This time, it's quantity that counts

If we were to rank all the minerals within Earth by their abundance, and use that ranking to pick a candidate for our interplanetary adventure, then we would have an easy winner. This mineral—which makes up about 38% of the Earth's volume—is thought to be the most abundant on our planet. It has been known for some time that this silicate mineral—(Mg,Fe)SiO₃—has what is known as an orthorhombic ABO₃ perovskite structure. Until very recently, however, it did not actually have a name. But now, based on work described in a new study published a few weeks ago in Science, the first detailed characterization of this long-known mineral phase has been conducted, and the mineral has finally been named.

The 'ABO₃ perovskite structure'.  Credit: T. Imai et al./NTT Photonics Laboratories

Through the study of seismic waves, i.e., the energy generated by earthquakes, scientists can learn about Earth's interior structure. At certain depths—known as discontinuities—below the planet's surface the propagation of these waves changes abruptly. It is these changes in seismic wave velocity that help us build up a picture of the Earth's innards, in which a solid inner core, a liquid outer core, the lower mantle, and the upper mantle lie beneath the crust.

Interior structure of the Earth, showing the dominant mineral species in each layer.
Credit: P. Huey/Science

As I discussed in a previous postcard, getting samples from deep within the Earth is not easy. Such materials normally take the form of diamonds or xenoliths, and tend to come from the upper mantle. But most material from the lower mantle—the region where our previously elusive mineral resides—does not survive the trip to the Earth's surface, and is therefore not readily accessible for geologic study. A combination of very high-pressure experiments, thermodynamic calculations, and first-principle modelling studies are therefore required to discern the major mineral constituents in each of these unreachable layers.

Official mineral names are approved by the International Mineralogical Association. But to suggest a name for a mineral through these official channels, its structure must be characterised from a naturally occurring sample. And this is the reason why our abundant mineral from the lower mantle has remained nameless for so long. In the absence of a suitable sample, researchers have been unable to conduct the required structural characterizations and therefore no name could be assigned. 

But in the recently published work by Oliver Tschauner (University of Nevada, Las Vegas) and colleagues, a meteorite sample (Tenham) has come to the mineralogical nomenclature rescue. The authors have studied part of this chondrite, which fell and was then recovered from a remote part of western Queensland, Australia, in 1879. During a brief impact event, the parent body (i.e., an asteroid) of this meteorite experienced a period of very high pressures (more than 25 GPa) and temperatures (about 2500 K). These conditions are comparable to those in parts of Earth's deep interior. During this impact, some material melted and formed so-called shock veins. It is within these veins that Tschauner et al., found clasts of the 'missing' mineral to study.

Having found the required sample they next undertook a series of synchroton micro-X-ray diffraction and electron probe microanalysis measurements to definitively determine the mineral's crystal structure and composition. Their results were sufficient to meet the stringent requirements of the International Mineralogical Association, and the authors were finally able to propose a name for Earth's most abundant mineral. The name they settled on—bridgmanite—was chosen to honour the Nobel Prize laureate Percy W. Bridgman (1882–1961). His serendipitous machinery malfunction led to pioneering work in the field of high-pressure experiments, which paved the way for the field of modern-day mineral physics and our understanding of Earth's interior.

Percy W. Bridgman. Credit: Smithsonian Institution

The Tenham meteorite—together with the newly named bridgmanite it contains—is yet another example of a geologic specimen I am choosing for our cosmological journey, which is not quite of the Earth. But with a hefty 38% of the mineralogical vote, brigmanite is a serious contender that cannot be ignored. So until we devise a way to get a terrestrial sample of Earth's most abundant mineral from the lower mantle itself, this meteorite will just have to do.

A thin section from the Tenham meteorite, from which newly-named bridgmanite was identified and characterized. Credit: Chi Ma

Planetary poetry

The inspiration for this blog—the 'Golden disk' on Voyager—is not the only example of a human message sent out on an interplanetary journey. The early days of space exploration were filled with declarations from us humans here on Earth. For instance, both Pioneer 10 and Pioneer 11 in the early 1970s (sent to explore the outer planets and leave the solar system) featured gold-anodized aluminium plaques designed by Carl Sagan. These plaques show illustrations of nude men and women to represent the human race, as well as other information, in case the spacecrafts were ever intercepted by extraterrestrial life.

