Postcard from the Moon

One morning, not many weeks ago, a postcard floated gently through my letter box, onto my doormat, and brought a smile to my face. For we all know that I love me a postcard. This specimen—sent by two friends/colleagues (the indomitable duo Paul Byrne and Christian Klimczak)—was especially exciting, however, because it could almost have come from the Moon. And we all know that I love me some Moon.

My very own postcard from the Moon, or at least from the Craters of the Moon National Monument, Idaho. "Violent eruptions in the recent past have created an unearthly landscape where visitors can walk to the top of the cinder cone, hike over lava flows, or drop beneath the surface into the large caves known as lava tubes."
Original image credit: Dave Clark Photography

And at first glance (if you ignore the clouds in the very Earthly blue sky), the landscape does look vaguely lunar. We can compare and contrast: 

Lunar landscape, captured during the Apollo 17 mission. Credit: NASA

But, alas, my postcard does actually have a terrestrial origin. Rather than bearing a lunar postmark, it was sent from the Craters of the Moon National Monument and Preserve in Idaho, where my friends were carrying out fieldwork. The protected region is volcanic in nature and a presents a well-preserved example of flood basalts (another example of flood basalts—the extensive Siberian Traps—featured in one of my early postcards). The site encompasses three major lava fields (about 1000 km² in area) that include a huge variety of basalts (in terms of composition) and excellent examples of many different volcanic features (such as lava tubes and scoria cones). 

When I asked Paul why he and Christian chose this site for their fieldwork, he replied that it includes "some neat interactions between volcanic and tectonic structures (e.g., scoria cones riven by fissures, fractures, and a rift zone). Mainly we went to see what these features look like and how they might compare with similar landforms on Mars." In particular, he noted that they found "a bunch of pit craters" that look very similar to those we see on Mars. As planetary geologists, Paul and Christian are thus interested in studying the terrestrial Craters of the Moon region as a planetary analogue, i.e., to gain better insight into the similar-looking features on Mars and elsewhere.

A pit crater (about 130 m in diameter) on Mars. This example lies on the flanks of the volcano Elysium Mons. The dark pit crater is clearly different from the many surrounding small impact craters that are covered by dust and sediment. Credit: NASA/JPL-Caltech/Univ. of Arizona

A pit crater, like a sinkhole, is a depression that forms via the collapse of a surface overlying an empty chamber. Unlike impact craters that have raised rims and sloped walls, pit craters have steep/almost vertical walls. In planetary imagery they thus appear as dark, approximately circular, shadowed holes, and their floors can only be seen when the Sun is at a high angle of illumination. One way that a pit crater may form—especially in volcanic environments—is through the collapse of a lava tube (which is essentially a tunnel formed by lava flowing underground). This process may begin with the buckling of the tube's roof at a location where the roof is thinnest. These craters are often known as 'skylights' because light can flood through them into the darkness of the connected cave.

In 2007, such skylights were discovered on Mars. Scientists looking at the pictures form NASA's Mars Odyssey and Mars Global Surveyor satellites were puzzled by the very dark, circular features. By combining the images with thermal information from Mars Odyssey's infrared camera, however, they concluded these pits were indeed windows into underground caves. Soon after, in 2009, the first lunar skylight discovery was made by a team working on high-resolution images returned from the Japan Aerospace Exploration Agency's SELENE satellite. 

High-resolution image from NASA's Lunar Reconnaissance Orbiter Camera showing a 'skylight' in the Moon's Mare Ingenii region. This pit has a diameter of about 130 m.
Credit: NASA/Goddard/Arizona State University 

These skylight discoveries have got many in the planetary science community quite excited. It is thought that the subsurface structures they provide a window into, could provide a potentially habitable environment. On Mars, organisms could have perhaps flourished under the protection of the ancient lava tubes (i.e., acting as a shield against harmful ultraviolet radiation). Moreover, if the lava tubes are structurally stable at a large enough size, they could be suitable as shelters for human explorers. Indeed, the maximum potential diameter of lunar lava tubes has been estimated by yet another friend/colleague of mine. While a PhD student at Purdue University, David Blair led a study in which he calculated the stresses and strains that would be present around lunar lava tubes. Given the size of the lava tubes inferred from NASA's GRAIL gravity data (i.e., with diameters of more than 1 km), the team estimated that the structures could remain stable even at widths of more than 1.6 km.

