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'.

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.