CAST your mind back to 1977. If you weren’t around then, here are some pointers: Elvis died, Star Wars was released and flared trousers were big in all senses.
But two lower-profile and seemingly unconnected things also happened that year – a deep-sea dive off the Galapagos Islands and a pair of rocket launches from Cape Canaveral in Florida. Their consequences are converging now in the depths of the outer solar system.
Those events marked the beginning of a revolution in our understanding: a story in which the Cassini probe, now reaching the end of its life, has played a distinguished part. Just 40 years ago, we never would have suspected that the secrets of how life formed on Earth, and whether it exists elsewhere, may lie in the icy moons of the outer solar system. That revelation, for which we have Cassini and its forebears to thank, is set to shape the next decades of space exploration – and perhaps lead to one of the biggest upheavals in human knowledge we can possibly imagine.
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Cassini’s Grand Finale:
Join us as we count down to the fiery end of the Cassini spacecraft’s mission to Saturn
The rockets launched in 1977 were NASA’s Voyagers 1 and 2. They weren’t our first probes to the outer solar system: Pioneers 10 and 11, launched in 1972 and 1973 respectively, hold that mantle (see “Nine probes reached the outer solar system: Where are they now?“). But the Voyagers were special because they gave us the blueprint for multi-instrument, deep-space probes able to build a comprehensive picture of distant worlds.
For the first time, we got close-up views not just of Jupiter and Saturn, but also of their moons. We discovered that Jupiter’s moon Io was the most geologically active world in the solar system, with its volcanic eruptions and lava-strewn surface. Voyager 1 confirmed a suspicion that Titan, Saturn’s largest moon, possessed a thick, hydrocarbon-rich atmosphere. And there were a host of other excitements including the icy visages of Jupiter’s moons and Ganymede, the solar system’s largest moon.
Intrepid exploration paid dividends. These mini-worlds have in many ways eclipsed their parent planets for scientific interest, thanks to the discovery there of organic molecules and water – the ingredients on which all known living things depend. Researchers came to believe these moons could be essential in solving the mystery of life’s origins.
Perhaps more than anyone else, in the 1980s the US astronomer Carl Sagan came to champion Titan as a time capsule that might tell us how life on Earth began. His interest in the chemical origins of life was sparked as a student at the University of Chicago in 1952, when Stanley Miller and Harold Urey performed landmark experiments. They took a sealed flask containing water, methane, ammonia and hydrogen – chemicals thought to be the composition of the atmosphere on the early Earth – and passed a spark through it to simulate lightning. Famously, they produced a tarry residue that contained amino acids, the building blocks of proteins that are themselves the building blocks of life.
Sagan endlessly tweaked and reran the experiment over a decade to find out the range of conditions under which amino acids would form. In 1979 this culminated in him and colleague Bishun Khare proposing the name tholins for the organic sludge.
A year later came the Voyager revelations about Titan’s atmospheric composition. Sagan pointed out that the mix was almost identical to that in the laboratory set-up – and the idea of Titan as a replica of early Earth was born.
“Cassini suggests that virtually all moons and planets were seeded with the ingredients of life”
The Cassini mission was conceived in part to investigate this connection. The Huygens lander, built by the European Space Agency, descended to Titan’s surface in 2004, and data from it and Cassini’s many fly-bys has only strengthened most astronomers’ belief that Titan has significant astrobiological interest (see “Cassini’s Grand Finale: The spacecraft that unveiled Saturn“). “Titan has the most complicated atmospheric chemistry in the solar system,” says Ravi Desai of University College London.
Cassini has made low passes through Titan’s upper atmosphere several times in recent years. Passing at altitudes from 950 to 1300 kilometres, it used its plasma spectrometer instrument, known as CAPS, to “sniff” the molecules there. In July this year, Desai and his colleagues reported the presence of chemical species known as carbon chain anions, and also longer organic molecules. These molecules became more abundant deeper down – just as the anions began to thin out. “It was a definitive correlation,” says Desai: the carbon-chain anions were coming together in the creation of longer chain organic molecules. These were Sagan’s tholins in the process of being formed.
