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

Impacting young minds

I've been quiet on the blogging front recently, but I've been pretty busy behind the scenes. This has included engaging in what I see as one of the most important (and fun) parts of my job as a research scientist: outreach.

One of my close friends teaches first grade at Somerset Elementary School in Montgomery County, MD, and she arranged for me to be one of the speakers at their recent Careers Day. I found myself sharing a stage with people from a host of different professions and careers. There was a salad dressing maker, an events organizer, an investment banker, an advisor to President Obama... and me—the planetary geologist.

I was given three 20-minute slots to speak to students who had chosen to hear what I had to say about space. And I was happily surprised that many of these children were eager to ask all kinds of questions. I was even more impressed that some of their more knowledgeable (or geeky) peers were able to step in and proffer answers of their own.

I used my time to show a PowerPoint presentation (full of fun images of space, volcanoes, and such) that I had put together for the occasion, and to let the children have an interactive experience with the MESSENGER postcard mosaic. This 'game' is a great way for kids (and adults) to learn about different features on Mercury and to think about how planetary scientists map the entire surface of another planet.

Fun images depicting aspects of planetary geology.
Credits: Alexander Belousov, Earth Observatory of Singapore, NASA, NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

I also took some rock samples to pass around, including an example of each of the three different types of meteorites: a stony meteorite, an iron meteorite, and a stony-iron meteorite. It is these kinds of rocks that help us understand the interior structure of rocky planets. They come from proto-planets or asteroids that were at some point blasted apart. Before their violent demise, however, these bodies experienced similar evolutionary processes to the planets of our solar system that we know and love. Stony meteorites (in this case represented by an ordinary chondrite) are the equivalent of the exterior crusts of planets.

An ordinary chondrite (about 7 cm across) that was found in Romania. Ordinary chondrites are the most common type of meteorite found on Earth. Credit: ASU/CMS

Iron meteorites—consisting almost entirely of iron and nickel—represent the dense cores of planets, and the stony-irons (or pallasites) come from the boundary between the rocky outer layers of a planetary body and the metal-rich core, known as the core-mantle boundary.

Example of a pallasite (Springwater). These beautiful meteorites contain orange grains of the mineral olivine within a matrix of shiny iron-nickel. Credit: KD Meteorites

The iron meteorite that I had available was a one of a group of samples known as Canyon Diablo. These are the small pieces that remain after their much larger (about 50 m in diameter) parent chunk of space debris smashed into Earth about 50,000 years ago and created the Barringer Meteorite (aka Meteor) crater in Arizona (click here for a 3D-flyover of the crater). This well-preserved impact crater is just over 1 km in diameter and continues to be a site of scientific investigations.

A single piece (a few centimeters across) of the remaining Canyon Diablo meteorite.
Credit: Meteorites Australia

Back in the 1950s isotopic analyses on samples of this meteorite group were used to refine the estimate of Earth's age. Radiometric dating has revealed that many different solar system materials have concordant ages, which we use to provide Earth's age. The estimate given by Claire Paterson in 1956 (4.55 ± 0.07 billion years), based on the Canyon Diablo meteorite is very close to the current estimate of 4.54 ± 0.05 billion years. I propose, therefore, that we send one of these iron meteorite pieces forth for our imaginary alien planetary geologists to discover. Afterall, what piece of information about our home planet and our home solar system is more fundamental than its age?

I'm not sure society has the need for all of the 50+ children I met at Somerset to one day become planetary geologists. But I do hope that at least a handful of them will translate their wonder and joy for things space-related to future careers in a science-focused field. I'm proud to play a very small role in shaping the next generation of civil engineers, meteorologists, and brain surgeons of this world.

A thank-you note from one satisfied customer.

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

Tardi and hardy space travellers

Recently, this BBC headline caught my eye:

Dinosaur asteroid 'sent life to Mars'

The piece focuses on an Astrobiology research article by Worth et al. In the paper the authors consider the likelihood that rocks ejected from Earth (or Mars) during large meteor impacts launched life-bearing materials into space. It is possible, they claim, that such rocks could reach other planets or moons in the solar system, where the life could resettle and colonize. This concept—known as lithopanspermia—isn't new, but their statistical approach to the problem is.

Back in 1996 a meteorite found in Antarctica, but which originated from Mars, known as Allan Hills 84001 (ALH 84001) hit the news. NASA scientists claimed that this rock contained evidence of life once having existed on Mars.

Electron microscope image of 'fossilized microbes' in the Allan Hills 84001 meteorite. Credit: NASA

Obviously this controversial subject has been disputed ever since and it has been shown that all the rock's 'biosignatures' can be produced inorganically, i.e., without the need to invoke life.

Whether or not this particular meteorite contains ancient martian lifeforms, it prompts an interesting question that links back to the subject of lithopanspermia. Is it possible for life from Earth to be exported? If we were to pick an organism to undertake this journey, what would it be? And what rock would we encase this organism in, to try and ensure its survival?

I've never been a big fan of bugs, in fact I'm pretty terrified of anything with more than four legs. However, I recently became aware that a group of organisms known as tardigrades exist. And even I have to admit that these little monsters are cool.

A tardigrade in moss. Credit: Eye of Science / Science Source Images

Tardigrades are classified as extremophiles, organisms that can survive in some of Earth's most difficult conditions. But these guys take the term to the extreme—they're pretty much indestructible. They can withstand temperatures that range from just above absolute zero to more than the boiling point of water, pressures greater than at the deepest ocean trenches, and the vacuum of space along with its harsh radiation conditions. So my vote goes to the tardigrades as the space-faring guinea pigs that get to be sent on a one-way (and potentially suicidal) mission to somewhere.

The encasement of rock around our tardigrades must also be chosen carefully. The material needs to be strong enough to survive two impacts: the large meteor event that expels the material into space, and the sample landing on the surface of its destination. Hardy quartzite, the metamorphic product of simple sandstone, would probably be a good option. Or perhaps a material similar to that of ALH 84001, i.e., an igneous rock called orthopyroxenite, would meet the requirements.

I don't know if life on Earth originated through lithopanspermia, or if life exists elsewhere because of it, but I do quite like the idea of the hardy tardigrades ruling some distant planet.