We are what we breathe

Oxygen: where would we be without it? 

The air we breathe today—inhaled from the Earth's lower atmosphere—is composed predominantly (about 78%) of nitrogen and a nice, agreeable amount (about 21%) of oxygen. Small amounts of argon, carbon dioxide, and other gases make up the rest. But this plentiful supply of oxygen— sufficient to sustain humans and the other oxygen-breathing species with which we share the biosphere—has not always enveloped the planet. Indeed, Earth's atmosphere has been evolving since it first formed early in the planet's history (more than 4 billion years ago). Scientists believe that Earth's earliest atmosphere was created from the outgassing associated with the volcanic activity that was prevalent during the planet's early differentiation (cooling of the molten Earth that formed the core, mantle, and crust). This earliest iteration of the atmosphere likely contained abundant carbon dioxide and water vapor (common volcanic gases), as well as other relatively heavy molecules (e.g., nitrogen and sulfur gases). Additional, lighter gases such as hydrogen would have escaped the Earth's gravitational field and escaped to space.

Artist's impression of the Earth's early atmosphere and oceans. Outgassing volcanoes released a mixture of gases (mostly carbon dioxide and water vapor) from the planet's interior to the atmosphere. As the surface of the Earth slowly cooled, water vapor condensed to form some part of the oceans. Credit: Lunar and Planetary Institute

During this period it is thought that the atmosphere contained less than 0.001% of the amount of oxygen that we find in our air today. Any 'free' molecules of this rare oxygen would have been chemically captured by dissolved iron or organic matter. Meanwhile, the oldest known fossils show that life has existed on Earth for at least 3.5 billion years (and possibly longer). These earliest preserved lifeforms are cyanobacteria fossilized within Archean-aged rocks from western Australia.

Two forms of fossilized cyanobacteria from the Bitter Springs chert (Australia).
Credit: J. William Schopf 

Cyanobacteria, which still exist today, are peculiar because they obtain their energy from photosynthesis. This process—more commonly known as the way plants use sunlight to convert water and carbon dioxide into chemical energy—meant that the cyanobacteria could thrive in Earth's early anoxic environment. Importantly, the photosynthesis of the ancient cyanobacteria also produced oxygen as a by-product. With the rise of the cyanobacteria, therefore, the amount of oxygen in the atmosphere steadily began to rise. For the first time, the previous 'oxygen sinks' (i.e., organic matter and iron) became saturated. In other words, the overall rate of oxygen production surpassed the rate of its removal by iron oxide sedimentation (a result of both increased bacterial colonization and of decreased volcanic activity) and the oxygen could accumulate in the atmosphere. By about 2.4 billion years ago, the Great Oxidation Event had occurred and the geologic record indicates that the concentration of atmospheric oxygen had significantly increased. 

Although there are various strands of evidence for the Archean's low atmospheric oxygen levels (e.g., the lack of oxidized iron in fossilized soils and large volumes of banded iron formations in Archean sedimentary rocks), the geologic information all relates to the lower atmosphere in particular. Until now, there has been no way to examine the makeup of Earth's upper atmosphere during the Archean. A new paper published last week in Nature, however, has provided a new twist to the story of our atmosphere's evolution.

In this new study, led by Andrew Tomkins from Monash University in Australia, 60 'fossil' micrometeorites were extracted from layers of limestone in the 2.7-billion-year-old Tumbiana Formation (in the Pilbara region, northwest Australia). Micrometeorites are extraterrestrial dust particles (up to about 2 mm in size) that survive entry through the atmosphere and are collected on the Earth's surface. As these particles fall through atmosphere, they experience maximum temperatures at altitudes between about 75 and 90 km (i.e., within the upper layers of the modern atmosphere). The small size of the micrometeorites means that many will completely melt during this passage and then rapidly re-crystallize. This 'quench-crystallization' occurs over a timespan of just a couple of seconds and the micrometeorites therefore chemically interact only with the upper layers of the atmosphere. Tomkins and his colleagues have thus used this understanding of modern micrometeorite behavior and applied it to their fossil samples so that they can probe the Earth's ancient upper atmosphere. 

