperse so is more densely packed and hence gives enhanced rates: a meteor outburst.
Several independent research teams studying the past behaviour of the shower have all come to the same conclusion. On August 31, 2021, the Earth will once again intersect that narrow band of debris and an outburst may occur, with predictions it will peak around 21:17 UTC or 21:35 UTC.
Such an outburst would be short-lived. The dense core of the debris stream is so narrow it will take the Earth just ten or 20 minutes to traverse. So you’ll have to be lucky to see it.
The forecast outburst this year is timed such that observers in Eastern Europe and Asia will be the fortunate ones, with the radiant above the horizon. The waning Moon will light the sky when the radiant is above the horizon, washing out the fainter meteors from the shower.
The Aurigids tend to be fast and are often quite bright. Previous outbursts of the shower have featured large numbers of bright meteors. It may just be worth getting up and heading outside at the time of the predicted outburst, just in case the Aurigids give us a show to remember.
Active: December 4–17
Maximum: December 14, 7am UTC = 6pm AEDT (NSW/ACT/Vic/Tas) = 3pm AWST (WA) = 2am EST
The Geminid meteor shower is truly a case of saving the best until last. By far the best of the annual meteor showers, it graces our skies every December, yielding good numbers of spectacular, bright meteors.
The shower is so good it is always worth observing, even in 2021, when the Moon will be almost full.
Over the decades, the Geminids have gradually become stronger and stronger. They took the crown of the year’s best shower from the Perseids in the 1990s, and have continued to improve ever since.
For observers in the northern hemisphere, the Geminids are visible from relatively early in the evening, with their radiant rising shortly after sunset, and remaining above the horizon for all of the hours of darkness.
As the night progresses, the radiant gets very high in the sky and the shower can put on a truly spectacular show.
For those in the southern hemisphere, the situation is not quite as ideal. The further south you live, the later the radiant will rise, and so the later the show will begin.
When the radiant reaches its highest point in the sky (around 2am–3am local time), it sits closer to the horizon the further south you are, so the best meteor rates you observe will be reduced compared to those seen from more northerly locations.
Despite these apparent drawbacks, the Geminids are still by far the best meteor shower of the year for observers in Australia, and are well worth a look, even on the moonlit nights of 2021.
Peak Geminid rates last for around 24 hours, centred on the official peak time, before falling away relatively rapidly thereafter. This means that observers around the globe can enjoy the display.
The best rates come when the radiant is highest in the sky (around 2–3am) but it is well worth looking up at any time after the radiant has risen above the horizon.
So wherever you are on the planet, if skies are clear for the peak of the Geminids, it is well worth going outside and looking up, to revel in the beauty of the greatest of the annual meteor showers.
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].
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 lies in the constellation Boötes, the Herdsman, relatively near the tail of Ursa Major, the Plough or Great Bear.
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.
Lyrids [N/S; N preferred]
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)
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.
Eta Aquariids [S]
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
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
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.
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)
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.
Such fireballs are not rare events, and serve as yet another reminder that Earth sits in a celestial shooting gallery. In addition to their spectacle, they hold the key to understanding the Solar system’s formation and history.
Crash, bang, boom!
On any clear night, if you gaze skyward long enough, you will see meteors. These flashes of light are the result of objects impacting on our planet’s atmosphere.
The larger the object, the more spectacular the flash. Where your typical meteor is caused by an object the size of a grain of dust (or, for a particularly bright one, a grain of rice), fireballs like those seen this week are caused by much larger bodies – the size of a grapefruit, a melon or even a car.
Such impacts are rarer than their tiny siblings because there are many more small objects in the Solar system than larger bodies.
That was probably the largest impact on Earth for 100 years, and caused plenty of damage and injuries. It was the result of the explosion of an object 10,000 tonnes in mass, around 20 metres in diameter.
On longer timescales, the largest impacts are truly enormous. Some 66 million years ago, a comet or asteroid around 10km in diameter ploughed into what is now the Yucatan Peninsula, Mexico. The result? A crater some 200km across, and a mass extinction that included the dinosaurs.
