What killed the dinosaurs?
Astronauts have to keep the space station clean to keep microbes under control
The object that caused Earth’s last mass extinction may not have been an asteroid
have broken away from a much larger cometary body. “The model tries to answer this question by identifying a special population of comets that can produce the necessary size and rate of impactors to explain Chicxulub in a way that is consistent with all of the data that we have,” continues Siraj.
Loeb and Siraj calculated the expected impact rate of an object with the size of the Chicxulub impactor by considering the incidence rate of long-period comets from the Oort Cloud – a shell of icy debris called planetesimals lurking at the very outskirts of the Solar System, which Loeb colourfully describes as the bricks left over from the construction project of the planets.
“We showed that a fraction of these long-period comets from the outskirts of the Solar system can be broken into pieces when they pass near the Sun,” says Loeb. “Their break-up produces a collection of small fragments that cross the orbit of Earth.” The pair found that these fragments increase the rate of giant rocks raining down on the surface of Earth from space significantly. This has the effect of making the rate of incidence consistent with the age of the Chicxulub crater.
There is also other evidence that suggests the origin of the Chicxulub impactor was a piece of a larger body which broke away before striking Earth. Some of this comes from two similar craters – one smaller, and another considerably larger.
“The cometary origin of the Chicxulub impactor also explains the composition of the largest impact crater on Earth, Vredefort, located in South Africa, which at 305 kilometres (190 miles) across is twice the size of the Chicxulub crater,” Loeb
explains. The researcher says that the shrapnel model implies that fragments smaller than the Chicxulub impactor, but with a similar make-up, should strike Earth every million or so years.
Loeb adds that this could account for the existence of the Zhamanshin crater – the remains of a meteorite crash in Kazakhstan which is around 14.5 kilometres (nine miles) in diameter and formed within the last million years. It’s ten times smaller than the Chicxulub crater, yet shows signs of an object that has a similar composition.
The composition of the objects at the centre of the three impact events is likely to be a source of further study. The paper assumes that 20 to 50 per cent of all long-period comets have a carbonaceous chondritic composition, consisting mainly of water, silicates, oxides and sulphides with the minerals olivine and serpentine, for which there isn’t currently a great deal of empirical evidence.
Fortunately, the duo’s findings are consistent with other investigations, including some that concern the origin of the Solar System itself.
“Our hypothesis is consistent with Solar System formation theories in which there exist wellseparated reservoirs of carbonaceous chondritic material beyond the orbit of Jupiter in the solar protoplanetary disc,” Siraj says. “Future observations of the chemical compositions of comets will allow for our hypothesis to be tested.”
Siraj reflects on his surprise that the model he and Loeb developed connected to these other craters: “The fact our hypothesis explains the largest confirmed crater in Earth’s history, Vredefort – the only one larger than Chicxulub
– was a surprise to me,” the astrophysicist says.
“As was the fact that it also explains the largest recently formed confirmed crater, Zhamanshin.”
Siraj also describes the process that could have fractured the parent body of the Chicxulub impactor, connecting it with the phenomenon that allows us to study these objects. “When the orbits of long-period comets, originating from the Oort Cloud, bring them through the inner Solar System, the Sun heats up their volatile ices and allows them to develop these stunning cometary tails, which help astronomers locate and study them,” Siraj says. “Since they spend most of their time far away from the Sun, comets are particularly sensitive to gravitational interactions with the
Sun and the planets,” he continues. “This is much like the way a great musician performing with an orchestra becomes attuned to the musical gestures coming from both the conductor and the individual sections.”
The pair’s paper describes what happened when the parent body of the Chicxulub impactor flew too close to the Sun and experienced a much more powerful gravitational influence on one side than the other, giving rise to a tidal force within the body. “This force was so strong that it ripped the comet apart, producing a field of cometary shrapnel,” says Siraj, pointing out that our knowledge of this process is still somewhat vague. “The tidal disruption of comets is not yet fully understood, including the number of fragments that are produced in such events.”
The Sun wasn’t the only Solar System body that played a role in the disastrous meeting between the Chicxulub impactor and Earth. The researchers point out that its second and third-largest bodies were also significant in the orchestration of this calamitous event by pulling long-range comets from the Oort Cloud towards their demise. “Like a pinball machine, the Jupiter-Sun system shapes the orbits of comets that enter the inner region of the Solar System, pushing a fraction of all cometary orbits very close to the Sun,” Siraj explains. “During these passages close to the Sun, large comets are
tidally disrupted, producing fields of cometary shrapnel.”
Most of these Chicxulub-sized fragments miss Earth, but the chance that any one of them hits Earth in a given period of time is actually greater than the chance that a similarly sized asteroid or comet strikes. “In other words, the break-up of enormous comets by the Sun actually results in a higher rate of Chicxulub-sized impactors than the rate provided by impactors that were originally that size,“Siraj adds.
