Web Site: http://www.andrewrushby.com
Bio: Andrew Rushby is a PhD student in the School Of Environmental Science at the University of East Anglia (UEA) in the United Kingdom. His research, which is supervised by Professor Andrew Watson FRS and Dr Mark Claire at the Laboratory for Global Marine and Atmospheric Chemistry, is focussed on constructing Earth system models to investigate the biogeochemical processes taking place both on the early Earth and extrasolar planets, in particular the oxidation of the atmosphere of the early Earth and extrasolar analogues, and the implications this may have for astrobiology.
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When viewed from space, the Earth glows like a blue marble under the light of the distant Sun. Azure oceans lap against the jagged coastlines and pale clouds swirl gracefully across its face, temporarily obscuring from view the brown-green landmasses beneath. From this vantage point, there is little to suggest that intelligent bipedal apes are scuttling around the coasts; confident of their centrality to all the workings of the cosmos, yet mostly unaware of the intricate complexities of its operation.
With the exception of five hundred operational satellites amidst a sea of orbital debris, one permanently occupied space station in low Earth orbit and two intrepid robotic explorers on the planet next door (Opportunity and Curiosity), humans have little visible presence outside of the Earth. In spite of our delusions of grandeur, we assume that no evidence of our global civilisation could be detected from light-year distances.
However, if we imagine that somewhere in the menagerie of stars that make up our local neighbourhood in the Milky Way, on a planet not too dissimilar from ours, an alien astronomer was perched at his (or her) telescope one night staring out into the dark when our Solar System happened into view. What would they see? Just another star on their survey, if relatively young and brighter than most, but perhaps one of many observed that evening. Initially, the blinding glare of the Sun would obscure our family of planets from direct view. Luckily, there are a number of ways to circumvent this problem. Using indirect planet detection techniques familiar to us such as radial velocity measurements or transit timings, the planetary companions of this curious yellow dwarf star are revealed: four gas giants and four smaller worlds. If the exo-astronomer ran their observations through their superior spectrometer however, chances are they may be intrigued by the results from one tiny blue planet in the orbit of this humdrum star.
Spectrometers measure the properties of light, first emitted by stars but then altered by the constituent gases of the planetary atmospheres through which the beam passes on the way to the receiving instrument. Different gases absorb light at different wavelengths to produce characteristic spectra and the composition of the atmosphere mirrored in the light can be teased out of the noise with sufficient skill. The high levels of water vapour, oxygen, methane and other gases associated with biological activity discovered in the atmosphere of this planet should result in the alien equivalent of a raised eyebrow. Methane is a ‘reduced’ gas and is usually rapidly destroyed in the presence of oxygen, meaning that detecting an appreciable amount of both may suggest that a biological mechanism is responsible for their continual replenishment. This mismatch is identified as a ‘biosignature‘ – a sign that this planet may harbour life.
Planetary atmospheres are something we are all intimately familiar with. The Earth’s is flush with life-giving oxygen, greenhouse gases essential (in the right balance) to maintaining a clement climate and an ozone layer that shields us from the Sun’s harmful rays. Most of us will never leave its gaseous embrace, and without it life would be extremely difficult. However, we take for granted the atmosphere’s ability to act as a mirror of our activities detectable from astronomical distances, able to reflect the unique signatures of the gases injected into it and hold them there for those with the correct instruments to see.
Further studies by the inquisitive alien astronomer would reveal a soup of exotic chemicals in the atmosphere of this distant little planet: increasing levels of carbon dioxide along with a suite of destructive, industrially produced compounds like chlorofluorocarbons (CFCs). There is no known biological pathway for producing CFCs, so their detection in the atmosphere of this planet is a strong indication of the activities of industry. They have struck gold (or the equivalently rare element on their planet) by discovering compelling evidence for the existence of another technologically advanced species. In doing so, they may have forever altered the way their civilisation views itself – one of perhaps many in a vast, galactic family.
Cloaked in an imaginative example, this is the theory that lies behind using spectroscopy as a method of detecting life, and perhaps even advanced civilisations, across the depths of space. Two promising space telescopes, TPF (NASA) and Darwin (ESA), were cancelled due to budgetary constraints, so for now at least interstellar planetary spectroscopy remains out of our grasp. However, the hope is that instruments of the near-future will be able to examine the atmospheres of exoplanets to search for these signs of life. Until they can, it might be worth remembering that we might not be the only ones able to gaze into the Earth’s atmospheric mirror.