Even that is not the largest impact Earth has experienced. Back in our planet’s youth, it was victim to a truly cataclysmic event, when it collided with an object the size of Mars.
When the dust and debris cleared, our once solitary planet was accompanied by the Moon.
Smaller impacts, like those seen earlier this week, come far more frequently – indeed, footage of another fireball was reported earlier this month over Illinois in the United States.
In other words, it is not that unusual to have two bright fireballs in the space of a couple of days over a country as vast as Australia.
Pristine relics of planet formation
These bright fireballs can be an incredible boon to our understanding of the formation and evolution of the Solar system. When an object is large enough, it is possible for fragments (or the whole thing) to penetrate the atmosphere intact, delivering a new meteorite to our planet’s surface.
Meteorites are incredibly valuable to scientists. They are celestial time capsules – relatively pristine fragments of asteroids and comets that formed when the Solar system was young.
Most meteorites we find have lain on Earth for long periods of time before their discovery. These are termed “finds” and while still valuable, are often degraded and weathered, chemically altered by our planet’s wet, warm environment.
By contrast, “falls” (meteorites whose fall has been observed and that are recovered within hours or days of the event) are far more precious. When we study their composition, we can be confident we are studying something ancient and pristine, rather than worrying that we’re seeing the effect of Earth’s influence.
Tracking the fireballs
For this reason, the Australian Desert Fireball Network has set up an enormous network of cameras across our vast continent. These cameras are designed to scour the skies, all night, every night, watching for fireballs like those seen earlier this week.
If we can observe such a fireball from multiple directions, we can triangulate its path, calculate its motion through the atmosphere, and work out whether it is likely to have dropped a meteorite. Using that data, we can even work out where to look.
In addition to these cameras, the project can make use of any data provided by people who saw the event. For that reason, the Fireballs team developed a free app, Fireballs in the Sky.
It contains great information about fireballs and meteor showers, and has links to experiments tied into the national curriculum. More importantly, it also allows its users to submit their own fireball reports.
As for this week’s fireball over southern Australia, NASA says it was probably caused by an object the size of a small car. As for finding any remains, they are now likely lost in the waters of the Great Australian Bight.
A catastrophic event occurred on Earth 66 million years ago. A huge meteorite struck our planet in what is now Mexico, triggering mass extinctions of the dinosaurs and most other living creatures.
A new paper shows the first recorded victims of this impact were fish and other marine animals, stranded by a wave that left them high and dry in an ancient river in North Dakota, at a site called Tanis.
For scientists unpacking the evidence around the event, a full picture of the cataclysm has involved looking into the details of planetary surface physics during giant impacts.
If you’ve never heard of a form of wave called a seiche, this is your chance to catch up.
Waves of damage
The Chicxulub meteorite crater in coastal Mexico is strongly associated with the mass extinction of the dinosaurs (and 75% of all species), 66 million years ago.
The first victims were right at the site. Any marine creatures close to the point of impact would have been instantly vaporised (sadly leaving no fossil record), along with much of the surrounding rock.
Around the periphery, the energy of the impact melted and ejected tonnes of molten rock, which together with condensing rock vapour, formed little glass beads (“impact spherules”) that can be found in a layer around the world at this time.
The shock wave itself pulverised the adjacent rock enough to metamorphise it, forming features like “shocked quartz” – fractured quartz indicative of enormous pressures. It carried the energy equivalent of a magnitude 11 earthquake – 1,000 times more energy than the 2004 Boxing Day quake which killed almost 230,000 people.
Vast inland sea now gone
North Dakota is more than 3,000km away from the Chicxulub crater, and was a similar distance at the time of the meteorite impact event.
Separating them back then, however, was a vast inland sea that covered much of midwest USA, from Texas up to the Dakotas. Feeding into that inland sea was a river system upon which the Tanis site in North Dakota was formed. This site has preserved the earliest recorded deaths of the Chicxulub impact.
The site itself is unusual. The deposition of sediments can tell us about the flow of water in the river.
