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

A peace of Earth offering

In recent weeks this interesting science story has caught my attention. A group of scientists has been tasked with deciding whether we are in the midst of a new geologic epoch, and if so, what historical event can be used to mark its beginning. The potential new epoch is known as the anthropocene—the 'human epoch'—and is defined as the period in which human activities have had a significant impact on the Earth and its ecosystems.

Several sensible options have been put forward as possible start points for the anthropocene. These include the invention and rise of agriculture about 10,000 years ago, or the start of the Industrial Revolution in the 18th Century. It seems, however, that these choices have lost out to the might and power of an atomic option.

A group of researchers, led by Jan Zalasiewicz at the University of Leicester, have proposed that the new epoch began with the dawn of the atomic age. They suggest that the first nuclear bomb test, on 16th July 1945 in the Jornada del Muerto desert of New Mexico, marks the moment in time when humankind left its first permanent and global imprint on Earth.

Rising fireball and forming mushroom cloud, nine seconds after the world's first atomic bomb was detonated on 16th July 1945 in New Mexico. Credit: U.S. Department of Defense

Earth's geologic timescale is divided into units of various lengths that stretch back to the planet's formation about 4.54 billion years ago. Any given eon, era, period, or epoch consists of characteristic rock strata that can be distinguished from those that are directly above (younger) and below (older) it. Often, these stratigraphic boundaries are marked by major compositional or paleontological changes, such as mass extinctions. If the anthropocene is to be added to the top of Earth's stratigraphic column, it therefore makes sense to identify its beginning with a similarly global signature.

The geologic timescale. Credit: Science Education Resource Center at Carleton College

Luckily, as Zalasiewicz and colleagues point out, the nuclear tests that took place between 1945 and 1963 caused an unambiguous and detectable change in the Earth's atmosphere. The nuclear explosions were the primary source of man-made radionuclides (i.e., radioactive isotopes) found in the atmosphere. The most abundant of these nuclides—caesium-137—has no natural sources and is the product of nuclear fission processes. The first pronounced increase in the atmospheric concentration of Cs-137 occurred in 1954, and there was an additional peak in 1963. Man-made radionuclides therefore represent the best chronological markers for the atomically-heralded anthropocene.

Fallout of anthropogenic radionuclides (including caesium-137). Credit: Hancock et al., 2014, Geol. Soc. London

I think it is pretty obvious that a piece of the anthropocene Earth should be sent into the cosmos as our next geological postcard. This period, in which humans have begun to leave an indelible mark on our planet, is uniquely representative of Earth's capacity to support intelligent life. Caesium-137, however, with a half-life of only 30 years, is not a great choice. It will not be able to journey very far into the vast expanse of the universe before it decays away to almost nothing. So instead, I'd like to send an actual rock that is a direct product of the first nuclear test.

After the bomb detonation on 16 July 1945, a glassy residue was left on the desert floor at the 'Trinity' test site. The glassy deposit is composed mainly of feldspar and quartz-rich sand grains that were melted during the blast. This rock—known as trinitite, after its type locality—is usually light green in colour, and is mildly radioactive.

A trinitite hand specimen, produced by the first nuclear bomb detonation. Credit: Paul M. Schumacher

 

I like to think that our hypothetical alien planetary geologists are a friendly race and would extend a hand of peace if we ever encounter them. As evidenced by the anthropocene, we are now able to alter the workings of our own planet. So let those aliens beware the destruction we would unleash—Independence Day style—if they turn out to be not quite that welcoming. A piece of trinitite should therefore serve as a suitable example of the awesome and destructive power humankind can wield when we put our mind to it.