Even though Mars receives most of the spotlight today when it comes to searching for places in the Solar System—besides Earth—where life might have arisen, there are many in the scientific community who think there are better prime candidates. And new studies and analyses of old data from the Galileo mission at Jupiter suggest we should turn the spotlight on Europa as well.
The idea that the seeds of life are common throughout the Universe, being propagated between planets and stars, has been around since antiquity. This notion of “panspermia” was first postulated by the Greek Ionian philosopher and natural scientist Anaxagoras in the 5th century BC. Although largely neglected for more than two thousand years afterward, interest in the panspermia hypothesis was rekindled during the 19th and 20th centuries by notable scientists like Jacob Berzelius, Hermann von Helmholtz, and Svante Arrhenius, and later popularised by Fred Hoyle in the 1970s. Although once viewed as a fringe idea, panspermia slowly gained acceptance and credibility within the scientific community. The discovery of hundreds of meteorites found to have come from Mars, for instance, showed that the exchange of material between planets is indeed happening. And the discovery that organic molecules that constitute the basis of life are common throughout the interstellar medium, and are also found in meteorites that have fallen to Earth, made scientists start hypothesising as to whether life on Earth was delivered from space.
A key mechanism by which life is transferred between worlds, according to the panspermia hypothesis, is by asteroid and comet impacts. Although the frequent exchange of material between the neighboring Earth and Mars had been proved, many scientists have been sceptical about the possibility of such collisions taking place between distant planets, or that any organic compounds or living microbes inside asteroids or comets would endure the shock of impact. A new study made by Rachel J. Worth, an astronomy graduate student at Penn State University, and her colleagues aimed to address the first leg by showing that meteoroid exchange between the inner planets and the moons of the outer Solar System could indeed be possible.
What Worth and her team did was to calculate the number of meteoroid impacts throughout the Solar System with material coming from Earth and Mars. The team used an N-body simulation to determine how possible the collisions of 100,000 of such meteoroids would be, on a timescale of 10-30 million years. What the results showed was that the majority of the ejecta coming from Earth or Mars would fall toward the Sun and back to their parent planets in a short amount of time. According to the study, that would ensure Earth would be re-seeded with life, following big, cataclysmic events like the Late Heavy Bombardment that happened approximately 4 billion years ago, when the space environment around the Earth was dominated by untold numbers of big asteroids, following the formation of the Solar System. A large portion of the ejected material would also fall toward the planets that were inward from those of their origin, so that ejecta from Mars would fall toward Earth, Venus, and Mercury, and ejecta from Earth would fall toward Venus and Mercury. When Worth’s team calculated the probability of collisions on Jupiter and Saturn, it found that thousands of collisions on the gas giants would indeed take place and as much as 10 collisions for each of the moons of Jupiter and Saturn.
Worth and her colleagues are quick to point out that the results of the simulations do not mean that life actually did arise on the outer moons, but rather that it is possible it could have. “We find that transfer of rock capable of carrying life has likely occurred from both Earth and Mars to all the terrestrial planets in the Solar System and Jupiter, and transfer from Earth to Saturn is also probable. Additionally, we find smaller but significant probabilities of transfer to the moons of Jupiter and Saturn from Earth, and to the moons of Jupiter from Mars,” the authors state in the study. ”The probability of life surviving such a journey or finding a tenable environment on arrival, is beyond the scope of our research. Ultimately, we conclude that the possibility of transfer of life from the inner Solar System to the outer moons, cannot be ruled out based on current knowledge. Any planned missions to search for life on Titan or the moons of Jupiter, will have to consider whether any biological material found, represents an independent origin, rather than another branch in the family tree populated by Earth life.”
Although theoretical, Worth’s study is supported by new observational evidence found on the surface of Jupiter’s moon, Europa. A new study to be presented in the 2013 Fall meeting of the American Geophysical Union analysed old data from the Galileo spacecraft that explored the Jovian system from 1995-2003. The study was led by Jim Shirley, a research scientist at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. Shirley and his team examined photographs of Europa that had been taken with Galileo’s Near-Infrared Mapping Spectrometer, during the spacecraft’s numerous flybys of the Jovian moon. The team discovered clear signs in the photographs of phyllosilicates on the moon’s surface, covering an area approximately 40 km wide.
Phyllosilicates are clay-like minerals, formed in the presence of a neutral-pH water, and are indicative of a habitable environment. They have also been found to be present on Mars, during a series of recent ground-breaking observations by Mars orbiters and by rovers like Opportunity and Curiosity. Phyllosilicates can also help in fossilising records of past life, if life was present. But on Europa, a habitable environment is more likely to be found in its underground ocean, rather than its surface, which is covered by a 100-km-thick layer of ice. The questions that arise are how could the phyllosilicates be found on the surface and how could they be transported to the ocean through all this ice?
What Shirley and his colleagues noticed by examining Galileo’s images is that the observed narrow ring of phyllosilicates on Europa lies 120 km away from a crater site on the moon’s surface. This crater was measured to be approximately 20 km wide. The explanation was apparent. “Organic materials, which are important building blocks for life, are often found in comets and primitive asteroids,” says Shirley. “Finding the rocky residues of this comet crash on Europa’s surface, may open up a new chapter in the story of the search for life on Europa.” Shirley’s team concludes that the observations point toward a shallow impact, at an angle of 45 degrees. This shallow angle would permit some of the ejected material from the collision to fall back on Europa. And because Europa’s ice layer is believed to extend for at least 100 km below the surface, it is considered improbable that the observed phyllosilicates would have risen from the moon’s interior. Thus, the most likely cause is an impact from a comet or asteroid, with a diameter of approximately 1,100-1,700 meters.
“Understanding Europa’s composition is key to deciphering its history and its potential habitability,” says Bob Pappalardo, a senior research scientist at JPL’s Planetary Science Section and pre-project scientist for the proposed Europa Clipper mission. “It will take a future spacecraft mission to Europa to pin down the specifics of its chemistry and the implications for this moon hosting life.”
With recent studies and observations strengthening the case for a habitable environment and potential for life on Europa, the need for a dedicated mission to this distant and enchanting moon becomes imperative.