The Indian Tunnel lava tube in Craters of the Moon National Monument and Preserve. This tube is about 9 m high, 15 m wide, and 245 m long. Lava tubes on the Moon are thought to be substantially larger because of the reduced gravity. Credit: Kurt Allen Fisher/Universities Space Research Association.

If Dave and his team are right, these lava tunnels may one day present a useful extraterrestrial habitat for humans—big enough to house sizable settlements. When living here on Earth is no longer feasible, perhaps we will decamp as a race to the lunar lava tubes where we will shelter from the bombardment of radiation and escape temperature extremes. I think we can therefore justify sending a piece of (basaltic) rock from the Craters of the Moon for this Postcard From Planet Earth. As a terrestrial analogue for these potentially habitable extraterrestrial settings, it can be our message to the alien planetary geologists when they arrive for their inevitable visit: 'if we're not in, try elsewhere'.

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.

With a leaden heart: Oliver Sacks

As countless tributes and obituaries for neurologist Oliver Sacks appear in the wake of his death, I find myself among the many fans saddened by our loss and the void that now exists in his place. As he himself recently wrote:

"When people die, they cannot be replaced... they leave holes that cannot be filled."

His Musicophilia, I think, will always remain one of my most favourite books. Indeed, his writings of human disfunctions were as much about the humans as they were about the disfunctions. And I am sure that is the reason of his extensive and mainstream popularity. In each of his patients he saw a whole person, and he possessed a wonderful—perhaps unique—ability to convey that insight to his readers. When I read his books, I find myself harbouring a rather mystifying wish to be ill myself. He makes neurological problems sound almost appealing.

Also well worth reading are his recent pieces published by the The New York Times. In one—My Periodic Table—he describes his way of dealing with loss. Even as a child he would turn to the nonhuman, to the chemical elements and numbers that he saw as his friends. And as a geologist—a physical scientist—I can relate to this approach. He writes of his collection of the elements and relates them to his age. Sacks died yesterday at the age of 82. His so-called lead (element 82 in the periodic table) birthday was his last. And so my response to his passing is to turn to this chemical, to this physical element that "has no life, but also no death".

The late Oliver Sacks. Credit: Adam Scourfield / AP

It is difficult, however, to pick a single element in this way—which is not unique in its occurrence to Earth—as a Postcard from Planet Earth. And although lead is abundant and common on Earth, its geological importance is not immediately obvious (i.e., it is not normally considered as a major rock-forming element).  In fact, my trusty geological dictionary has just one entry for lead:

Lead–lead dating: A radiometric dating method based on the proportion of radiogenic ²⁰⁷Pb and ²⁰⁶Pb, the former of which accumulates six times more rapidly than the latter.

Lead actually features in additional radiometric dating systems (e.g., uranium–lead and thorium–lead), but I want to stick with this lead mother/daughter system in this postcard. The most important use of the Pb–Pb dating system is to determine the age of the Earth. As I have pointed out in a previous postcard, the age of the Earth—or indeed any planet—must be considered as one of its most fundamental properties.

