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

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.

Wake up and taste the water

Comets have made the news a number of times in recent months. These interplanetary travellers, which were once thought to herald doom, are now known to be among the most primitive objects in the solar system. And of course, comets periodically make visits from the far reaches of the solar system (regions known as the Oort Cloud and the Kuiper belt) to our more local neighbourhood.

Back in December, Comet ISON—the 'comet of the century'—made a much-watched and disappointing suicide plunge into the Sun. And just this week, the European Space Agency successfully 'woke up' its Rosetta spacecraft from its two-and-a-half-year hibernation.

Currently more than 400 million miles away from home, Rosetta is journeying towards the comet 67P/Churyumov-Gerasimenko. Once it arrives, it will first orbit, and then land on the comet's surface. Together, the orbiter and lander portions of the spacecraft are equipped with over 20 scientific instruments that will make important characterizations of the comet. Scientists working on the mission hope to find out if comets such as this, which contain complex organic molecules, may have played a role in seeding life on Earth.

Artist's impression of the European Space Agency's Rosetta spacecraft orbiting the comet 67P/Churyumov-Gerasimenko. Credit: ESA, C.Carreau / ATG medialab

There has also been a long-lived debate over whether or not comets contributed significantly to the delivery, early in its history, of Earth's vast water inventory.

Because water is such a vital ingredient for the sustenance of life here on Earth, we target our search for extraterrestrial life on places where water exists. Water therefore seems like a pretty obvious choice for an Earth-material to send as one of our interplanetary postcards. But how might an alien scientist be able to tell our Earth-water apart from any other foreign H₂O? We all know that water can vary drastically in its colour, salinity, taste, etc. So is there a characteristic signature of our water that portrays its Earthly provenance?

Earth: A water world. Credit: NASA

Lucky for us, chemistry has an answer. Hydrogen (H), like all elements is defined by the number of protons it contains. 'Normal' hydrogen contains just one proton in its nucleus. However, it is possible for a stable hydrogen atom to contain one or two neutrons in addition to the lone proton. The hydrogen isotope that has one proton and one neutron is known as deuterium (D). Heavy water is enriched in molecules that contain deuterium in place of the normal hydrogen.

Planetary scientists have shown that D/H ratios in water vary throughout the solar system. Measurements for a number of Oort Cloud comets reveal that they have D/H ratios which are more than twice the value for Earth's oceans, and are thus unlikely to have been the source of water on our planet.

Range of deuterium/hydrogen (D/H) ratios in solar system objects.
Credit: A. E. Saal et al. 2013, Science

Paul Hartogh and colleagues, however, showed in 2011 that a Jupiter-family comet (103P / Hartley 2), which probably originated from the Kuiper belt, has a D/H ratio that is much more consistent with that of Earth. This means that at least some of Earth's water may have been delivered by comets. Although in more recent work, Conel Alexander et al. argue that CI chondrites (the class of meteorite whose composition most closely resembles that of the Sun) were the principal source of terrestrial water.

And so the great water debate continues.  But no matter how, and from where, the water got here; get here it did. And we wouldn't be alive without it.