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

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.