Carl Sagan holding the Pioneer plaque. Credit: www.daviddarling.info

Apollo 11—probably the most famous space mission of all—included a plaque that was bolted onto the lower part of the Eagle Lunar Module. This landing stage still sits on the Moon and can even be seen in modern-day Lunar Reconaissance Orbiter Camera images.

The landing stage of Apollo 11's Eagle Lunar Module can still be seen in images from NASA's Lunar Reconnaissance Orbiter Camera. The flight hardware is at the centre of this image, with its shadow to the left. Credit: NASA/GSFC/Arizona State University

The Apollo 11 plaque reads as follows:

Here Men from the Planet Earth First Set Foot Upon the Moon

July 1969 A.D.

We Came in Peace for all Mankind

Apollo 11 plaque attached to the ladder of the Lunar Module. Credit: NASA

But another, much less formal, but equally enduring and touching message was left also on the Moon by the astronaut Gene Cernan. Cernan was the commander of Apollo 17 and the last man—to date—to have walked on the Moon. He writes in his autobiography of the small way in which he honoured his daughter during his final moments on the lunar surface:

"... I drove the Rover about a mile away from the LM [Lunar Module] and parked it carefully so the television camera could photograph our takeoff the next day. As I dismounted, I took a moment to kneel and with a single finger, scratched Tracy's initials, T D C, in the lunar dust, knowing those three letters would remain there undisturbed for more years than anyone could imagine."

Just this week, NASA's MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft has entered into orbit around Mars. This mission will be the first to study the upper atmosphere of the Red Planet, and how it has evolved with time. As part of the mission's education and public outreach activities, the University of Colorado ran a public competition. In this contest, people young and old were invited to write a haiku that could be sent along with the poet's name onboard MAVEN to Mars. This type of programme is a great way to inspire children to think about science—and poetry—and I even submitted an entry myself.

MAVEN, you raven

pray, tell, with your expert ways

is Mars life's haven?

This was actually my first attempt at haiku, and I thought not a bad first effort. It even gained the approval of my talented poet friend who had first brought the competition to my attention. Unfortunately, however, it didn't make the final cut. The winners can be read here, and my favourite is probably this one by Greg Pruett:

distant red planet

the dreams of earth beings flow

we will someday roam

I haven't picked a piece of Earth today to represent our planet to unknown aliens, but these poems, plaques, and traced initials are all beautiful examples of the ways in which we humans try to communicate our place in the universe. As MAVEN starts its orbital mission, I hope it succeeds in unraveling some of Mars' atmospheric mysteries. Perhaps we will learn if our planetary neighbour could ever have supported intelligent life, and what caused its evolution to diverge so drastically from that of our own Earth.

Artist's conception of the MAVEN spacecraft in orbit around Mars. Credit: NASA/Goddard

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.

Bedrocks in the city that never sleeps

This past weekend I made the short trip from Washington DC up to New York. I went to visit friends and to spend some time enjoying the city itself. But as I sat on the bus and departed New York on Sunday evening there were tears in my eyes. You see, for the past three and a half years while my body has been based here in DC, my heart has lain firmly in the grasp of Manhattan. As it has since the first time I visited New York back in the summer of 2008. On that trip I was fortunate enough to stay in my friend's south-facing Harlem high-rise apartment, which had this for a view:

View south from Harlem.

On my first evening in the city we made a short foray into the Bronx and relaxed on a rooftop with some delicious mojitos. My love affair with New York City had truly begun. Since that week, it has been my ambition—sometimes a nagging one at the back of my mind and sometimes the overriding purpose of my day—to live and work in the crazy confines of the amazing metropolis.

In the next few weeks, however, I will be moving back across the Atlantic Ocean to London, my home. As much as I am looking forward to living again in my first city-love and to starting the next chapter of my life, I am sad that I will no longer be within arm's reach of New York. I don't know when I'll next be able to walk its streets, relish its anonymity, and savour the familiarity of all its sights, smells, and sounds. My ambition remains unfulfilled. Those were the reasons for my tears.

After that rather sappy introduction, I should explain how my feelings for New York are relevant to this postcard from planet Earth.

