Postcard from the Moon

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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.

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.

What's up Sun?

For this postcard from planet Earth I've decided to cheat a bit on my own rules and pick a rock that isn't necessarily of the Earth, even if it is on the Earth. Mainly because I wanted to write about something close to my heart.

Let me explain.

I recently read this article in Scientific American. The research it highlights describes how interactions between solar wind and interplanetary dust particles can produce water. This got me thinking about how the Sun is a vital piece in creating our habitable little part of the solar system / galaxy / universe. (It also links nicely to my previous postcard, where I discuss how water might originally have been brought to Earth.)

The solar wind is a stream of charged particles (mostly electrons and protons) released from the Sun's upper atmosphere. This stream can vary, over time and from location to location around the Sun, in its density, temperature, and speed.

During solar flares, strong blasts of solar wind are fired through the solar system.
Credit: NASA

Now, even though I am a geologist and I spend most of my working hours thinking about rocks on planets other than our own, I also spend a fair bit of time thinking about the Sun. And worrying about the Sun.

You see, as a PhD student I waited (not necessarily patiently) for sunspots to erupt on the Sun's surface and for solar flares to fire X-rays through the solar system, towards the Moon's surface where an orbiting spectrometer onboard India's Chandrayaan-1 lunar satellite would detect the resulting X-ray fluorescence and provide me with some much needed data to analyze for my thesis research. Unluckily for me, I was doing my PhD when the mission was active, during the deepest solar minimum in over a century (solar cycles normally last about 11 years and most solar flares occur during the peaks of activity). Needless to say, my desired events were few and far between. Indeed that solar minimum lasted much longer than had been anticipated and the current cycle was almost a full year 'overdue' by the time it started.

Nowadays I still keep a watchful eye on the Sun's activity. Mostly because I work on the analysis of similar X-ray fluorescence data from NASA's MESSENGER mission that is currently orbiting Mercury. And it seems that predictions for the length and strength of the cycle change from week to week. For instance, this recent article discusses whether the Sun might be headed into another 'Maunder Minimum'. This was an approximately 70-year period (1645–1715) when the Sun was almost completely devoid of sunspots. The Maunder Minimum coincided with the middle of the Little Ice Age, during which there was a series of particularly frigid northern hemisphere winters.

Schematic illustration of MESSENGER's X-Ray Spectrometer in operation around Mercury. Credit: NASA / The Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

The beautiful northern and southern lights, or the aurorae borealis and australis, occur when the energetic charged particles of the solar wind are directed by the Earth's magnetic field into the atmosphere at high latitudes, where they undergo collisions with atmospheric atoms. But besides acting as a long-term forecast tool for keen aurora hunters, much of today's solar physics research is focused on aspects of the Sun's activity that affect our lives here on Earth.

The magical northern lights. Credit: Bjorn Jorgensen / National News & Pictures

For example, coronal mass ejections (CMEs) occur most frequently during the peak periods of a solar cycle, and induce geomagnetic storms. Threats to Earth-orbiting telecommunication satellites in high, geosynchronous orbits are posed during these storms. The high currents that are discharged to the satellites can damage their components. Additionally, geomagnetic storms have been known to cause the temporary loss of electrical power over large regions, such the 1989 Quebec event. Understanding when and why CMEs occur can help plan for, and mitigate the effects of, geomagnetic storms on our telecommunication and electricity networks.

And some research has shown that the variable output of ultraviolet radiation through the course of a solar cycle can be tied to terrestrial climate changes. Climate scientists are now trying to make reliable climate predictions on decadal timescales, therefore sound solar predictions are important inputs for their models.

So with all this research in solar physics, why are predictions for the Sun's activity so seemingly unreliable? Physicists have observations from a host of solar satellites at their disposal, yet they seem to still be in the metaphorical dark.  Perhaps this complex problem will just a little bit longer to unravel, or maybe the timescales of study are too short?

And that's where my rule-bending rock postcard comes in.

I wonder if we can use material that the Apollo astronauts brought back from the Moon to increase the length of time over which we can study the Sun and its solar wind output. Back in 1970, scientists made measurements of noble gases (such as helium and argon) that were trapped inside tiny pieces of the lunar soil. And it is thought that those noble gases were implanted into the soil as the solar wind bombarded the Moon's ancient surface. So by studying these trapped pieces of the solar wind we can learn more about how the Sun has changed through time. If we had enough samples from discrete layers in the lunar surface we could even build up a record of this solar wind material that might help place the Sun's modern activity into a larger context and give the solar physicists a helping hand.

Color photograph of Apollo 11 lunar soil sample 10084. These grains are between 9000 and 10,000 mm. Credit: NASA / Johnson Space Center

We might still be learning and then re-learning things about our Sun, but I think it is important that we send one of these tiny Moon pebbles, complete with its trapped solar cargo, to our alien planetary geologist friends. They should know it is the Sun king who rules over us and our whole solar system. Maybe they could even help us decipher its mysteries.