This suggests tholins are easy to make, explaining why we are now beginning to find these molecules in abundance across the solar system. The New Horizons mission, which flew past Pluto in 2015, showed that they give a red colour to it and its moon Charon, and they are also present on the largest asteroid, Ceres. “Our work seems to be showing that there is a universality to creating large complex organic molecules,” says Desai.
But it’s still a long way from organic molecules to actual life, and here this particular juggernaut comes to a shuddering halt. Huygens found no evidence of life on Titan, and suggestions of possible life signs on the frosty moon – which would have to be life not as we know it, based on liquid methane instead of water – are tenuous. Cassini suggests that virtually all moons and planets in the solar system might have been seeded with the ingredients of life, but “how chemistry turns into biology is probably the biggest open question in science,” says Desai.
“127 fly-bys of Titan by the Cassini mission”
Better, perhaps, to take one step back. “You cannot start with the chemistry,” says of NASA’s Jet Propulsion Laboratory in California. “Right down underneath everything, you’ve got to know the physics. Life is not simply an agglomeration of organic molecules.” Besides the building blocks, you need a source of free energy. That takes us back once again to 1977, this time to that deep-sea dive off the Galapagos Islands.
In February of that year, oceanographers Jack Corliss of Oregon State University, and Tjeerd van Andel of Stanford University travelled to the sea floor in Alvin, the submersible best known for exploring the wreck of the Titanic. They were looking for hydrothermal vents, which jet warm water out from beneath the seabed into the cold ocean. They not only found these “black smokers” – so-called for the colour of the minerals that precipitate out of the hot water – but also an extraordinary abundance of life around them.
Extraterrestrial tests
Corliss and others concluded that such places could have been the backdrop for the origin of life, pitting them against those advocating the idea of organic chemistry in the atmosphere. Russell, then a deep sea vent geologist working on underwater mineral deposits, soon joined the discussion. He thought the black smokers were too energetic: fragile organic molecules would be as easily destroyed as created. Instead, he proposed that life began at the calmer, warm water vents, in the mineral deposits he was studying.
The trouble was how to test the hypothesis. To simulate the sea floor would require water at pressures 10 times that at sea level. It would have to be acidic to represent the higher concentration of carbon dioxide in Earth’s early atmosphere, and the whole lab would have to be more sterile than an operating theatre to ensure that no ordinary Earth bugs were inside – and nothing that sprang to life inside could get out.
The Galileo probe, which reached Jupiter late in 1995, showed the experiment might already be going on elsewhere. In particular, its images of Europa showed huge cracks in the moon’s icy surface and areas where the ice blocks had moved, as if transported by currents in a subsurface ocean. Readings from Galileo’s magnetometer revealed that a churning saltwater system encircled the whole moon. This ocean probably contains more water than Earth, and must be powered by a heat source at Europa’s centre. That gives rise to the hope of hydrothermal vents – and a perfect place to put Russell’s theory through its paces. “I’m lucky enough to be at the beginning of a big extraterrestrial test of these ideas,” says Russell.
NASA and ESA are both planning return missions to Jupiter’s moons. NASA’s is called Europa Clipper and will launch in the 2020s. Because of the intense radiation around Europa, in the form of high-energy electrons trapped in Jupiter’s powerful magnetic field, the spacecraft will not orbit the moon, but circle Jupiter and perform 45 fly-bys at altitudes varying from 2700 kilometres to just 25 kilometres. Cameras and spectrometers will determine the surface ice composition, radar will determine its thickness, and a magnetometer the depth and salinity of the underlying ocean. Together, those readings should confirm whether the conditions are suitable for life to have arisen.
But Europa is not the only object of interest. The ESA mission, called for Jupiter Icy Moons Explorer, is planned for a 2022 launch. It will dive close to Europa to take similar readings to the Europa Clipper, but also enter orbit around Ganymede. Galileo’s observations suggested this satellite, too, might have a hidden ocean, not so active as Europa’s and perhaps 100 kilometres down. “It might have more liquid water than Earth and Europa,” says of JUICE. “It is the largest moon of the solar system, so there must be something special about it. I will not be surprised if by the end of the JUICE mission you find that Ganymede is maybe a more interesting object than Europa.”