In July 2010 I had a week-long New York visit, primarily for the the Meteoritical Society's annual meeting. Before the conference started, however, I was spending some time with my friend (whose Harlem apartment I stayed in, and with whom many of my New York stories are entangled)—we'll refer to her as 'Mrs Cowboy' (I'm repaying the favour here after I was featured on her blog a while back)—in Brooklyn. As we sat on a park bench in Brooklyn Heights gazing over the East River to Manhattan our conversation meandered around. At one point, I posed the question as to why the Manhattan skyscrapers have a bimodal distribution. Both the Financial District downtown and the landscape of midtown are dominated by buildings reaching up for the heavens, but the architecture between them is distinctly challenged in the vertical dimension.

The twin peaks of downtown and midtown Manhattan. Credit: Jason Barr

Mrs Cowboy, a seasoned New Yorker, didn't have a particularly good answer and neither did I at the time. But I had a geologist's hunch that maybe it was something to do with the rocks that lie beneath. After all, those huge buildings must need some almighty foundations. Then one afternoon during the conference I participated in a lovely and informative tour of Central Park with a local geologist, Sidney Horenstein. I learnt all kinds of interesting facts about the city's geology, including an answer to my skyscraper question.

Many of Manhattan's bedrocks consist of original sediments (clays and muds) that underwent major mineralogical changes due to metamorphism. The actions of heat and pressure during the Taconic and Acadian orogenies (mountain-building episodes) changed the original sedimentary rocks to metamorphic rocks. This took place between about 400 and 500 million years ago, during the Devonian and Silurian geologic periods, and was part of the construction of the ancient supercontinent Pangaea. As Earth's tectonic plates shifted and jostled for position, the sedimentary strata were squeezed and squashed. The resulting metamorphic alteration created new rocks, including what is known as the Manhattan Schist. Today, these formations are not nice flat strata, but rather they have been folded to form a series of anticlines and synclines, or peaks and troughs.

A folded outcrop of the Manhattan schist (with a pegmatite intrusion) in Central Park.

One of these synclines dips down below the surface between the approximate city levels of Washington Square Park and Chambers Street, i.e., the gap between the midtown and downtown skyscrapers. This syncline was filled during the Pleistocene ice ages with much softer glacial sediments. So I had been correct with my geologist's hunch. New York's geology really does play a vital role in shaping the city's architecture. The ancient metamorphic bedrocks are sturdy and strong enough to support the huge skyscraper structures, but the younger and less consolidated glacial deposits just can't do the job.

A beautiful old illustration from A Geological History of Manhattan or New York Island by Issachar Cozzens. The geologic profile shown in the middle panel depicts some of the folded rock formations that make up the island of Manhattan.

I definitely believe that one of the most characteristic things about our Earth is its capacity to sustain complex and intelligent life. Great cities are amazing examples of the engineering feats that human civilization can achieve. So I can't think of of a better way to represent the human race, and Earth, to fellow sentient beings than with a piece of Manhattan's bedrock. For it is on these solid foundations that the (perhaps) greatest city of them all has been built.

The view from the Staten Island Ferry on Sunday. Midtown's Empire State building peaks through the urban canyons of downtown, just to the right of the Freedom Tower.

Journey from the centre of the Earth

One of the questions I am most frequently asked by non-planetary scientists is "How did the Moon form?". My answer invariably involves an explanation of what has become the most commonly accepted scenario and is known as the 'giant impact hypothesis'.

The early history of our solar system was pretty violent. The amount of dust, debris, and projectiles flying around the Sun would have made collisions and impacts a common occurrence. In the giant impact hypothesis scenario, it is thought that an almighty collision between an early-formed (or proto-) Earth and another Mars-sized body—known as Theia—took place approximately 4.5 billion years ago.

Artist's impression of the hypothesized Moon-forming giant impact event between the proto-Earth and a smaller body known as Theia. Credit: NASA

The immense amount of energy associated with this event would have been enough to blast apart the impacting Theia and to melt a significant amount of the proto-Earth. The remaining pieces of the obliterated Theia would have been hurled into space and subsequently re-coalesced to form our beloved Moon in orbit around Earth. Computer simulations of the giant impact are varied, but most suggest that Theia impacted Earth at an oblique angle. They also predict most of the material that accreted to form the Moon (more than three quarters) was derived from Theia. The rest of the Moon should therefore have been built from proto-Earth material.