Most ripples (or flame structures) indicate a southerly flow of the river before and after the Tanis deposit. However, these flow indicators point the wrong way during the time the Tanis unit formed. Water was flowing upstream, fast.
At the site are also found the fossilised remains of species, like sharks and rays, that occupied brackish water, rather than the freshwater of the stream. These had to be brought inland from the sea by something, and left to die, smothered in sediment, on a riverbank.
Stranded in Dakota
The obvious candidate is an impact tsunami. Perhaps the impact of the meteorite hitting the ocean generated a huge wave that carried fish from the inland sea, and against the flow of fresh water, to leave the creatures stranded in Dakota?
But there are problems with this hypothesis. The tiny impact spherules that formed in Chicxulub can be found throughout the deposit (many clogging the gills of fish), and pockmarks in the sedimentary layers means rocks were still raining down. This means the surge of water occurred within around 15 minutes to two hours of the impact itself.
For a tsunami to travel the 3,000km from the point of impact, to the Tanis site across the inland sea, would have taken almost 18 hours. Something else killed these creatures.
The seismic waves from the impact would have travelled through the Earth much faster than a tsunami travelled across water – and arrived near Tanis between 6-13 minutes later. The authors of the Tanis study suggest these seismic waves may have triggered an unusual type of wave in the inland sea, called a seiche.
Seiches are standing waves in bodies of water, and are often found in large lake systems during strong winds. The winds themselves cause waves and water displacement, which can have a harmonic effect, causing the water to slosh side to side like an overfull bathtub.
However, earthquakes are also known to cause seiches. Particularly dramatic seiches are often seen in swimming pools during large quakes. The interaction of the seismic wave’s period (the time between two waves) with the timescale of waves sloshing in a pool can amplify their effect.
But seiches can affect larger bodies of water too.
During the 2011 Tohuku earthquake in Japan, seiches over 1m high were observed in Norwegian fjords more than 8,000km away. With an energy more than 1,000 times greater, the Chicxulub event could quite conceivably have generated bigger than 10 metre swells in the North American inland sea – the scale implied by the deposition of the Tanis site.
Given a seiche can be driven by seismic waves, it’s conceivable that one drove the surge that stranded marine creatures at Tanis, resulting in the short time between the impact debris and the surge deposit.
Still lots of questions
But a lot remains unclear regarding exactly what did happen 66 million years ago.
Could the fish stranding have been driven by the first seismic activity to appear at Tanis (the P and S waves in science parlance, which travel through the interior of the Earth, arriving at Tanis 6 and 10 minutes after impact, respectively), or the more destructive but slower surface waves at the top of the Earth’s crust, which arrived 13 minutes after impact?
Would the period of sloshing of a seiche be consistent with the scale of the inland sea? (The inland sea was much larger than most lakes seiches are traditionally observed in – and may or may not have been open to the ocean). Given so little is really known about the dimensions of the inland sea, this is hard to constrain.
The Tanis site has given us an incredible window into the first few hours of a mass-extinction. But it has also highlighted how little we have probed into the fatal surface physics of these extreme events.
A bright fireball lit up the night sky around Kati Thanda (Lake Eyre South) in South Australia on November 27, 2015.
But how to find the impact site of that meteorite? And how can we know where in the solar system the object came from?
Thankfully, a new meteorite tracking system we’ve installed in Australia has enabled us to answer these questions, helping us better understand the history and composition of our solar system.
Meteorites are the oldest rocks in existence. They contain a unique physical record of the formation and evolution of the solar system, and the processes that led to terrestrial planets.
They sample hundreds of different heavenly bodies, a compositional diversity that spans the entire inner solar system.
But the most basic piece of data – context – is absent. In almost all cases, meteorite researchers have no idea where their samples came from.
What they need are orbits and the ability to track meteorites back to their place of origin in the solar system. The goal of the Desert Fireball Network is to provide that data.
A network of ‘eyes’
This is a project that started in 2012 and since then we’ve installed a network of 32 automated observatories in remote areas of Australia. They are capable of operating for 12 months without maintenance, storing all imagery collected over that period.