Perhaps we should try to keep it clean?
Imagination will often carry us to worlds that never were. But without it we go nowhere.
– Carl Sagan (Cosmos, 1980)
Since the dawn of civilisation, humans have gazed up at the stars and planets overhead. Even now, separated from our forebears by an expansive gulf of time, technology and knowledge, the stars remain distant, esoteric but evocative targets. Our curiosity and thirst for understanding drives us on, pushing the limits of human endurance, engineering and science to the point where 528 humans from 38 nations have flown beyond the tenuous envelope of gases clinging to the surface of the Earth into wilderness of space. A first, unsteady and cautious step into the vast unknown that surrounds our tiny globe. Of these, only 12 have stepped foot on the surface of the Moon. At over 385,000 km away, reaching the desolate face of our lunar companion remains the pinnacle of manned spaceflight capability, yet it is a mere stone’s throw from Earth in astronomical terms. We peer out from the relative safety of our home, edge into the abyss that surrounds us and tentatively contemplate its content.
The delicate squishiness of the human form is not conducive to the hostile environment of space. Fleshy bags of meat and fluids don’t travel well in a vacuum, the near absolute-zero temperatures dessicate skin and lung and our fragile bones snap and break easily under undue strain. Bombarded by radiation, and far from the protective effect of the ozone layer, our cells mutate and die. Ingenuity and engineering have surmounted these problems in the short term by wrapping our bodies in spacecraft and suits, but the frailties of our terrestrial form remain.
As with many aspects of our lives, we have increasingly outsourced the monumental task of space exploration to robotic envoys. Obedient, unfaltering and better able to withstand the hardships of space travel, these metallic pioneers are our eyes and ears in the depths of space, straddling the boundary of the known and unknown to help us elucidate the mysteries of our near and distant planetary neighbours. Beacons in the fog, they light the way out into space.
Moreover, these scientific emissaries are more than merely (very expensive) collections of navigational equipment, cameras, sensors and propulsion. They are more than laboratories, more than the experiments they conduct, or the raw data they return. More too than the images they record, most never seen by the eyes of a human. These magnificent machines, representative of the peak of human exploratory technology are much greater than the sum of their parts. Often the result of years of international collaboration, teamwork, anguish and joy, these are the ambassadors of our knowledge, the manifestations of the spirit of human curiosity and the first steps of a lonely species wandering out into the darkness. Whilst they wander space in isolation, they have the dreams and imagination of many people behind them.
This is why, when a launch fails or an unmanned probe goes missing, the loss is felt by us all. The cost can be counted in dollars or euros, but the real price is the setback to the campaign for understanding that our failed or lost probe was spearheading. A scout lost to the enemy. I’ve heard stories of folks who cried at the loss of Beagle II (the British-built Mars lander lost to the Martian atmosphere in 2003/4), and who amongst us are not moved by xkcd‘s wonderful homage to the late (but very successful) MER Spirit rover?
On the eve of the landing of MSL Curiosity, the most complex rover ever designed, it is worth bearing in mind the hard work and dedication that it took for the latest generation of scientists and engineers to push the limits of our understanding and put a car-sized robot on Mars. I wish all those involved in the construction and operation of this wonderful machine the best of luck. Earth is rooting for you!
Follow the landing live at JPL’s site here
As Voyager 1 cradles the edge of our Solar System, poised to enter the vacuous expanse of deep space, we are approaching a milestone that many on this planet are not aware of. As this magnificent example of human engineering leaves the confines of the warm embrace of our Sun, at ~120 AU a now faint and distant beacon in the enveloping darkness, we will become an interstellar species. The gravitas of this monumental achievement should not be overlooked.
Whilst it remains theoretically feasible that our universe may be teaming with life, intelligence of space-faring calibre may be exceedingly rare. We, the product of a knife-edge balancing-act between biological, geochemical and astronomical implausibility, are lucky to be here at all. The inordinate complexity, the innumerable coincidences and the eventual culmination of 3 billion years of evolution – we stand on the peak of the impossible, gazing out into the void, with Voyager as our first envoy to the stars.