An example giant impact computer simulation. Within the first six hours after the collision, material is starting to form the Moon. Colours represent temperature; red regions would have been heated the most due to the energy of the impact and would have experienced the most melting. Credit: R. M. Canup 2004, Icarus

This hypothesis, at face value, is pretty simple to explain and easy to grasp. In any conversation with a non-specialist, however, I tend to leave out several of the problems that are associated with the scenario. And to understand at least one of those, we need to get a bit more technical and talk isotopes. Many of my previous postcards have referred, implicitly or explicitly, to isotopes and their usefulness in several aspects of geology. And I am sure they will continue to crop up, so I thought this was a good a chance as any to actually explain what they are.

We can start with a dictionary definition:

forms of the same element that contain equal numbers of protons, but different numbers of neutrons in their nuclei and hence differ in relative atomic mass, but not in chemical properties

We can also illustrate this with some helpful cartoons. In this case we will focus on oxygen isotopes because they are relevant to this postcard. The most common naturally occurring oxygen isotope—oxygen-16, or ¹⁶O—constitutes more than 99% of any given quantity of oxygen. It is known as ¹⁶O because it has 16 particles in its nucleus: eight protons (positively charged particles) and eight neutrons (particles with no charge).

Cartoon of an oxygen-16 atom. There are eight protons (red) and eight neutrons (green) in the nucleus, which is surrounded by eight orbiting electrons (blue).

The two remaining isotopes are ¹⁷O and ¹⁸O, which contain an extra one and two neutrons in their nuclei, respectively. All isotopes of a single element have the same number electrons and they therefore exhibit the same chemical characteristics. It is only the mass of the isotopes that changes, due to the variable number of neutrons.

Cartoons of oxygen-17 (left) and oxygen-18 (right) isotopes (note the additional neutrons).

So after that chemistry lesson we can get back to the story...

Based on measurements of rocks from the Earth, Mars, and several asteroids, scientists have long thought that the isotopic signatures (e.g., the relative amounts of the three oxygen isotopes) of planetary bodies within the early solar system were variable. They therefore expect Theia and the proto-Earth to have contained different proportions of ¹⁶O, ¹⁷O, and ¹⁸O. If that were indeed the case, and because the majority of Theia ended up in today's Moon, the relative amounts of the three oxygen isotopes that we measure today in lunar and terrestrial rocks should be different. But up until now such differences have not been detected.

In a new study led by Daniel Herwatz at the University of Göttingen, however, small differences between the oxygen (and titanium) isotope abundances in rocks from the Earth and the Moon have now been found. The authors claim that their precise measurement results are therefore evidence to support the giant impact hypothesis.

I don't want to focus here on the details, issues, or implications of their work, but rather on the type of rocks from Earth that they chose to analyse. Herwartz and coauthors compared the composition of basalts from three of the Apollo landing sites with rocks that they decided best represented Earth (sound familiar?). Or at least the most primitive, unaltered part of the Earth. And for this they chose rocks known as mantle xenoliths.

These xenoliths are pieces of Earth's otherwise inaccessible mantle (the region between the crust and core). They are most commonly rocks known as peridotites, which are made up almost entirely of the ultramafic minerals (silicates that have high abundances of magnesium and iron) olivine and/or pyroxene. Xenoliths are brought to the Earth's surface when they are encapsulated and entrained inside other rocks, which crystallize from magmas that originated in the (upper) mantle. Kimberlites—common diamond-bearing rocks—are an example of this geological transportation system. They made the journey from the (almost) centre of the Earth and often contain several samples of this precious mantle cargo. 

My very own little piece of the mantle, a pretty green peridotite.

Mantle xenoliths are probably the most valuable samples we have for understanding the fundamental composition of the Earth. If they have journeyed this far already, I propose that at least one makes the next leg towards the stars and represents the whole Earth to those alien planetary geologists.

Impacting young minds

I've been quiet on the blogging front recently, but I've been pretty busy behind the scenes. This has included engaging in what I see as one of the most important (and fun) parts of my job as a research scientist: outreach.

One of my close friends teaches first grade at Somerset Elementary School in Montgomery County, MD, and she arranged for me to be one of the speakers at their recent Careers Day. I found myself sharing a stage with people from a host of different professions and careers. There was a salad dressing maker, an events organizer, an investment banker, an advisor to President Obama... and me—the planetary geologist.