Although they are high resolution intelligent imaging systems, they cost around A$5,000 each, which is only a fraction of the cost of previous systems. We’ve completely automated data reduction, so we can potentially scale up the system to arbitrary size without needing hordes of poor PhD students doing manual labour.
And members of the public can contribute by sending in their own reports via a smartphone app that we’ve developed called Fireballs in the Sky.
Trying to track an object moving at many kilometres a second, from the edge of the Earth’s atmosphere to the surface, isn’t easy. You have to account for everything from minor distortions in the camera lenses, to the effect of winds blowing the object off course when the light has gone out.
We would only know that it worked when we found a rock on the ground.
A green flash in the sky
When that fireball lit up the skies above South Australia in November, it was imaged by five Desert Fireball Network automatic observatories. The stations sent alerts to our server in Perth, attaching thumbnails of the fireball image.
With data from just a couple of cameras, we could tell pretty quickly that we had a meteorite on the ground. First, we had to get out to South Australia to pick up additional data from cameras that weren’t online, so that we could precisely triangulate the fireball.
We took a light aircraft flight from William Creek, which showed us that there was a feature on the surface that might be where the rock plunged into the mud. Now we had to get out on the lake.
Some of our team set to work pulling together all the data. The more accurately we could pinpoint the fall position, the easier any search would be. Their analysis showed that the object came in at a very steep angle, with a velocity of 50,000km/h, and punched down low in the atmosphere, still visible as a fireball at 18km altitude.
When it entered the atmosphere, it was about 80kg. At the end of the fireball it had more likely been whittled down to between 2kg and 6kg.
Alongside the effort to work all this out, we were putting together logistics for the trip. We knew we had to get there quickly. There had already been rain. Much more of it and any trace of the rock might be wiped away.
In addition, Kati Thanda has spiritual significance for the Arabana people. We would need their permission before we could go out on the lake. But the Arabana understood the urgency, and gave consent almost immediately. The Arabana guides, Dean Stuart and Dave Strangway, who came with us on the trip were a huge help.
The search is on
We got to the lake shore on December 29. But the lake doesn’t have a firm surface; it’s thick mud. We had to pick our way out to the fall site – almost at the centre of the lake – trying to find a route that would support a quad bike. Eventually, we found a way in.
Next day we got to the site, and searched the area, but didn’t find any trace of the feature that we’d seen a couple of weeks before from the air. Time was running out. Rain was coming in. We figured we might have just have one more day left.
So we decided to double down: one of our team would fly over the site, while two of us would search on the ground. If they saw anything from the air they would radio, circle the spot, and we could check it immediately.
It was overcast and drizzling as we headed out to the shore, but heavy rain held off long enough for us to get to the fall site. For an hour, the plane just circled.
Then we got a call that they’d seen it. We ran to the spot, and found the last remnant of the feature that our friend had seen a couple of weeks before. The meteorite had punched a deep hole in the mud.
Digging down through that pipe my fingers eventually touched a rock. We’d found our meteorite. The rock is 1.6kg in weight, a bit lighter than we’d expected, and it’s probably an ordinary chondrite, the most common type of meteorite. But we need to do some analyses to tell for sure.
An unexpected surprise
We didn’t know it when we built the network, but it turns out it can do a lot more than we ever expected. We can track satellites, space debris and rocket launches. We’ve even tested systems that will let us do fundamental astronomy. And, with a minor upgrade, we’ll have a facility that can spot supernovae and optical counterparts to gamma ray bursts.
But it’s the potential for planetary research that still gets us excited. Already, we’ve seen more fireballs than have ever been recorded up to now, giving us a unique window on what’s hitting the Earth.
As we recover more rocks, we will gradually build a geological map of the inner solar system. If we can link a meteorite to an asteroid, then we essentially have a sample-return mission to near-Earth asteroids, without the need for spacecraft.
This first rock we’ve recovered is just the start. In itself, it’s a research gold mine. But it also proves that our system works so there should be many more.