By Dr. Manish Sinha
Since time immemorial, scientists have been perplexed by an array of questions about the Earth’s past and the European Space Agency’s Rosetta mission, launched on 2 March, 2004 attempted to elucidate one of our planet’s most ancient mysteries: just how was Earth seeded with water? Whilst Earth-based instruments have successfully decoded cosmic fingerprints emanating from the early solar system, certain information requires physical samples that can be obtained from interplanetary missions alone. Rosetta represents the nineteenth mission to explore comets and a paradigm shift from prior endeavours in terms of technological advancement and sheer audacity of its undertaking. Preceding missions amounted to mere fly-bys. Rosetta, by contrast, is the inaugural mission designed to orbit around and landing upon a comet blazing a trail at speeds of 85,000 mph as well as conduct the first in situ tests of material extracted directly from the comet surface. Additionally, it will travel side-by-side with the comet for over one year as it undergoes a dramatic metamorphosis during its approach to the Sun, the latter’s heat causing hundreds of kilograms of material to be expelled every second – thereby forming the comet’s tail. After a ten-year expedition through 6.4 billion miles of interplanetary space aided by four gravity assist fly-bys of Earth and Mars, on 6 August 2014 Rosetta finally arrived at its destination. Would it reveal the source of Earth’s waters and what new conundrums would it pose?
Whilst perhaps not as spectacularly beautiful or striking in appearance as Saturn’s rings or Jupiter’s Great Red Spot, comets and asteroids provide something crucial that planets do not. Unlike planets, comets possess no atmosphere and have thus remained fundamentally static in shape and chemical composition since their formation. The passage of 13.7 billion years has been circumvented and comets consequentially provide a privileged and untouched window into the earliest days of the solar system. In short, they represent a celestial time machine into the past that has escaped the influence of chemical processes and geological transformations that drive planet’s structure and topology. In stark contrast to Earth, what you see today is precisely what you would have seen billions of years ago – welcome to the birth of the solar system.
The comet selected for this mission is known as Comet 67P after its discovery in 1969 by Klim Churyumov and Svetlana Gerasimenko, two astronomers at the Alma-Ata Astrophysical Institute in Kiev, Ukraine. Whilst billions of comets reside within our solar system, they persist predominantly in two locations. Firstly, the Oort Cloud, which is the most distant repository found beyond the outer fringes of Pluto and home to approximately 12 billion comets and, secondly, the Kuiper belt, located beyond the orbit of Neptune. Comets typically remain in either the Oort Cloud or Kuiper Belt until they collide with one another, at which point the impact dislocates them from their orbit, thereby commencing a journey toward the Sun. Due to their proximity with Jupiter, comets emanating from the Kuiper Belt are influenced by Jupiter’s powerful gravity and begin a modified orbit; Comet 67P hails from such a category. Cometary collisions are known to have been much more ubiquitous during the early solar system than today. As comets and asteroids contain water (locked-up in the form of ice) these primordial relics are thought to provide a potential mechanism through which water was transferred to Earth as ice and which subsequently melted. Any early water present within the pre-solar nebula that condensed to form the Earth would have evaporated due to the heat; the ‘external carrier’ theory therefore holds appeal. By comparing the structure of water on Earth to that found within comets, scientists aim to put this theory to the test.
The mission’s name bears historical significance. The spacecraft possesses two components; an orbiter named Rosetta and a lander named Philae. The orbiter is named after the Rosetta stone discovered in 1799 in Egypt and which contained, by royal decree, carved inscriptions in three languages: Egyptian hieroglyphics, Egyptian Demotic and Ancient Greek. These inscriptions would ultimately allow scientists to understand a lost Egyptian culture. However this was not achieved independently. The lander is named Philae after an island on the Nile River where an obelisk was discovered whose inscriptions, coupled with those on the Rosetta stone, helped decipher knowledge about the ancient civilisation. Correspondingly, mission scientists hope that the scientific synergies, yielded by instruments on board both Rosetta and Philae, will permit a much greater understanding about the origins of Earth’s waters and more.