As staggering in size as the crater left behind by the collision between Earth and the Chicxulub impactor is, the impact has left an even more startling legacy on our planet. The elimination of the dinosaurs allowed the rise of mammals, and in turn the evolution of humans. Were it not for the Chicxulub impactor and the mass extinction it sparked, we likely wouldn’t even be here today. Just as such an event allowed the rise of humankind, scientists are painfully aware that a similar impact could potentially cause its fall.
“Knowing the source of the impactor can better inform us in our future predictions of the risks posed by large impactors,” Manasvi Lingam of the Florida Institute of Technology, who helped review the study, tells us. “Most of the focus has been on asteroids, but this study suggests that longperiod comets may also pose long-term hazards to humans and Earth’s biosphere.”
The fact that Loeb and Siraj’s work suggests a higher incidence of impacts from large pieces of cometary shrapnel like the Chicxulub impactor should be a concern for scientists and governments across the globe. But it may not be time to start doomsday preparations just yet. Loeb suggests our species’ investment in science could be its redemption.
“The human body is much smaller than that of a dinosaur, but the human brain is much more valuable for long-term survival,” Loeb says. “It allows us to build telescopes that would warn us about incoming rocks and deflect them away from Earth.” Loeb points to the upcoming Large Survey of Space and Time (LSST) to be conducted at the Vera C. Rubin observatory, due to begin monitoring the Chilean sky in 2023. Part of the LSST’s mission is to better map objects in the immediate vicinity of Earth, projected to result in the identification of two-thirds of all rocks larger than a football field
– a percentage the size of the Chicxulub impactor. “Our future is brighter than that of the helpless dinosaurs,” Loeb concludes.
International Space Station astronauts don’t have time to sleep in on Saturday mornings. They’re far too busy sticking to their strict cleaning schedule. Saturday is their big ‘cleaning day’, when surfaces have to be sanitised and dust removed. On 30 April, European Space Agency (ESA) astronaut Thomas Pesquet, who arrived at the orbital outpost aboard Dragon Crew Endeavour on 24 April, shared insights into what a space station cleaning routine looks like.
The inhabitants of the International Space Station are kept as safe as possible from the ongoing COVID-19 pandemic: strict preflight quarantine and other safety measures are in place for crew members, as well as newly delivered equipment, Pesquet said.
However, keeping potentially harmful microorganisms in check is still an important part of safe living in space, and improper hygiene habits have no place on board the station. “We have to disinfect all the surfaces we touch every week,” Pesquet said. “We also have a lot of measures on the space station that are similar to those you can find in hospitals or at airports that are designed to prevent the propagation of bacteria.”
While microbe populations – like the microbiomes in our guts – are necessary for human health, the growth of harmful microorganisms in a closed environment such as the International Space Station, where air is constantly recycled, could create the risk of serious health problems. Therefore the space station currently hosts several experiments that test various antibacterial and antiviral materials designed to prevent the growth of microorganisms that come into contact with them, Pesquet mentioned. Several space agencies are studying advanced materials that could make future space travel safer. Such materials could do the same job on Earth, protecting door knobs, elevator buttons and other objects that humans frequently touch.
Earlier this year, a Boeing-led experiment began at the space station testing a new type of antimicrobial surface coating that could help to prevent the spread of microbes, including the SARS-CoV-2 virus, in aeroplanes. The experiment requires astronauts to regularly touch two sets of objects, including an aeroplane seat buckle, a piece of seat belt fabric and a tray table. Only one set of objects is treated with the coating. As the astronauts touch the objects, they transmit microbes that naturally occur on human skin. After the samples return to Earth, Boeing will analyse how effectively the coating stopped the spread of the microorganisms.
The ESA previously ran a similar set of experiments to test the ability of five types of surface materials to repel microbes. The experiment, called MatISS (Microbial Aerosol Tethering on Innovative Surfaces in the International Space Station), returned to Earth earlier this year after more than 12 months in space. Previous studies have shown that many types of microorganisms thrive on the space station, with some doing even better in the microgravity environment than on Earth. A
NASA study in 2020 also found that the station’s microbiome changes as crews rotate, which makes sense – each astronaut brings their own unique set of microbes with them, which then colonise the station’s interior.
Speaking about the weekly cleaning routine, Italian astronaut Samantha Cristoforetti said that astronauts use disinfectant wipes to sanitise handrails, handheld microphones, computers and anything else they touch to minimise their bacterial trail. She further added that “the toughest modules to clean are certainly Node 3, where we have the toilet and the exercise equipment, and Node 1, where we eat”.
Astronauts also have to regularly vacuum ventilation grids, which can “get pretty dirty, because all the little debris that floats in the cabin eventually gets taken by the airflow to a return grid,” said Cristoforetti. A blocked ventilation grid could impair the station’s carbon dioxide scrubbing mechanisms, making the air inside unfit for breathing.
“We also have a lot of measures on the space station that are similar to those you can find in hospitals”