A hand sample of galena, the most common lead-bearing mineral on Earth.
Credit: Fabre Minerals

As element 82, lead has 82 protons and 82 electrons, but its number of neutrons varies such that there are four naturally occurring stable isotopes of lead on Earth (i.e., ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb). Indeed, three of these isotopes (²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb) are the 'daughter' products that result from the natural radioactive decay of particular uranium and thorium isotopes. As time goes on, the final decay products of these sequences (i.e., the lead isotopes) accumulate at a constant rate and the ratio of this radiogenic lead to non-radiogenic lead (i.e., ²⁰⁴Pb) increases. As such, the age of a geological specimen can be determined if two factors are known: the initial radiogenic lead to non-radiogenic lead ratios, and the present-day ratios. Furthermore, if the sample has remained a closed system, a graph of ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb should form a straight line.

Clair Cameron Pattern famously applied this Pb–Pb dating technique to various meteorites in 1956. He measured the lead ratios of stony and iron meteorites so that he could determine the age of the planetesimals from which they originated. The dense iron cores of planets are depleted in uranium and thorium (because they tend to stay with silicon rather than iron in rock-forming processes), whereas the more rocky parts of planets (i.e., crusts and mantles) have greater concentrations of these elements. The iron meteorites Patterson dated were pieces of planetesimal cores, and the stony meteorites derived from the outer layers of these bodies.

A piece (few centimetres across) of the Canyon Diablo meteorite, samples of which Patterson used to determine the age of the Earth. Credit: Meteorites Australia

The iron meteorite U/Pb measurements were so low that no radiogenic decay was detected. These isotopic values therefore represent the primeval lead isotope composition of the solar system. In contrast, the stony meteorites had very high ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb ratios. Put together, all these results define an isochron, the slope of which gives an age of 4.55 billion years for the meteorites. Furthermore, Patterson measured the isotopic composition of pelagic sediments that had been collected from Earth's ocean floor. The lead isotope values of these terrestrial samples plotted on top of the meteorite isochron, which indicates that Earth and the meteorites have the same age.

The lead-lead isochron obtained by Patterson (1956) to determine the age of the Earth.

In addition to being widespread, lead is relatively easy to extract from its ores, highly malleable, and easy to smelt. As such, it has been used by humans through the millenia. Metallic lead beads dating to 6400 BCE have been discovered in Turkey. Lead was used, along with antimony and arsenic, in the early Bronze Age. And the Romans commonly used lead in their plumbing systems and building structures. Amongst its modern uses, lead is often found in battery electrodes. As well being an essential player in the determination of our Earth's age, it is clear that lead has been—and continues to be—a particularly versatile element for human activities. 

Sacks wrote shortly before his death that he was sad he would not witness future breakthroughs in the physical and biological sciences. Whatever those breakthroughs turn out to be, however, I am certain that lead will play a role in at least a few of them. I would be happy, therefore, to send a piece of lead into space as a representative of Earth. For it exemplifies the human ability to utlilise and manipulate the natural resources we find around us. And although we, as a race, are poorer for no longer counting Sacks as one of our number, we are richer for the wisdom he left behind.

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.

Dark new horizons shed light on an old Earth

My last postcard was about context. In that postcard, I explained how MESSENGER's exploration of Mercury has helped us learn more about the planetary neighbourhood in which our Earth sits. And for this latest offering, I want to follow a similar theme. But first, we need to take a pretty huge leap (about 5.85 billion km) across the Solar System. Where we will find ourselves in the vicinity of Pluto.

Pluto—once famous for being the ninth and most distant planet from the Sun—is now more famous for being the planet that isn't a planet. Following its discovery in 1930, Pluto—which has a diameter of about 2,300 km—enjoyed more than 75 years at the planet level of the Solar System hierarchy. But in 2006, members of the International Astronomical Union (IAU) decided to demote Pluto, and assign it a new status as a dwarf planet. This decision was prompted when it became clear that Pluto is just one of many large, Sun-orbiting icy bodies in the outer Solar System. The astronomers therefore decided to officially define the term planet, specifically so that Pluto (and other bodies like it) would be excluded from this class.