I was given three 20-minute slots to speak to students who had chosen to hear what I had to say about space. And I was happily surprised that many of these children were eager to ask all kinds of questions. I was even more impressed that some of their more knowledgeable (or geeky) peers were able to step in and proffer answers of their own.

I used my time to show a PowerPoint presentation (full of fun images of space, volcanoes, and such) that I had put together for the occasion, and to let the children have an interactive experience with the MESSENGER postcard mosaic. This 'game' is a great way for kids (and adults) to learn about different features on Mercury and to think about how planetary scientists map the entire surface of another planet.

Fun images depicting aspects of planetary geology.
Credits: Alexander Belousov, Earth Observatory of Singapore, NASA, NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

I also took some rock samples to pass around, including an example of each of the three different types of meteorites: a stony meteorite, an iron meteorite, and a stony-iron meteorite. It is these kinds of rocks that help us understand the interior structure of rocky planets. They come from proto-planets or asteroids that were at some point blasted apart. Before their violent demise, however, these bodies experienced similar evolutionary processes to the planets of our solar system that we know and love. Stony meteorites (in this case represented by an ordinary chondrite) are the equivalent of the exterior crusts of planets.

An ordinary chondrite (about 7 cm across) that was found in Romania. Ordinary chondrites are the most common type of meteorite found on Earth. Credit: ASU/CMS

Iron meteorites—consisting almost entirely of iron and nickel—represent the dense cores of planets, and the stony-irons (or pallasites) come from the boundary between the rocky outer layers of a planetary body and the metal-rich core, known as the core-mantle boundary.

Example of a pallasite (Springwater). These beautiful meteorites contain orange grains of the mineral olivine within a matrix of shiny iron-nickel. Credit: KD Meteorites

The iron meteorite that I had available was a one of a group of samples known as Canyon Diablo. These are the small pieces that remain after their much larger (about 50 m in diameter) parent chunk of space debris smashed into Earth about 50,000 years ago and created the Barringer Meteorite (aka Meteor) crater in Arizona (click here for a 3D-flyover of the crater). This well-preserved impact crater is just over 1 km in diameter and continues to be a site of scientific investigations.

A single piece (a few centimeters across) of the remaining Canyon Diablo meteorite.
Credit: Meteorites Australia

Back in the 1950s isotopic analyses on samples of this meteorite group were used to refine the estimate of Earth's age. Radiometric dating has revealed that many different solar system materials have concordant ages, which we use to provide Earth's age. The estimate given by Claire Paterson in 1956 (4.55 ± 0.07 billion years), based on the Canyon Diablo meteorite is very close to the current estimate of 4.54 ± 0.05 billion years. I propose, therefore, that we send one of these iron meteorite pieces forth for our imaginary alien planetary geologists to discover. Afterall, what piece of information about our home planet and our home solar system is more fundamental than its age?

I'm not sure society has the need for all of the 50+ children I met at Somerset to one day become planetary geologists. But I do hope that at least a handful of them will translate their wonder and joy for things space-related to future careers in a science-focused field. I'm proud to play a very small role in shaping the next generation of civil engineers, meteorologists, and brain surgeons of this world.

A thank-you note from one satisfied customer.

Moontalkers

This week I'm attending the 45th Lunar and Planetary Science Conference in Houston, which is basically an annual gathering of the world's biggest space geeks. And today we were treated to some space geek royalty. Not one, but two, of the 12 Apollo 'moonwalkers' spoke to captivated audiences over the lunchtime period.

First, Harrison 'Jack' Schmitt (the only geologist to have walked on another planet) talked about the scientific discoveries that his mission, Apollo 17, has yielded in the subsequent 30+ years. 

Jack Schmitt sampling conducting fieldwork on the Moon during Apollo 17. Credit: NASA/Eugene Cernan

And then we were treated to a fascinating planetary travelogue as Dave Scott, the commander of Apollo 15, challenged the scientists in the room to go forth and work with the engineering community in a synergistic way to design more capable, exciting, and scientifically meaningful lunar (and martian) missions for a new century.

Commander of Apollo 15, Dave Scott, saluting the American flag in 1971. Credit: NASA

So today, instead of choosing a rock from Earth to send forth into the cosmos, I'm including two quotes with which Dave Scott ended his presentation: 

"Man must rise above the Earth—to the top of the atmosphere and beyond—for only then will he fully understand the world in which he lives."