Scanning electron microscope images of a selection of the fossil micrometeorites extracted from the Tumbiana limestone formation. Credit: Tomkins et al., 2016, Nature

Upon examination of the 60 sampled micrometeorites, it was found that they had diameters of between 8.6 and 50 micrometers and 'cosmic spherule' morphologies. The rounded form of the particles indicates that they all had fully melted during their journey through the atmosphere. In addition, all but one of the samples had compositions that were almost entirely iron-nickel metal (with no silicate minerals). Analyses for the interiors of 11 of the spheres were also conducted as part of the study and they revealed compositions dominated by the iron oxides magnetite (Fe₃O₄) and wüstite (FeO), as well as minor amounts of iron-nickel metal. These results thus demonstrate that most of the micrometeorites' original iron-nickel material had been oxidized during their encounters with the Archean upper atmosphere.

Given the long-held view that the Archean atmosphere had a very low oxygen content, these new results are quite a surprise. The highly oxidized nature of the micrometeorites suggests that there must have been abundant oxygen at altitudes above 75 km, at the time the micrometeorites fell to Earth. Tomkins et al., therefore propose a model for the Archean atmosphere in which an oxgyen-rich upper layer—with a similar oxygen concentration to today's—experienced minimal levels of mixing with an oxygen-poor lower atmospheric layer.

So for this postcard, I propose we send a piece of the Tumbiana Formation—along with its hidden treasure of micrometeorites—into space, as a representative of our special Earth and the life that it contains. Not only do the microscopic spherules tell an important story about the history of our life-sustaining atmosphere, but the limestone formation itself may be uniquely capable of telling this tale. The mineral wüstite is rarely found on Earth's surface, and is crucial to interpreting the extraterrestrial origin of the micrometeorites. Luckily, these microscopic particles were laid down in a rock formation that was once a system of highly alkaline lakes. In such pH conditions, wüstite has a low solubility and was thus able to survive in these micrometeorites for 2.7 billion years. Unfortunately, such conditions are rarely found within the geologic record and this set of micrometeorites from the Pilbara region may indeed be unique. Surely, therefore, they deserve the adventure of our interplanetary mission.

Simon says "send a smoke signal"

I am currently in the midst of some rather hectic few weeks. I seem to be flying back and forth across the Atlantic slightly more than usual. But a couple of sandwiched weeks in London thankfully coincided with my favourite author's time in the UK for the promotion of his new book. Indeed, hearing Simon Winchester speak, or reading his work, is one of (my) life's great pleasures. Every word he writes or utters seems to simultaneously educate and entertain. He truly is one of our world's great polymaths.

Simon Winchester talking about his new book, Pacific: The Ocean of the Future, at Daunt Books in London, October 2015.

His latest book—Pacific: The Ocean of the Future—is the last installment of a trilogy that also includes Atlantic: A Vast Ocean of a Million Stories and The Men who United the States. In this new offering he writes a recent biography of the Pacific, based around several major events that have shaped the largest of our oceans. He has made a conscious decision to eschew the ancient history of the Pacific, and instead focus on stories that have occurred since 1st January 1950. This date defines the present in the 'before present' (BP) timescale, which is mainly used in geology and other scientific disciplines to quantify when past events took place. Of course, it was in the 1950s that nuclear weapons testing first altered the proportion of carbon isotopes in the Earth's atmosphere and thus changed the way radiocarbon dating is conducted (read a previous postcard to learn about one proposed start of the Anthropocene at about this same time, i.e., when nuclear radionuclides became detectable around the globe).

Given that Simon was talking about one of the Earth's greatest natural features—covering about one third of our planet's surface area—I could not let the chance slip by to ask him a question. My question.  So at the end of his talk, I sought his opinion about what piece of Earth—specifically from within the Pacific Ocean—he would send into space to represent our planet to hypothetical alien planetary geologists. Obviously, I was looking for inspiration for a new postcard, but I was also genuinely interested as to what he (as a fellow Oxford geology graduate, no less!) would choose. And despite me putting him absolutely on the spot, I do believe he came up trumps with his answer (as I had no doubt he would): "Black smokers".

He thought that black smokers—rather mysterious landforms (and the exotic ecosystems they harbour) deep within the Pacific Ocean—would be a wonderful geological emissary for the hypothetical cosmic journey. I'm not sure I could have come up with a better answer.

