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.

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

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.