Photographic plates used for the discovery of Pluto. The arrows mark Pluto's position. Pluto clearly moved against the background of stars in the six days between the two observations, which were made by Clyde Tombaugh in 1930. Credit: Lowell Observatory Archives

Under the IAU's new formal definition, a planet must meet three requirements:

1. The celestial body must orbit the Sun.
2. The body must have a large enough mass to give it a nearly round shape. 
3. The body must have cleared the neighbourhood (of other material) of its own orbit.

Unfortunately—for Pluto at least—the former ninth planet could not meet this third requirement. And a global public outcry—which continues today—followed.

The International Astronomical Union's decision to reclassify Pluto and strip it of its planet status hit the headlines in 2006 and caused a huge public outcry.

But the question of Pluto's planethood is currently being pushed aside, as the level of excitement surrounding NASA's New Horizons mission rapidly grows, prior to the spacecraft's Pluto fly-by. New Horizons—first launched in 2006—is the first spacecraft to visit Pluto and its system of five known moons (Charon, Styx, Nix, Kerberos, and Hydra). The probe will not go into orbit around Pluto, but will instead zoom by a week from now, on 14 July 2015. The fly-by will only last about eight or 10 hours, but at its closest approach the spacecraft will be about 12,500 km from the surface of Pluto. 

Photograph of Pluto and its five moons taken with the Hubble Space Telescope in 2012. Credit: NASA, ESA, and L. Frattare (STScI)

The scientific payload of the spacecraft consists of seven instruments that were chosen so that the geology, surface composition and temperature, and atmospheric characteristics of Pluto and its moons could be investigated. The bulk of the scientific data will be obtained during a period of about 24 hours around the time of the fly-by. The best pictures should reveal features as small as 60 metres across on Pluto's surface.

Map of Pluto released by the New Horizons team on 7 July 2015. The map was created from images obtained with the spacecraft's Long Range Reconnaissance Imager (LORRI) instrument, which were combined with low-resolution colour data obtained with the Ralph instrument. The map clearly shows an intriguing pattern of bright and dark markings on Pluto's surface. The brightest region may contain fresh deposits of methane, nitrogen, and/or carbon monoxide frost. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

This newest installment in the history of human exploration of the Solar System is all very nice, but how is it relevant to my original brief? How can learning about this far-distant world help us convey the uniqueness of Earth to a hypothetical alien planetary geologist? Well, if all goes to plan, then the fly-by of Pluto will not be the end of the New Horizons mission. It  should just be the end of the beginning. Pending approval from NASA for an extended mission, New Horizons will be sent on an onwards journey to study another Kuiper belt object.

The Kuiper belt is a region that extends outwards from the orbit of Neptune for about 20 AU (astronomical unit, equal to about 150 million km). It is similar to the asteroid belt (which lies between the orbits of Mars and Jupiter), as it contains many—relatively small—bodies that are remnants from the formation of the Solar System. Most Kuiper belt objects are icy bodies, composed mainly of substances such as methane, ammonia, and water. Pluto is the largest known object in the Kuiper belt, but about 100,000 objects (with diameters of more than 100 km) are expected to exist in this region, and more than one thousand have been discovered since 1992.

The path of the New Horizons spacecraft (yellow line) through the outer Solar System and the Kuiper belt. The orbits of the planets are shown in blue. The largest Kuiper belt objects are labelled. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

By studying Pluto and its Kuiper belt companions, we can potentially learn about two aspects of Earth's earliest history. As remnants of the Solar System's formation, the Kuiper belt objects are seen as akin to planetary embryos or protoplanets. Planetary accretion is believed to begin with the condensation of solids from the gas cloud that surrounds a star. Accretion of gas and dust then produces bodies that have diameters of 1–10 km, which are known as planetismals. The Kuiper belt objects can help us understand this early accretionary stage and thus what processes went into building our Earth. Violent impacts that occurred during the stage of runaway growth allowed the many planetismals to coallesce and form the large planets we know today.