The Sully 'black smoker' hydrothermal vent, part of the Main Endeavour Vent Field in the northeast Pacific Ocean. Credit: NOAA

These black smokers, to which Simon referred, are a specific example of underwater hydrothermal vents. Such vents are fissures in the Earth's surface from where geothermally heated water can escape. On land, hydrothermal vents give rise to features such as hot springs, fumaroles, and geysers. But in the sea, they can form black smokers. Until 1977, however, these sea vents were unknown. They were first discovered on the East Pacific Rise (a mid-oceanic ridge tectonic plate boundary) by scientists from Scripps Institution of Oceanography, who were using a deep submergence vehicle. Since then hydrothermal vents—and the associated smokers—have been found at almost all active spreading ridges (i.e., tectonic boundaries where plates move apart). Over 500 active submarine vent fields are now known.

Map of known active submarine hydrothermal vent fields. Credit: InterRidge Vents Database

The characteristic black 'smoke' that emanates from these vents is actually hot (about 350°C) liquid containing a thick suspension of dark, fine-grained particles. These metal-rich fluids are a product of reactions that take place between seawater and hot basalt, the latter of which is created at the spreading ridges. When the super-heated, metal-laden waters escape from the vents they mix with the frigid waters of the deep ocean. This abrupt mixing causes rapid precipitation of the metals and gases that were suspended in the water. Large amounts of various sulphide minerals (such as pyrite, chalcopyrite, and sphalerite), as well as silica and anhydrite (a calcium sulphate mineral) precipitate and form the chimney-like structures of the smokers themselves.

Cross-section through a black smoker 'chimney'. The concentric rings represent zones of different precipitated minerals. Credit: Rachel Haymon

But in addition to the pure geological excitement of these seafloor regions, the black smoker environments represent nutrient-rich oases in the deep ocean. As such, the areas surrounding submarine hydrothermal vents are much more biologically active than most of the dark, abyssal sea. They play host to complex ecosystems full of foreign species.

A well-developed hydrothermal vent ecosystem in the Pacific Ocean, which includes tube worms (red) and mussels (yellow shellfish). Tube worms such as these can grow to be up to
2 metres in length. They no mouth or stomach, but billions of symbiotic bacteria living inside the tube worms produce sugars from carbon dioxide, hydrogen sulphide, and oxygen.
Credit: Woods Hole Oceanographic Institution.

Because no light reaches the depths of the ocean floor (hydrothermal vents in the Atlantic and Pacific oceans exist at an average depth of 2100 metres), these species are not based around photosynthesis. Instead, these ecosystems are chemosynthetic. The alien-like species, which include varieties of clams, limpets, shrimp, and giant tube worms (specifically mentioned by Simon in his reply to my question), mostly exist by consuming the sulphide minerals that are available.

So as well as representing an exotic part of the Earth's deep sea environment, the black smokers illustrate the true variety and abundance of life that exists on Earth. To an alien planetary geologist, observing from afar, our surface biological communities are likely to be the most obvious. But without digging deeper, into our giant ocean domains, Earth's amazing diversity cannot totally be revealed. Wherever we look, our world literally teems with life.

A whole world's wake-up call

The past few weeks in the world of space have been pretty hectic. Most especially because of the fantastic new views of Pluto we've been receiving, courtesy of the New Horizons flyby (which I wrote about in my last postcard). We've also been hearing about the "frozen primordial soup" of organic compounds detected by the European Space Agency's Philae lander on comet 67P/Churyumov–Gerasimenko, as detailed in a new special issue of Science. Some of these compounds may be important for the prebiotic synthesis of amino acids, sugars, and nucleobases, i.e., the very ingredients of life. 

The surface of comet 67P/Churyumov–Gerasimenko, as imaged from 9 metres away. Credit: ESA

But there are two other recent news items I want to focus on in this postcard. First, the new photograph of the Earth captured by NASA's new Deep Space Climate Observatory (DSCOVR) satellite. And second, the recent discovery of an exoplanet that is being billed as Earth's 'twin'.

On 6 July 2015, the Earth Polychromatic Imaging Camera (EPIC) instrument on DSCOVR returned its first view of the entire sunlit Earth. Safe in its gravitationally stable location one million miles away—at a so-called Lagrange point—the satellite was able to obtain this kind of full-Earth portrait for the first time since the famous 'Blue marble' photograph was snapped by the Apollo 17 astronauts whilst on their way to the Moon in 1972. I've mentioned that older, stunning photo in a previous postcard, but as the most reproduced image in history, I think that it is more than worth showing again.