It is unlikely, but not impossible, that any interstellar civilisation has come before us. We’ve been listening for our galactic neighbours, via the enormous ear of SETI, for over 50 years to no avail. No radio chatter, no xenoarchaeology nor ambassadorial spacecraft. Given the ubiquity of planet forming material, and what we consider the relative normality of our watery home, the emptiness – the silence, is paradoxical.
The galaxy is ~13.2 billion years old and our 4.5 billion year-old Solar System has orbited its centre ~25 times. This planet has been habitable for around 4 billion years, and based on our best estimates, we have another half a billion or so to go before the evolution of the Sun renders the planet uninhabitable. We’ve been hitching a ride through space for one hundredth of one percent (5 million years) of the age of our planet, and have had space technology for one thousandth of that time (50 years). Assuming this is the case for most habitable planets, and knowing as we do that exponential colonial growth is impossible, it seems likely that if intelligent civilisations had arisen at any point in the history of our galaxy, and at some coordinate closer to the galactic core, there has been little evidence to suggest that they ever made it out this far. Given that colonisation infers a survival value, the fact that nearby planets give no indication of being inhabited leads to the conclusion that there are likely to be no other colonisers out there.
What conclusions can we draw from the silence? Well, conjecture abounds. Perhaps the galaxy is teaming with civilisations who have consciously hidden themselves from us until we overcome some technological or societal hurdle that would usher our entry into the ‘galactic club’ – perhaps superluminal travel or the formation of a world government? Who knows. In the immediate future, and without too much speculation, we can possibly infer that we may be the only intelligent civilisation ever to have arisen, in this neighbourhood anyway. If so, that places quite a burden on us, whether we realise it or not, to protect our planet and each other until such time that we can make our own way through the stars. We, or most likely our distant descendants, may be the sole custodians of the true meaning of existence, nature and the universe; the formulators and keepers of the ‘theory of everything’. Their success, and ours in the meantime, depends on the decisions we make now.
We are the pioneers, but we are also most certainly endangered by our own machinations. Up to this point, some of those decisions have been rather poor and have possibly compromised the very habitability of the planet we draw life from. Others, like Voyager et al. have been great. This humble, unassuming vessel represents the first step of an infant civilisation adopting a truly universalist, extrospective outlook. With 10 – 15 years of power left, Voyager will continue to take measurements and beam information back to Earth on the transition through the heliopause and the composition of the interstellar medium. After its batteries have died and its instruments have gone silent Voyager will continue to obediently sail through the depths of space on a mission lasting an eternity; a mission with no end and no more formal objectives. The spacecraft will not decay in the vacuum of space and its form and technology will be preserved indefinitely as a timecapsule to the stars. Long after the Earth has ceased to exist, Voyager will remain.
What a truly magnificent thought! It is humbling to be part of the first generation of interstellar human beings and an honour to have Voyager as our flagship.
This article was originally posted on my personal blog here.
Undoubtedly the most exciting exoplanet news of the past week is the discovery of a star system with a total of 9 potential planets, surpassing even our own Solar System in terms of planetary diversity. University of Hertfordshire astronomer Mikko Tuomi discovered the bustling planetopolis around the enigmatic star HD 10180, a Sun-like G-type main sequence star 127 light years distant, using a probabilistic Bayesian analysis technique.
HD 10180 has been known as a multi-planet system since 2010, but the last analysis of the HARPS data available for the star, carried out by Christopher Lovis last year, seemed to indicate a 6 or 7-planet system was most likely. However, the novel probabilistic methods used by Tuomi are more computationally intense than those previously applied, and confirm the findings of Lovis whilst also adding a further two planets to the planetary inventory of HD 10180.
Tuomi’s Bayesian method, which seeks to evaluate a number of possible scenarios to determine which is most consistent with the observations, finds that an orbital configuration including an eighth and ninth planet, with masses 5.1 and 1.9 times that of the Earth respectively, returns a 99.7% probability.
The planets themselves, denoted HD 10180 b through h, are a diverse bunch, including two Earth-mass terran planets, one superterran, five neptunian and one jovian-sized planet, and all are contained within 3.5 AU – roughly the distance of the asteroid belt between Mars and Jupiter in our Solar System. Despite their proximities, the orbits are predicted to be stable over astronomical time.
The image above, from the Habitable Exoplanets Catalog, provides a visualisation of the orbital system and a comparison of the sizes of the planets. Note that one neptunian, HD 10180 g, is within the habitable zone but is unlikely to be habitable given its large mass, at least not by our definition.