In addition, the Kuiper belt is thought to be the region from which most short-period comets (i.e., those with orbits of less than 200 years) originate. As the European Space Agency's current Rosetta mission has wonderfully shown, comets are intriguing bodies in our Solar System. Indeed, many scientists believe that comets may have contributed a significant proportion of Earth's water inventory. In a previous postcard, I discussed how a Jupiter-family comet—which probably originated in the Kuiper belt—has a water signature that is a good match for that of Earth. 

New Horizons therefore provides us with a great opportunity to get to know the Kuiper belt better and to potentially understand the building blocks of Earth just a little bit more. But furthermore, the Kuiper belt may provide a big clue to any alien astronomers of our Earth's existence. Neptune, as a giant gas planet, exerts a great gravitational force on the cloud of dust that surrounds it in the Solar System (which includes the Kuiper belt). The gravity tugs on this cloud of dust and creates a distinctive ring structure. Computer simulations show that this ring contains a gap where Neptune itself resides. So even if the alien astronomers cannot directly image the planets of our Solar System from afar, they might be able to detect Neptune's presence. As such, they would know that our Sun possesses a planetary system, and we on Earth are here to be found.

Computer simulations show what the Solar System might look like to an alien astronomer. The gravity of Neptune creates this distinctive ring structure in the dust cloud. The planet itself resides in the gap that can be seen as the dark area in the right of the image. Credit: NASA/Goddard/Marc Kuchner and Christopher Stark

Requiem

Lacrimoso

Today will see the end in the life of a dear friend. A life in which I am proud to have played a small part. At the beginning of 2011 I moved across the Atlantic Ocean to Washington, D.C., to start a new job—working on the science team of NASA's MESSENGER mission. That move was one of the best decisions I have ever made. But later today, after more than four years in orbit around Mercury, and over 10 years in space, the mission is about to come to a very conclusive end. The spacecraft is now, well and truly, out of fuel. It will crash into the planet and it will form a new crater in the already pocked surface. But as I mourn the loss of our spacecraft, I can look back with pride and celebrate the wonderful achievements of this groundbreaking mission.

Artist's impression of the MESSENGER spacecraft in orbit around Mercury. Credit: NASA

MESSENGER—an acronym for MErcury Surface, Space ENvironment, GEochemistry, and Ranging—has been the first spacecraft to orbit Mercury, the innermost planet of our solar system. After more than four years of studying Mercury from orbit, MESSENGER has completely transformed our understanding of the planet. Back in the 1970s, Mariner 10—the only other spacecraft to have visited Mercury—made three flybys of the planet. Although Mariner 10 led to several important discoveries, substantial gaps were left in the Mercury cannon. Less than half the planet, for instance, was imaged up close by Mariner 10.

Mariner 10 image showing part of the Caloris basin (left), the largest well-preserved impact basin on Mercury. The basin has a diameter of about 1,550 km and its full extent was realized only during the MESSENGER mission. Credit: NASA

Following Mariner 10, many scientists believed that Mercury was geologically similar to the Moon, and therefore not worth an expensive and extensive follow-up mission. But a committed and insightful group of scientists and engineers, led by Principal Investigator Sean Solomon, were not so easily placated. They believed that Mercury could not be so easily dismissed and they set about making their MESSENGER dream a reality. The MESSENGER mission concept was finally accepted as the seventh of NASA's Discovery-class missions, in July 1999.

Several engineering challenges are presented in designing spacecraft to orbit Mercury. In addition to the extreme heating conditions the spacecraft must endure, the Sun's huge gravitational pull is a major issue. To enter orbit around Mercury, the spacecraft must be captured by the gravity of Mercury itself, which is tiny in comparison with that of our parent star. So the clever rocket scientists came up with a solution. Instead of sending the spacecraft on a direct course to Mercury, MESSENGER took a particularly circuitous route into the inner parts of the Solar System. To be captured by Mercury's gravity, MESSENGER's speed needed to be dramatically reduced as it approached the planet. But a body moving towards the Sun will be constantly speeding up. Of course, spacecraft thrusters (i.e., brakes) can be fired to reduce the velocity, but this requires a tremendous amount of fuel, and massively increases the weight and cost of launching the spacecraft from Earth. 

