DID LIFE COME FROM COMETS?
They race in and out of our Solar System, and according to evidence, they could have brought life with them
In the void between planets, huge snowballs tumble across the Solar System. These comets – relics from an age before large worlds circled the Sun – have been patrolling our local neighbourhood for 4.6 billion years. Hidden away in their icy layers are clues to what our Solar System was like all that time ago, along with teasers as to how our family of planets formed. But their real bounty could be knowledge of how one of the Solar System’s stand-out features came to pass: life. Could comets have played a role in delivering biology to Earth? Scientists are starting to think so.
Perhaps the single biggest factor in our planet’s suitability for life is the presence of water. Thanks to our temperate position around the Sun, water can exist largely in liquid form. Yet still fresh from the violent collisions that gave birth to it, the early Earth would have been too hot for any water molecules to escape evaporation. “The impact that formed the
Moon would have gotten rid of any ocean or atmosphere too,” says Kathrin Altwegg from the University of Bern in Switzerland. The fact that we live on a wet planet today suggests that more water must have arrived some time later. Given that Earth is thought to have developed oceans within 500 million years, and life is thought to have popped up within 800 million years, it must have come fairly quickly and in abundance.
Naturally astronomers turned their attention to the objects in space known to have a high water content: comets. Often compared to ‘dirty snowballs’, these small objects were formed far from the Sun when gravity gathered up grains of dust and ice into objects several kilometres across. Many of these comets crashed into the planets and their moons in the Solar System’s youth.
By looking at the ones left over today, scientists can judge whether their lost companions were the source of most of Earth’s water. “Although you’d have needed 10 million comet impacts to provide enough water,” explains Altwegg. And as far back as 1986, this idea hit another snag.
Water comes in two main varieties: the ordinary water that we are used to on Earth and a rarer type called ‘heavy water’. The difference is that in heavy water the two hydrogen atoms – water’s composition is H O – each boast an extra particle called a neutron. Scientists can easily measure the ratio of ordinary water to heavy water in Earth’s oceans – there are 160 molecules of heavy water for every 1 million molecules of ordinary water. If comets did indeed bring that water here, the surviving ones that didn’t bash into a planet or moon should also exhibit a similar ratio today.
In 1986 the European Space Agency (ESA) sent the Giotto probe to sidle up to Halley’s Comet during its jaunt through the inner Solar System. They found that heavy water is twice as abundant on the comet as it is on Earth. In 1999 telescopic analysis of Comet Hale-Bopp showed that it also has a significantly higher proportion of heavy water present than back here on Earth.
However, in 2011 astronomers used the Herschel Space Observatory to take a closer look at the Comet Hartley 2. There they found a good match between the heavy water content of the comet and Earth. Then came the ESA’s famous Rosetta probe, which carried the Philae lander on a decade-long
trek to Comet 67P/Churyumov–
Gerasimenko. On 12 November 2014, Philae made history by making humanity’s first landing on a comet. Meanwhile, Rosetta studied the ancient ice pile from orbit. Data from Rosetta’s ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instrument backed up the Halley and Hale-Bopp findings. “The proportion of heavy water on 67P is three-times higher than on Earth,” says Altwegg, lead scientist for the ROSINA instrument.
So it seems unlikely that comets are solely responsible for delivering water to our planet. “The average abundance of heavy water on comets is certainly higher than it is on Earth,” comments Altwegg. That rules out comets as the dominant source of our water. However, there are two main alternative explanations. The first is that most of the water on Earth was brought here by asteroids. The second is that over time Earth itself replenished its surface water from deep inside the planet. “For me this makes a lot more sense than comet or asteroid impacts, because you would need a lot of them,” explains Altwegg.
While comets might not have brought us the majority of our life-giving water, other results from Rosetta suggest these ice mountains might have played a different, yet still vital role. Altwegg and her team scrutinised the comet’s coma – the dusty cloud of material that cocoons 67P as it is warmed by the Sun. Tucked away inside they found glycine, the simplest version of molecules called amino acids, which are often referred to as the building blocks of life because on Earth they link together to form proteins, the workhorses of our cells. Glycine had previously been found in dust returned to Earth from Comet Wild 2 by NASA’s Stardust mission, but some researchers argued that the sample might have become
contaminated during the analysis.
“The fact that we live on a wet planet today suggests that more water must have arrived some time later”
The Rosetta team also found other chemicals important for life on 67P, including phosphorus and formaldehyde – both play a key role in the formation of DNA. Other molecules known to be important in building amino acids had already been found on Halley’s Comet, Hartley 2 and Hale-Bopp, but Rosetta was a step up. “We more than doubled the number of molecule types found on comets from 28 to 60,” says Altwegg. But how did these molecules end up there?
Zita Martins, an astrobiologist at Instituto
Superior Técnico, Portugal, believes she might have the answer: they were forged when comets were struck by other space debris. Along with colleagues including Mark Burchell at the University of Kent, she created mixtures of ices similar to those found on a comet and used a gas-powered gun to fire steel projectiles at them at seven kilometres (4.3 miles) per second in order to simulate the comet being hit. “It took us three years to get the ice mixtures right,” says Burchell. “But after the impact the ices contained amino acids like glycine.” They dubbed the effect ‘shock synthesis’.
In a separate study, a team at the Institut de Chimie de Nice in France also mocked up artificial comets using different mixtures of ices. They cooled water, methanol and ammonia ices to -200 degrees Celsius (-328 degrees Fahrenheit) and then shone UV light onto the blend to simulate the glow of newborn stars. They then brought it up to room temperature to mock up the comet getting close to the Sun later in its life. They found several complex molecules had formed during the process – the most exciting of which was ribose, which helps form RNA. It must be stressed, however, that ribose is yet to be found on a real comet.
If amino acids and sugars are really created in this way in space, and many comets once rained down on our planet, comets could have delivered them to Earth. The exciting thing about that idea is that our planet wasn’t the only place to get battered by comets in its infancy. The other planets, along with their bevy of moons, were also bombarded. Anywhere those molecules might have mixed with liquid water is an enthralling place to explore. That’s particularly true for icy moons with liquid oceans.
“These moons have a big problem in that they orbit giant planets that might sweep up most of the impacts,” says Duncan Forgan, an astrobiologist from the University of St Andrews, Scotland. “But all it takes is enough hits, and enough might not need to be big numbers.” That puts Jupiter’s moon Europa and Enceladus around Saturn top of the destination wish list for future robotic exploration.
But not all agree that comets are a big deal when it comes to kick-starting life. Lewis Dartnell from the University of Westminster believes amino acids were probably here on Earth all along, even before the comets hit. “If that chemistry is going on in space then it was almost certainly already going on in the primordial seas of planets like Earth and Mars,” he says. Glycine, for example, could have been present in the cloud of gas and dust that collapsed to form the Sun and the planets. That way glycine would have become incorporated into Earth whether comets bombarded us or not. Radio astronomers have attempted to detect glycine in similar gas clouds elsewhere in our galaxy with mixed results. Some have claimed a discovery, but others have questioned their findings.
The picture is still blurry. It just goes to show that the origin of life on Earth is a complex story with many twists and turns. Slowly but surely, however, we are starting to piece together the tale of how our planet came to host living things. In doing so we might also get a better idea of the chances of finding other life forms out there among the stars. Because if comets did indeed make a significant contribution to Earth’s complex chemistry, there could be new planets out there being showered with comets right now, setting the wheels in motion for a brand-new form of alien life to take hold.