That’s an extraordinary array of sizes and shapes crammed into a comparatively small area, and unseats our Solar System, with a certain 8 planets (excluding trans-neputunian objects, asteroids and dwarf planets – sorry Pluto fans!), from atop the pile of planetary richness, all the while adding to our understanding of the mechanisms of planetary system formation.
Whilst this is certainly an exciting discovery, should we be surprised by the apparent ubiquity of multi-planetary systems? It would be more unusual if this architecture wasn’t the norm, given model predictions. Writing for his Scientific American blog Life, Unbounded, astrobiologist Caleb Scharf notes that the combined masses of the HD 10180 planets would only amount to roughly half that of Jupiter, and given the star’s similarity to our own Sun, its proto-planetary circumstellar disk should have contained a similar amount of material. Therefore, it wouldn’t be surprising if more planets lurked in the HD 10180 system somewhere!
In fact, the same could be said for any of the planetary systems we have detected so far as well as those that we find in the future. Our detection techniques remain biased towards massive, short-period planets that produce readily identifiable signals, particularly when using the radial velocity method, and we suffer from the fact that we have only been collecting data for a few years and so may have missed more orbitally distant, longer period planets.
However, as with most exoplanet discoveries, the detection of this diverse family of worlds serves to put our planet into some wider perspective – to challenge the notion that Earth and this solar system are particularly unique, at least in an astronomical sense.
Solar systems, it seems, are everywhere.
Jupiter’s icy moon Europa has been of interest to astronomers for hundreds of years, and to planetary scientists since it was first imaged by the Pioneer spacecraft in the 1970s. Since then the NASA missions Voyager 1 and 2, Galileo and most recently New Horizons have all paid the moon a visit, and thanks to the remarkable high resolution imagery returned by Galileo in particular, it is possible that scientists may have uncovered the single most promising candidate for hosting astrobiology in our Solar System to date.
Europa is a moon with a difference. The smallest of the Galilean satellites, and the second most proximate to Jupiter, Europa has a surface topography matched by no known planet or moon. It is smooth and bright, and its young surface – only 40-90 million years old, is relatively devoid of significant impact craters when compared to its Jovian neighbours Ganymede and Callisto. The icy shell of Europa is pockmarked with a series of prominent crisscrossing surface features known as lineae, but also with domes, albedo features and surface ridges, all thought to be the geomorphological products of a subterranean ocean of liquid water churning slowly beneath the brittle crust. Jupiter and its extensive family of moons orbit the Sun at roughly 5 Astronomical Units (AU), a distance at which the light of our nearest star is faint and weak, and planetary surface temperatures are well below freezing. Nevertheless, it is thought that beneath Europa’s ~20km thick crust an ocean of salty liquid water exists, perhaps a hundred kilometres deep, warmed by the internal heat generated by Europa’s rapid transit around Jupiter, as well as from the gravitational interaction with its Laplace resonance partners, Ganymede and Io.
The Biogeochemistry of the Europan Ocean
The Europan ocean represents a completely unique geochemical system and a potential astrobiological repository unlike any other in our Solar System. Cut off from light and the planetary surface, the circulation of Europa’s salty subsurface ocean is most likely driven by convection from below, where heat and reductant input originates directly from the upper mantle. Oxidation products such as oxygen and hydrogen peroxide may be formed via radioactive decay within the ocean and ice shell, but also produced and deposited extensively on the surface crust via ion irradiation. These products are likely to be the Europan ocean’s main source of oxidising power, eventually penetrating the ice/ocean barrier under conditions of preferential or partial melting of the surface crust, possibly due to a localised plume of upwelling warm water from below. Ubiquitous lineae, surface ridges and ‘chaotic’ terrain on the surface of Europa are put forward as evidence of this process occurring over timescales of 10 – 100 million years. Whether a discreet boundary exists at the surface crust/ocean interface or if an intermediate layer of slushy material separates the two zones is not immediately clear.
Photosynthetic surface-dwelling life is considered extremely unlikely to exist on Europa due to high levels of magnetospheric irradiation and frigid surface temperatures of between 80 and 120 K; a very harsh environment for even the most hardy, radiation-resistant psychrophilic microorganisms. Brine deposits would not be stable anywhere on the Europan surface, further reducing the likelihood of surficial biology. However, the slightly alkaline (pH 8-9), sulphate-carbonate Europan ocean represents a relatively warm (~270 K) chemical-rich environment and easily forms the strongest candidate medium for hosting astrobiology within our Solar System.