Socrates 469–399 BC

"What was most significant about the lunar voyage was not that man set foot on the moon but that they set eye on the earth." 

Norman Cousins, Cosmic Search magazine, volume 1, number 1, January 1979

These beautifully encapsulate the reason why the hundreds of scientists in the audiences today (including myself) do what we do. For a few years back in the 1960s and 1970s Dave Scott, Jack Schmitt, and all the other Apollo astronauts were our very own human interplanetary messengers. They were sent forth to explore another world, and to bring back pieces of it so that we can learn about our home and its place in the universe. Their mission was not chosen for its ease, but because it was hard. Because that goal served to organize and measure the best of humankind's energies and skills. I think we are more than overdue a renewal of quests such as these. It's time to inspire a whole new generation of scientists and engineers.

A slightly blurry Dave Scott and me in Houston, 17 March 2014.

Today I got to meet one of my heroes. And he reminded us all that our Earth is a beautiful and special place. More wonderful, dare I say, than can ever be represented by one rock.

"The blue marble", taken from Apollo 17. Credit: NASA

Zirvivors

As a planetary geologist studying the surface of the Moon and Mercury, I think daily about old rocks—up to about 4.5 billion years old. These planetary bodies—smaller than our Earth—have ancient crusts, which have not been greatly disturbed or overhauled through their history. They do not have operating plate tectonic systems to create and destroy crust in long and regular cycles. Nor do they (currently, at least) have surrounding atmospheres with active climates to weather and erode surface materials. This means that the rocks on the Moon and Mercury are witnesses to the earliest part of the inner solar system's history—a record that has been almost completely erased from the surface of Earth.

The heavily cratered surfaces of the Moon (left) and Mercury (right) bear the scars of many meteoroid impacts that have occurred during their 4.5-billion-year lifetimes. Credit: NASA

When I first started thinking about this blog and its central theme, it seemed to me that one of the most obvious rocks to best represent the Earth would be the oldest. But deciding on what is truly the oldest terrestrial rock is not as easy as it sounds. Rocks are made up of minerals, and over time, the rocks may be changed through the actions of heat, pressure, and/or chemistry so that the original rock gets broken down. At least some of the constituent minerals, however, can survive and become incorporated into new rocks.

Some of my colleagues at the Department of Terrestrial Magnetism work on the analyses of rocks that are amongst the most ancient found on Earth, but I want to leave those as a subject for another postcard and another day. Instead, I want to focus here on the oldest minerals that have yet been discovered.

New work published this week in Nature Geoscience provides an age for what is thought to be the oldest fragment of material from Earth's crust. This grain is a fragment of the mineral zircon. Zircons are found ubiquitously in all kinds of rocks—igneous, metamorphic, and sedimentary. They are hard (with a value of 7.5 on the Mohs hardness scale), which together with their chemical inertness, means they are difficult to destroy. A very ancient zircon grain can therefore have inhabited a number of different rocks during its lifetime. In the new work conducted by John Valley, from the University of Wisconsin, and coauthors, a zircon grain from a sandstone outcrop in the Jack Hills of Western Australia is shown to be 4.4 billion years old.

Fragment of a 4.4 billion year-old zircon grain (about 0.5 mm in length). Credit: John Valley

Valley and his team used a radiometric dating technique to find the age of this rare mineral fragment. Although zircon is composed almost entirely of the three elements zirconium, silicon, and oxygen, other elements can be incorporated into its mineral structure in very small (trace) amounts as it grows. In particular, they measured the numbers of uranium and lead atoms in the sample. Certain isotopes of uranium decay at fixed rates to form isotopes of lead. If a specific sample has remained a 'closed system', the number of these measured uranium and lead isotopes can be used as a chronometer to tell us the age of the sample.

This zircon fragment dates to the Hadean eon—Earth's earliest geologic period—that was characterized by hot and violent conditions. The grain, along with other slightly younger zircons, is evidence that a solid crust formed soon (geologically speaking) after Earth's formation (about 4.6 billion years ago) and the giant impact event that likely formed the Moon and created an Earth-wide expanse of molten material, known popularly as a 'magma ocean'.