The gravity fields of the inner planets were therefore used as an alternative, natural, braking system. After MESSENGER was launched from Cape Canaveral on 3rd August 2004, the spacecraft undertook a series of 'gravity assist' flyby manoeuvres, which were designed to reduce its velocity. A year after launch, MESSENGER performed its first flyby, of Earth, on 2nd August 2005. Next up were two flybys of Venus in 2006 and 2007. Then in 2008 and 2009, MESSENGER made another three flybys, this time of Mercury itself, before it finally entered orbit on 18th March 2011.

 

The Earth, our home, as seen by MESSENGER during its gravity assist flyby on 2nd August 2005. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

It is images such as this one of Earth taken by MESSENGER, that remind us of the power of comparative planetology. Even with the fantastic capabilities of remote sensing, as exemplified by MESSENGER and other planetary satellites, there are certain geological investigations that can never be achieved if you do not have physical contact with a planet. The study of Earth, and the comparison of its geological features with those we observe on Mercury (and other planets), is therefore a vital part of our planetary science investigations. But furthermore, by studying Mercury (the end-member of the Solar System), we also gain a more thorough understanding of the neighbourhood in which our Earth sits.

My role in the MESSENGER mission, was as a postdoctoral fellow at the Carnegie Institution of Washington's Department of Terrestrial Magnetism. I worked with Larry Nittler on the analysis of data from MESSENGER's X-Ray Spectrometer (XRS), through which we are able to learn about the geochemical makeup of the planet's surface. In our first MESSENGER XRS paper, we analyzed data from the first three months of the orbital mission. These data provided the first glimpse of Mercury's major element composition, and showed us that Mercury's surface is not as like the Moon (or typical parts of the Earth's crust) as had previously been thought. 

Maps of magnesium/silicon and thermal neutron absorption across Mercury's surface, as measured with MESSENGER's X-ray and Gamma-Ray Spectrometers. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

It is these geochemical findings that allow theories of Mercury's formation to be constrained. In particular, scientists have long puzzled over the reason for Mercury's particularly high density (i.e., it has a disproportionately large core). Some scientists believe that the outer (and less dense) parts of Mercury were obliterated during a huge impact event early in the planet's history. The MESSENGER geochemistry results, from the XRS and the Gamma-Ray Spectrometer, however, have revealed that Mercury is not depleted in a group of chemical elements known as volatiles. These elements (including sulfur, sodium, and chlorine) should be lost (evaporated) during the heating that would have been associated with such a massive impact event.

It is more likely, think other geologists, that the major-element composition of Mercury is much more indicative of the original materials which accreted to form the planet. Perhaps those original materials had distinctive compositions, unlike the materials that built the other planets in the Solar System. In that original Science paper, we proposed materials akin to enstatite chondrite meteorites as the potential building blocks of Mercury. The jury is still out on what those precursor materials may have been. And in all likelihood, those materials may no longer exist and may not be present in our meteorite collection. But by studying Mercury in depth for the first time with MESSENGER, we have learned about the full diversity of the Solar System.

So this postcard isn't about sending a single rock from Earth to the alien planetary geologists. It is about the much bigger picture. For those aliens to really understand our wonderful home, they need to see Earth in the context of its planetary brothers and sisters. By sending spacecraft to visit Mercury, Venus, Mars, as well as the outer planets and moons of the Solar System, we are building up a panoramic postcard of our whole family.

Thank you MESSENGER for playing your part perfectly in that endeavour. You served us well and you will be missed.

The Earth and Moon, taken from the MESSENGER spacecraft at Mercury. The Earth is the bright object in the bottom-left of the image. The Moon is the smaller and fainter spot to its right. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Finis