Biogeochemical modelling studies of Europa’s ocean have been carried out over the last few years with interesting results. Tidal heating of the silicate mantle resulting from gravitational interaction with Jupiter and the other Galilean moons could produce volcanic and hydrothermal vent systems on the ocean floor analogous to those found on the seabed of the Earth. Here, potential chemotrophic microorganisms have the option of exploiting a host of different metabolic pathways, including the oxidation of methane, hydrogen, sulphur, ferrous iron and organic compounds, but also the reduction of ferric iron or by adopting anaerobic methanogenesis or acetogenesis. The link between these hypothetical exobiological processes and the geochemical cycles of carbon, iron and sulphur on Europa would, as on Earth, represent a fundamental biological recycling and feedback mechanism between geology and chemistry ultimately driven by the periodic replenishment of rocks and the steady trickle of reductant from the ocean floor.
The general consensus seems to be that whilst the Europan ocean’s chemical soup could support a putative chemoautotrophic biomass, the viability of this ecosystem is likely to be several orders of magnitude lower than what could be supported by photosynthesis. In light of this, any Europan life would most likely be very small, exhibit staggeringly low growth rates and limited motility.
The Search for Europan Life
Given the compelling evidence, several missions to Europa have been planned for the future. Some of these have unfortunately been cancelled, but perhaps the most exciting extant proposal is the Europa Jupiter System Mission – Laplace (EJSM/Laplace), a joint NASA/ESA unmanned campaign scheduled for launch in 2020, arriving at Jupiter in 2025. The EJSM will consist of two components, the Jupiter Europa Orbiter (JEO), built and operated by NASA, and ESA’s Jupiter Ganymede Orbiter (JGO). These instruments will be tasked with addressing some of the major science questions posed by the Jupiter system, but are heavily focussed on issues surrounding habitability and geochemistry. In particular, the objectives of the JEO instrument are centred on determining the chemistry and geology of Europa’s ice shell, its ocean and any ocean/mantle interactions. To do this JEO is equipped with a suite of 11 instruments, including ice penetrating radar, visible-infrared and UV imaging spectrometers, an ion and neutral mass spectrometer, several cameras, a magnetometer and a thermal instrument.
Orbital observation missions like EJSM are unlikely to be able to conclusively solve the puzzle of potential Europan biology, but they will provide more information on the geochemistry of this unusual moon and by implication improve our understanding of its potential habitability. Definitively answering this most pressing of questions may be out of the reach of contemporary or near-future exploratory technology. Some extremely imaginative proposals have been suggested, such as the Nuclear Europa Mobile Ocean (NEMO) mission. NEMO would consist of a lander and autonomous underwater vehicle termed a ‘hydrobot’ that would be deployed directly into the Europan subterranean ocean. However, contamination of the subsurface ocean is a serious concern, and the technological and budgetary hurdles that would need to be overcome for this mission to be successful essentially rule out any possibility of its inception in the foreseeable future.
The mysterious oceans of Europa, and any of its potential inhabitants, will most likely remain out of reach of science for some decades to come, but the attraction of this icy, scarred moon is undeniable. Europa is the first step on the long road to a better understanding the outer planets and their many diverse moons and, in terms of astrobiological potential, Europa is unmatched in our Solar System. It will therefore form the focus of many more mission proposals with habitability and biological detection at their core in the future. It is an exciting, if frustrating, time to be working in this area of science, but I believe that the secrets that this moon may have to offer will be worth the wait, no matter how long it takes.
NASA (2011). Europa Jupiter System Mission (EJSM) [Online]. Available here. (Last accessed on 25th January 2012)
Pappalardo, R. T. (2010). Seeking Europa’s Ocean. Galileo’s Medicean Moons: their impact on 400 years of discovery. Proceedings of IAU Symposium No. 269, 2010 pp. 101 – 114
Schmidt, B.E., Blankenship, D.D., Patterson, G.W. and Schenk, P.M. (2012). Active formation of ‘chaos terrain’ over shallow subsurface water on Europa. Nature. 479 pp. 502 – 505
Zolotov, M. Y and Shock, E. L (2004). A model for low-temperature biogeochemistry of sulfur,carbon, and iron on Europa. Journal of Geophysical Research 109 E06003