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.

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.