Russell is not so sure. For him, the focus remains on Europa because the radiation that makes spacecraft operation difficult is exactly what he thinks might be needed to trigger life. “Life is an electrical motor,” he says (see “What does life do?“). An electrical motor needs a battery to supply a flow of electrons. At hydrothermal vents on Earth, the battery is created by the alkaline waters of the vent gushing into the seawater, which is more acidic thanks to dissolved atmospheric carbon dioxide, as well as chemical reactions around the vent. All in all, the vents generate 500 millivolts to 1 volt.
Russell thinks that the electrons in Jupiter’s radiation field can do a similar job.”Europa has much of what one might expect from a battery,” he says. High-energy electrons hit the moon all time, oxidising the surface. “If you can get those electrons into the ocean, you have the disequilibrium that life requires,” he says.
The importance of finding life beyond Earth cannot be overstated. To find it elsewhere in our solar system would surely mean it is widespread throughout the entire galaxy. It would allow us to study the chemical composition of life: must it be based on DNA or is another molecule capable of carrying heritable information? It might also bolster the idea that not just planets, but moons in other solar systems might be profitable places to look for life.
“Beneath the ice, Europa’s heated subsurface ocean probably contains more water than Earth”
It may be a while before we find out. Russell was part of a 21-person team who published a NASA report earlier this year on a potential mission to search for evidence of life on Europa’s surface, concluding it would need to look for cells. “They can be dead, but whole cells would be what I would look for. That would be the killer evidence,” says Russell. These cells might be expelled in a plume of water and caught during a fly-by, or found scattered across the icy surface using an on-board microscope to image samples. But money to develop a Europa lander was cut in NASA’s 2018 budget, and the Europa Clipper and JUICE will only be able to analyse molecular compositions. In April, planetary scientists from both sides of the Atlantic called for ESA and NASA to work together to land on Europa, just as ESA developed the Huygens lander for the NASA Cassini mission.
Others would prefer to use any money to land on Titan and explore the organic chemistry there. Thanks to Cassini, we now know the moon has methane lakes and a hydrological cycle based on liquid methane. But with large missions taking around 20 years from conception to results, even if the money can be found, deciding which destination to gamble on first is a big deal.
But one thing is for sure. Whatever your viewpoint about the origin of life – whether as a product of atmospheric chemistry and the downward drift of organics, or underwater hydrothermal vents – the icy worlds of the outer solar system are now the place to be. “Europa has the ocean and possibly hydrothermal vents so is similar to Earth. Titan is a different environment and like the early Earth,” says Desai.
As we wave goodbye to Cassini, we are far from the end of an eye-opening voyage of discovery. “It is astounding in my lifetime what has changed,” says Russell. “Forty years ago we didn’t know what these moons looked like up close.”
What does life do?
When it comes to working out what life is, that’s the one question not to ask, says Michael Russell of NASA’s Jet Propulsion Laboratory. “Never ask what anything is. Ask what it does,” he says. “What does life do?”
We may be tempted to answer that life passes on heritable information by the process of reproduction. But that’s a limiting statement, according to Russell, because it looks at things from the point of view of our biology, rather than the underlying physical processes. “Life hydrogenates carbon dioxide,” he says. It takes hydrogen from water and joins it together with carbon dioxide, which originally emanated from volcanoes in abundance. This rights what would otherwise be an unresolvable chemical disequilibrium, and produces a supply of organic molecules in the process.
Doing this requires a source of free energy to drive the chemical reaction and a small engine to make it happen. In this picture, life is made of small electrical engines, driven by the movement of free electrons in our environment. Replication – or reproduction – is just something that evolved to keep that going. As the Hungarian Nobel prizewinner Albert Szent-Györgyi put it: “Life is nothing but an electron looking for a place to rest”. “Put electrons in the system and they will try to escape. That very flow of electrons is what drives these little engines to generate the organic molecules,” says Russell.
This article appeared in print under the headline “The next voyage”
