Dark new horizons shed light on an old Earth

My last postcard was about context. In that postcard, I explained how MESSENGER's exploration of Mercury has helped us learn more about the planetary neighbourhood in which our Earth sits. And for this latest offering, I want to follow a similar theme. But first, we need to take a pretty huge leap (about 5.85 billion km) across the Solar System. Where we will find ourselves in the vicinity of Pluto.

Pluto—once famous for being the ninth and most distant planet from the Sun—is now more famous for being the planet that isn't a planet. Following its discovery in 1930, Pluto—which has a diameter of about 2,300 km—enjoyed more than 75 years at the planet level of the Solar System hierarchy. But in 2006, members of the International Astronomical Union (IAU) decided to demote Pluto, and assign it a new status as a dwarf planet. This decision was prompted when it became clear that Pluto is just one of many large, Sun-orbiting icy bodies in the outer Solar System. The astronomers therefore decided to officially define the term planet, specifically so that Pluto (and other bodies like it) would be excluded from this class.

Photographic plates used for the discovery of Pluto. The arrows mark Pluto's position. Pluto clearly moved against the background of stars in the six days between the two observations, which were made by Clyde Tombaugh in 1930. Credit: Lowell Observatory Archives

Under the IAU's new formal definition, a planet must meet three requirements:

1. The celestial body must orbit the Sun.
2. The body must have a large enough mass to give it a nearly round shape. 
3. The body must have cleared the neighbourhood (of other material) of its own orbit.

Unfortunately—for Pluto at least—the former ninth planet could not meet this third requirement. And a global public outcry—which continues today—followed.

The International Astronomical Union's decision to reclassify Pluto and strip it of its planet status hit the headlines in 2006 and caused a huge public outcry.

But the question of Pluto's planethood is currently being pushed aside, as the level of excitement surrounding NASA's New Horizons mission rapidly grows, prior to the spacecraft's Pluto fly-by. New Horizons—first launched in 2006—is the first spacecraft to visit Pluto and its system of five known moons (Charon, Styx, Nix, Kerberos, and Hydra). The probe will not go into orbit around Pluto, but will instead zoom by a week from now, on 14 July 2015. The fly-by will only last about eight or 10 hours, but at its closest approach the spacecraft will be about 12,500 km from the surface of Pluto. 

Photograph of Pluto and its five moons taken with the Hubble Space Telescope in 2012. Credit: NASA, ESA, and L. Frattare (STScI)

The scientific payload of the spacecraft consists of seven instruments that were chosen so that the geology, surface composition and temperature, and atmospheric characteristics of Pluto and its moons could be investigated. The bulk of the scientific data will be obtained during a period of about 24 hours around the time of the fly-by. The best pictures should reveal features as small as 60 metres across on Pluto's surface.

Map of Pluto released by the New Horizons team on 7 July 2015. The map was created from images obtained with the spacecraft's Long Range Reconnaissance Imager (LORRI) instrument, which were combined with low-resolution colour data obtained with the Ralph instrument. The map clearly shows an intriguing pattern of bright and dark markings on Pluto's surface. The brightest region may contain fresh deposits of methane, nitrogen, and/or carbon monoxide frost. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

This newest installment in the history of human exploration of the Solar System is all very nice, but how is it relevant to my original brief? How can learning about this far-distant world help us convey the uniqueness of Earth to a hypothetical alien planetary geologist? Well, if all goes to plan, then the fly-by of Pluto will not be the end of the New Horizons mission. It  should just be the end of the beginning. Pending approval from NASA for an extended mission, New Horizons will be sent on an onwards journey to study another Kuiper belt object.

The Kuiper belt is a region that extends outwards from the orbit of Neptune for about 20 AU (astronomical unit, equal to about 150 million km). It is similar to the asteroid belt (which lies between the orbits of Mars and Jupiter), as it contains many—relatively small—bodies that are remnants from the formation of the Solar System. Most Kuiper belt objects are icy bodies, composed mainly of substances such as methane, ammonia, and water. Pluto is the largest known object in the Kuiper belt, but about 100,000 objects (with diameters of more than 100 km) are expected to exist in this region, and more than one thousand have been discovered since 1992.

The path of the New Horizons spacecraft (yellow line) through the outer Solar System and the Kuiper belt. The orbits of the planets are shown in blue. The largest Kuiper belt objects are labelled. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

By studying Pluto and its Kuiper belt companions, we can potentially learn about two aspects of Earth's earliest history. As remnants of the Solar System's formation, the Kuiper belt objects are seen as akin to planetary embryos or protoplanets. Planetary accretion is believed to begin with the condensation of solids from the gas cloud that surrounds a star. Accretion of gas and dust then produces bodies that have diameters of 1–10 km, which are known as planetismals. The Kuiper belt objects can help us understand this early accretionary stage and thus what processes went into building our Earth. Violent impacts that occurred during the stage of runaway growth allowed the many planetismals to coallesce and form the large planets we know today.