View on Earth during the Hadean eon? Credit: Mark Garlick - Space Art

The majority of Earth's surface may not be as old as what we see on some of our solar system neighbors, but these little zircon pieces from Australia are about as old as we are going to find. If they have survived this long, I think that they should definitely make the celestial trip to meet our alien planetary geologists. Perhaps our hypothetical friends are somewhere 4.4 billion light-years away and can even observe the Hadean Earth firsthand.

Please, sno' more

I'm sure I'm not alone as I happily anticipate the first stirrings of spring here on the US east coast. I'm not a fan of winter. Or snow. Sure, it looks pretty for a while, but then it just gets in the way of every day life. I'm more than ready for this week's thaw.

I was contemplating all this as I sat on the bus yesterday morning, watching the piles of dirty DC snow on the sidewalks rush past me. I realised how thankful I am that we live on an Earth that is not covered with snow ALL the time. But then I remembered, our world hasn't always necessarily been like this.

The grey, gloomy, and snow-covered National Mall; slippery conditions prevail.

If we were to go back 650 million years, and then some, we would probably find an Earth with a very different climate. Geologists have long thought that this ancient period was one of a 'snowball Earth', when the surface was entirely (or almost) frozen.

The term for this hypothesis was first coined by Caltech scientist Joseph Kirschvink in 1992, but the idea of a global glaciation had previously been proposed to explain, for instance, glacial rock deposits in places such as Greenland. These rocks, known as tillite, are formed when glacial till—basically the unsorted crud that glaciers pick up, drag along, and then deposit as they melt—becomes lithified. Finding this kind of material in chilly places like Greenland shouldn't really be much of a surprise. But the rocks in question are old. Very old. And the process of continental drift means that Greenland's current position in Earth's northern latitudes was not its location when these rocks formed.

By studying the magnetism of certain minerals, which capture the direction of the surrounding magnetic field as they form, 'paleomagicians' are able to reconstruct the position of ancient continents around the globe. Such paleomagnetic studies on the tillites from Greenland show that they formed at tropical latitudes, where the Earth receives more (because of Earth's tilt on its axis) of the Sun's warming radiation—hardly where you might expect glaciers to exist.

Tillite deposit in East Greenland. Credit: M. Hambrey

And there are plenty of other distinctive rock types (e.g., banded iron formations and cap carbonate rocks) that date from this same era, which appear to be evidence of a global glaciation. But how could this state of Earth-wide freezing have occurred?

Climate models show that if sea ice advanced far enough towards the equator then a positive feedback system would have been established. The bright albedo (reflectance) of sea ice versus seawater means that more radiation is reflected away from the water in its solid form. So if the area of Earth's surface that was covered by ice increased, more light would have been reflected away and the Earth would have cooled. This would have led to more ice formation and more reflection and more cooling... You get the idea.

How Earth may have looked during a 'snowball Earth' episode.
Credit: geology.fullerton.edu

So if the snowball Earth really did exist, how did the planet get itself into such a state in the first place and how did it manage to escape?

To start the ice formation, obviously there had to have been some kind of large-scale initial cooling event. Options include: a supervolcano eruption that may have thickened the atmosphere and reduced the amount of radiation received from the Sun, or perturbations in the Earth's orbital dynamics (which follow the Milankovitch cycles) that brought the planet into a particularly cold configuration.

And greenhouse gases—namely carbon dioxide and methane—were probably the route for escape from Earth's frozen hell. In much the same way that the build-up of these molecules in today's atmosphere is to blame for global warming, the steady increase of their concentration in our ancient atmosphere could have allowed the huge accumulations of ice to thaw and melt. It has been estimated that about 350 times the concentration of carbon dioxide in today's atmosphere were needed to achieve such a feat. And it seems that volcanic eruptions, occurring over tens of millions of years, could have emitted these gases in the required quantities to melt ice in the tropics. This would have initiated an opposite feedback loop and would bring sea ice levels down to the more modest quantities of more modern times.

Although the snowball Earth hypothesis is still disputed, the fact that it immediately precedes the large-scale development of multi-cellular life—popularly known as the Cambrian explosion—is an intriguing fact. I think, therefore, at least one of the rock samples that serve as evidence for this hypothesis warrants being sent into space for another planetary geologist to recover. They are testament to the winter of our Earth's discontent and the conditions that had likely stifled the blossoming of life.

Bring on the spring. I eagerly await the sight of Washington DC in full blossom again.