Despite the newfound status of Comet 67P it was not, in fact, Rosetta’s initial target. The primary destination was Comet Wirtanen but an unexpected one-year delay led to a forced trajectory revision. New targets were selected that maximized scientific return, minimized risks and conformed to budgetary constraints and Comet 67P was deemed to meet the best of all worlds. Rosetta represents the most daring cometary mission ever attempted but its conceptualization occurred over twenty-five years ago. During the 1986 approach to Earth by Halley’s comet a total of 5 missions were launched, the most prominent being the European Space Agency’s Giotto mission. Whilst being the first spacecraft to conduct investigations of a comet at close proximity, the probe’s sensors calculated that on approach to the Sun the comet ejected three tonnes of material every second; material that comprised eighty percent water. Could similar water have previously come to Earth through cometary collisions with our planet? Complex organic molecules compromising nucleic acids and amino acids – ingredients essential for life, as we know it – were also discovered within the comet. It became evident that subsequent missions were crucial for further insight. Both NASA and the ESA concurrently began developing new probes, however budgetary cutbacks in 1992 led to the cancellation of NASA’s contemporaneous mission, leaving the ESA to forge a lone path. The resulting enterprise is an extensive collaboration between fifty contractors spanning fourteen European countries as well as the United States. It required 1.4 billion euros of funding from inception to culmination and is the result of the dedicated efforts of 2,000 scientists and engineers spanning 20 years.
Brand new technology was developed for the mission. New generation solar panels enabled Rosetta to become the first mission beyond the main asteroid belt – where sunlight is 96 percent weaker than on Earth – to rely completely upon solar cells alone for power. Furthermore, the instruments developed were robust enough to operate in temperatures as low as -180°C and as high as +150°C. The Philae lander comprises eleven sophisticated instruments. The comet’s density and thermal properties will be captured and gas analysers will aid the detection and identification of any complex organic molecules. Additionally, a drilling machine will enable material 23 centimetres below the surface to be extracted and examined. The denouement of a seven-hour descent was a soft landing on 12 November 2014 but all did not go precisely to plan. Owing to the comet’s small size, approximately 2 miles in length, its gravitational pull is several thousand times weaker than that of Earth. In order to avoid bouncing off the surface upon touchdown, harpoons were designed to anchor Philae to the rocky surface. Unfortunately, a technical mishap prevented this from performing as expected and the lander, rather fortuitously, appears to have bounced off the icy surface and landed safely a second time. It is on such fine lines that missions can fail in the blink of an eye.
Whether water could have been transported to Earth from comets was deciphered as follows. Water molecules comprise three atoms: two atoms of hydrogen and one atom of oxygen. Occasionally, one hydrogen atom is replaced by deuterium – an isotope element similar to hydrogen but slightly heavier in weight. In the case of water molecules found in Earth’s oceans, one water molecule in every 6,420 molecules contains one hydrogen atom replaced by deuterium. It is thought that all the deuterium found in nature was produce during the Big Bang 13.7 billion years ago and, chemically, once the ratio of deuterium to hydrogen is established it is difficult to alter. Thus, if comets were indeed providers of Earth’s waters the ice found within comets should, at least, possesses a similar prevalence of deuterium to water found on Earth. Unfortunately for Rosetta’s scientists, initial results indicated that the ice within Comet 67P had an abundance of deuterium that was three times higher than that found on Earth – higher, in fact, than in any previously studied comet. This was an unexpected discovery as another comet, Hartley 2, that also originates from the Kuiper Belt was investigated by ESA’s Herschel mission in 2011 and displayed an abundance of deuterium similar to that of Earth’s waters. Two facts thus became clear: first, the chemical composition of comets within the Kuiper Belt is significantly more diverse than previously thought and, secondly, it is unfortunately not yet possible to conclude with any certainty whether or not Earth's waters possess a cometary origin.
The search for the answer to the origin of Earth’s waters will therefore continue. The reward will be an improvement to our understanding of forming solar systems and discovery of how materials combine to create environments in which life might thrive. Unlike the Rosetta stone and the obelisk in Philae, scientists have been unable to combine Comet 67P’s time machine-like properties to decipher the mission’s principal goal. Rosetta’s negative findings do not preclude the possibility that comets delivered water to Earth billions of years ago but several more comets within the Kuiper Belt and possibly the Oort Cloud may need to be investigated before a definitive answer can be established. If so, it may take several decades, perhaps more than a century, to unravel one of Earth’s oldest and most primitive mysteries.