In addition, the Kuiper belt is thought to be the region from which most short-period comets (i.e., those with orbits of less than 200 years) originate. As the European Space Agency's current Rosetta mission has wonderfully shown, comets are intriguing bodies in our Solar System. Indeed, many scientists believe that comets may have contributed a significant proportion of Earth's water inventory. In a previous postcard, I discussed how a Jupiter-family comet—which probably originated in the Kuiper belt—has a water signature that is a good match for that of Earth. 

New Horizons therefore provides us with a great opportunity to get to know the Kuiper belt better and to potentially understand the building blocks of Earth just a little bit more. But furthermore, the Kuiper belt may provide a big clue to any alien astronomers of our Earth's existence. Neptune, as a giant gas planet, exerts a great gravitational force on the cloud of dust that surrounds it in the Solar System (which includes the Kuiper belt). The gravity tugs on this cloud of dust and creates a distinctive ring structure. Computer simulations show that this ring contains a gap where Neptune itself resides. So even if the alien astronomers cannot directly image the planets of our Solar System from afar, they might be able to detect Neptune's presence. As such, they would know that our Sun possesses a planetary system, and we on Earth are here to be found.

Computer simulations show what the Solar System might look like to an alien astronomer. The gravity of Neptune creates this distinctive ring structure in the dust cloud. The planet itself resides in the gap that can be seen as the dark area in the right of the image. Credit: NASA/Goddard/Marc Kuchner and Christopher Stark

Planetary poetry

The inspiration for this blog—the 'Golden disk' on Voyager—is not the only example of a human message sent out on an interplanetary journey. The early days of space exploration were filled with declarations from us humans here on Earth. For instance, both Pioneer 10 and Pioneer 11 in the early 1970s (sent to explore the outer planets and leave the solar system) featured gold-anodized aluminium plaques designed by Carl Sagan. These plaques show illustrations of nude men and women to represent the human race, as well as other information, in case the spacecrafts were ever intercepted by extraterrestrial life.

Carl Sagan holding the Pioneer plaque. Credit: www.daviddarling.info

Apollo 11—probably the most famous space mission of all—included a plaque that was bolted onto the lower part of the Eagle Lunar Module. This landing stage still sits on the Moon and can even be seen in modern-day Lunar Reconaissance Orbiter Camera images.

The landing stage of Apollo 11's Eagle Lunar Module can still be seen in images from NASA's Lunar Reconnaissance Orbiter Camera. The flight hardware is at the centre of this image, with its shadow to the left. Credit: NASA/GSFC/Arizona State University

The Apollo 11 plaque reads as follows:

Here Men from the Planet Earth First Set Foot Upon the Moon

July 1969 A.D.

We Came in Peace for all Mankind

Apollo 11 plaque attached to the ladder of the Lunar Module. Credit: NASA

But another, much less formal, but equally enduring and touching message was left also on the Moon by the astronaut Gene Cernan. Cernan was the commander of Apollo 17 and the last man—to date—to have walked on the Moon. He writes in his autobiography of the small way in which he honoured his daughter during his final moments on the lunar surface:

"... I drove the Rover about a mile away from the LM [Lunar Module] and parked it carefully so the television camera could photograph our takeoff the next day. As I dismounted, I took a moment to kneel and with a single finger, scratched Tracy's initials, T D C, in the lunar dust, knowing those three letters would remain there undisturbed for more years than anyone could imagine."

Just this week, NASA's MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft has entered into orbit around Mars. This mission will be the first to study the upper atmosphere of the Red Planet, and how it has evolved with time. As part of the mission's education and public outreach activities, the University of Colorado ran a public competition. In this contest, people young and old were invited to write a haiku that could be sent along with the poet's name onboard MAVEN to Mars. This type of programme is a great way to inspire children to think about science—and poetry—and I even submitted an entry myself.

MAVEN, you raven

pray, tell, with your expert ways

is Mars life's haven?

This was actually my first attempt at haiku, and I thought not a bad first effort. It even gained the approval of my talented poet friend who had first brought the competition to my attention. Unfortunately, however, it didn't make the final cut. The winners can be read here, and my favourite is probably this one by Greg Pruett:

distant red planet

the dreams of earth beings flow

we will someday roam

I haven't picked a piece of Earth today to represent our planet to unknown aliens, but these poems, plaques, and traced initials are all beautiful examples of the ways in which we humans try to communicate our place in the universe. As MAVEN starts its orbital mission, I hope it succeeds in unraveling some of Mars' atmospheric mysteries. Perhaps we will learn if our planetary neighbour could ever have supported intelligent life, and what caused its evolution to diverge so drastically from that of our own Earth.

Artist's conception of the MAVEN spacecraft in orbit around Mars. Credit: NASA/Goddard

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.