The famous and historic 'Blue marble', taken during the Apollo 17 mission in 1972. Credit: NASA

It might come as a surprise that it has taken more than 40 years to recapture Earth in a similar view. The pictures you've seen of Earth's full disc in the meantime have either been this Apollo 17 photograph, or composite images (i.e., several smaller images that have been stitched together). It is difficult to obtain these images because many variables come into play. The camera must be between the Earth and the Sun, and far enough away to capture the whole planet in its field of view. Although weather satellites—in geosynchronous orbits—get similar views, they cannot normally see an entire hemisphere without shadow.

The Earth, from one million miles, as seen by the Deep Space Climate Observatory on 6 July 2015. Credit: NASA

The data from EPIC will primarily be used to measure changes to the ozone and aerosol levels in Earth's atmosphere, as well as cloud height, vegetation properties, and ultraviolet reflectivity characteristics. But these new, beautiful, images of a whole Earth remind us how powerful it is to see our entire home in one go. As pointed out by John Grunsfeld, associate administrator of NASA's Science Mission Directorate, "these new views of Earth give us an important perspective of the true global nature of our spaceship Earth."

Indeed, I'm reminded of an excellent book I read several years ago by Robert Poole. In Earthrise: How Man First Saw The Earth, Poole tells the story of how images of Earth—such as the Blue marble and the equally famous Apollo 'Earthrise'—taken during the dawn of the space age, played a huge role in the birth of the now-popular environmental and conservation movements.

'Earthrise' photograph taken by astronaut Bill Anders during the Apollo 8 mission, on 24 December 1968. Credit: NASA

It is another aspect of these images of our blue Earth, however, that strikes me most. It is the human capacity for intelligence and creativity that enables space exploration and capturing of Earth-selfies from afar. Yet we do not see evidence of our presence in these pictures. In many ways, we are invisible to the universe. It is not life that makes Earth special. It is the blue oceans, the green forests, and the white wispy clouds in our lovely oxygen-rich atmosphere that make our world habitable. So for this postcard to our hypothetical alien planetary geologists, I want to send a snapshot of our whole world. Let them see the Earth and all its systems intertwined.

The uniqueness of Earth, however, might be under threat if a new discovery from the Kepler space telescope is anything to go by. On 23 July 2014, scientists working on the Kepler mission announced that they have found the most Earth-like extrasolar planet yet. The new planet—known as Kepler-452b—is located about 1,400 light years away, and is a similar size to Earth. In addition, Kepler-452b orbits a Sun-like star at a distance that is similar to that of Earth around the Sun. The planet is being hailed as "the first possibly rocky, habitable planet around a solar-type star". And it will thus, likely, become the focus of an intense search for extraterrestrial life. Perhaps we'll even find those alien planetary geologists there waiting for us.

Artist's concept of Kepler-452b in orbit around its parent star. Credit: NASA Ames/JPL-Caltech/T.Pyle

At a time when humanity seems to be as fractured as ever, perhaps we need a wake-up call like these ones from NASA. We need to be reminded every once in a while that we are all one family, stuck together here on our little spaceship Earth. We should do our utmost to look after it—and each other.

Good morning Earthshine

When I first mentioned the overall idea for this blog to a friend and colleague, he immediately found an interesting way to slightly reframe the question. Instead of considering how alien planetary geologists might recognize rock specimens representative of Earth, he wondered how Earth might look to an alien astronomer observing us with a faraway telescope.

This question is actually a pretty obvious one, especially given the popular pursuit of extrasolar planets in current astronomical research. The first confirmed detection of an 'exoplanet'—a planet that orbits a star other than our own Sun—was not made until 1992, but this field of research has now, almost literally, exploded. More than 1800 exoplanets have since been discovered, and this has largely been possible because of NASA's Kepler mission. The aim for this space observatory was to discover Earth-like exoplanets that are located in, or near, the 'habitable zone' of their parent star. This habitable—or Goldilocks—zone is the region around a star where planets (with sufficient atmospheric pressure) can support liquid water at their surface.

The habitable zones—where liquid water can exist on the surface of a planet—of different size and temperature stars. Credit: NASA Kepler Mission

This is all part of humankind's everlasting desire to discover life—potentially sentient—elsewhere in the universe. As our home continues to be the sole 'datapoint' for life, it is natural that we use Earth-like planets as a base for our search. And it is the special feature of liquid water at the surface that makes our planet so hospitable. The modern-day search for extraterrestrial life is therefore often focused on the hunt for this precious H₂O. A discovery of a planet hosting water at its surface within another star's habitable zone, however, does not equal the discovery of life elsewhere in the cosmos.

To be more certain of a water-hosting planet's potential to harbor life, an additional telltale detection is required. The spectra from the observed exoplanets must include signs of life—biosignatures—along with the sign of water. But what would an astronomical biosignature look like, and would we even recognize such a signal from an exoplanet? To begin to answer this question, we first need to understand how Earth's atmosphere looks from afar and which of its properties hint at the rich biosphere that lies beneath. This information can then be used as a reliable baseline with which to compare exoplanet detections.

 

Spectra for Venus, Earth, and Mars illustrate Earth's unique biosignatures. All three planets have a strong atmospheric absorption caused by carbon dioxide (CO₂), but only Earth's atmosphere has signals due to water (H₂O) and ozone (O₃) that can be representative of life. Credit: Mark Elowitz

As detailed in a 1993 study by Carl Sagan and colleagues, observations of Earth's atmosphere from space—in this case from the Galileo spacecraft—reveal several biosignatures. These include abundances of molecular oxygen and methane that are far from chemical equilibrium, as well as a sharp increase in albedo at wavelengths longer than 700 nm, which is caused by vegetation. It is also known that as light passes through Earth's atmosphere it can be polarized due to scattering by aersols and cloud particles, and reflected at variable amounts by oceans and land. In a more recent paper, Michael Sterzik et al. use a technique known as spectropolarimetry to make a detailed analysis of Earth's atmospheric properties.

Instead of using space-based measurements of Earth, Sterzik and co-workers made observations of 'Earthshine' by pointing their telescopes at the Moon. This rather romantic sounding light originates from the Sun before being reflected by the Earth onto the Moon, and then back to Earth again. It is the reason you can sometimes to see the 'dark' part of a non-full Moon.

Earthshine illuminates the 'dark' portion of the Moon. Credit: Will Gater

Sterzik et al. used a technique known as spectropolarimetry (a combination of spectroscopy and photopolarimetry) to conduct a detailed investigation of Earth's atmosphere. This methodology is better than standard spectroscopy for characterizing exoplanet atmospheres. The Earthshine observations could be used to determine the fractional contribution of cloud and ocean contributions within the reflecting surface, and were sensitive to relatively small areas of vegetation.

It is measurements such as these, using the Moon as a handy mirror, that can be used as a benchmark for diagnosing the atmospheric composition and surfaces of potential life-bearing exoplanets. They also serve as a clue to what an alien astronomer might see when they glance in our direction. I hope they too can recognize how special our Earth is.

Wake up and taste the water

Comets have made the news a number of times in recent months. These interplanetary travellers, which were once thought to herald doom, are now known to be among the most primitive objects in the solar system. And of course, comets periodically make visits from the far reaches of the solar system (regions known as the Oort Cloud and the Kuiper belt) to our more local neighbourhood.

Back in December, Comet ISON—the 'comet of the century'—made a much-watched and disappointing suicide plunge into the Sun. And just this week, the European Space Agency successfully 'woke up' its Rosetta spacecraft from its two-and-a-half-year hibernation.

Currently more than 400 million miles away from home, Rosetta is journeying towards the comet 67P/Churyumov-Gerasimenko. Once it arrives, it will first orbit, and then land on the comet's surface. Together, the orbiter and lander portions of the spacecraft are equipped with over 20 scientific instruments that will make important characterizations of the comet. Scientists working on the mission hope to find out if comets such as this, which contain complex organic molecules, may have played a role in seeding life on Earth.

Artist's impression of the European Space Agency's Rosetta spacecraft orbiting the comet 67P/Churyumov-Gerasimenko. Credit: ESA, C.Carreau / ATG medialab

There has also been a long-lived debate over whether or not comets contributed significantly to the delivery, early in its history, of Earth's vast water inventory.

Because water is such a vital ingredient for the sustenance of life here on Earth, we target our search for extraterrestrial life on places where water exists. Water therefore seems like a pretty obvious choice for an Earth-material to send as one of our interplanetary postcards. But how might an alien scientist be able to tell our Earth-water apart from any other foreign H₂O? We all know that water can vary drastically in its colour, salinity, taste, etc. So is there a characteristic signature of our water that portrays its Earthly provenance?

Earth: A water world. Credit: NASA

Lucky for us, chemistry has an answer. Hydrogen (H), like all elements is defined by the number of protons it contains. 'Normal' hydrogen contains just one proton in its nucleus. However, it is possible for a stable hydrogen atom to contain one or two neutrons in addition to the lone proton. The hydrogen isotope that has one proton and one neutron is known as deuterium (D). Heavy water is enriched in molecules that contain deuterium in place of the normal hydrogen.

Planetary scientists have shown that D/H ratios in water vary throughout the solar system. Measurements for a number of Oort Cloud comets reveal that they have D/H ratios which are more than twice the value for Earth's oceans, and are thus unlikely to have been the source of water on our planet.

Range of deuterium/hydrogen (D/H) ratios in solar system objects.
Credit: A. E. Saal et al. 2013, Science

Paul Hartogh and colleagues, however, showed in 2011 that a Jupiter-family comet (103P / Hartley 2), which probably originated from the Kuiper belt, has a D/H ratio that is much more consistent with that of Earth. This means that at least some of Earth's water may have been delivered by comets. Although in more recent work, Conel Alexander et al. argue that CI chondrites (the class of meteorite whose composition most closely resembles that of the Sun) were the principal source of terrestrial water.

And so the great water debate continues.  But no matter how, and from where, the water got here; get here it did. And we wouldn't be alive without it.

Tardi and hardy space travellers

Recently, this BBC headline caught my eye:

Dinosaur asteroid 'sent life to Mars'

The piece focuses on an Astrobiology research article by Worth et al. In the paper the authors consider the likelihood that rocks ejected from Earth (or Mars) during large meteor impacts launched life-bearing materials into space. It is possible, they claim, that such rocks could reach other planets or moons in the solar system, where the life could resettle and colonize. This concept—known as lithopanspermia—isn't new, but their statistical approach to the problem is.

Back in 1996 a meteorite found in Antarctica, but which originated from Mars, known as Allan Hills 84001 (ALH 84001) hit the news. NASA scientists claimed that this rock contained evidence of life once having existed on Mars.

Electron microscope image of 'fossilized microbes' in the Allan Hills 84001 meteorite. Credit: NASA

Obviously this controversial subject has been disputed ever since and it has been shown that all the rock's 'biosignatures' can be produced inorganically, i.e., without the need to invoke life.

Whether or not this particular meteorite contains ancient martian lifeforms, it prompts an interesting question that links back to the subject of lithopanspermia. Is it possible for life from Earth to be exported? If we were to pick an organism to undertake this journey, what would it be? And what rock would we encase this organism in, to try and ensure its survival?

I've never been a big fan of bugs, in fact I'm pretty terrified of anything with more than four legs. However, I recently became aware that a group of organisms known as tardigrades exist. And even I have to admit that these little monsters are cool.

A tardigrade in moss. Credit: Eye of Science / Science Source Images

Tardigrades are classified as extremophiles, organisms that can survive in some of Earth's most difficult conditions. But these guys take the term to the extreme—they're pretty much indestructible. They can withstand temperatures that range from just above absolute zero to more than the boiling point of water, pressures greater than at the deepest ocean trenches, and the vacuum of space along with its harsh radiation conditions. So my vote goes to the tardigrades as the space-faring guinea pigs that get to be sent on a one-way (and potentially suicidal) mission to somewhere.

The encasement of rock around our tardigrades must also be chosen carefully. The material needs to be strong enough to survive two impacts: the large meteor event that expels the material into space, and the sample landing on the surface of its destination. Hardy quartzite, the metamorphic product of simple sandstone, would probably be a good option. Or perhaps a material similar to that of ALH 84001, i.e., an igneous rock called orthopyroxenite, would meet the requirements.

I don't know if life on Earth originated through lithopanspermia, or if life exists elsewhere because of it, but I do quite like the idea of the hardy tardigrades ruling some distant planet.