A whole world's wake-up call

The past few weeks in the world of space have been pretty hectic. Most especially because of the fantastic new views of Pluto we've been receiving, courtesy of the New Horizons flyby (which I wrote about in my last postcard). We've also been hearing about the "frozen primordial soup" of organic compounds detected by the European Space Agency's Philae lander on comet 67P/Churyumov–Gerasimenko, as detailed in a new special issue of Science. Some of these compounds may be important for the prebiotic synthesis of amino acids, sugars, and nucleobases, i.e., the very ingredients of life. 

The surface of comet 67P/Churyumov–Gerasimenko, as imaged from 9 metres away. Credit: ESA

But there are two other recent news items I want to focus on in this postcard. First, the new photograph of the Earth captured by NASA's new Deep Space Climate Observatory (DSCOVR) satellite. And second, the recent discovery of an exoplanet that is being billed as Earth's 'twin'.

On 6 July 2015, the Earth Polychromatic Imaging Camera (EPIC) instrument on DSCOVR returned its first view of the entire sunlit Earth. Safe in its gravitationally stable location one million miles away—at a so-called Lagrange point—the satellite was able to obtain this kind of full-Earth portrait for the first time since the famous 'Blue marble' photograph was snapped by the Apollo 17 astronauts whilst on their way to the Moon in 1972. I've mentioned that older, stunning photo in a previous postcard, but as the most reproduced image in history, I think that it is more than worth showing again.

The famous and historic 'Blue marble', taken during the Apollo 17 mission in 1972. Credit: NASA

It might come as a surprise that it has taken more than 40 years to recapture Earth in a similar view. The pictures you've seen of Earth's full disc in the meantime have either been this Apollo 17 photograph, or composite images (i.e., several smaller images that have been stitched together). It is difficult to obtain these images because many variables come into play. The camera must be between the Earth and the Sun, and far enough away to capture the whole planet in its field of view. Although weather satellites—in geosynchronous orbits—get similar views, they cannot normally see an entire hemisphere without shadow.

The Earth, from one million miles, as seen by the Deep Space Climate Observatory on 6 July 2015. Credit: NASA

The data from EPIC will primarily be used to measure changes to the ozone and aerosol levels in Earth's atmosphere, as well as cloud height, vegetation properties, and ultraviolet reflectivity characteristics. But these new, beautiful, images of a whole Earth remind us how powerful it is to see our entire home in one go. As pointed out by John Grunsfeld, associate administrator of NASA's Science Mission Directorate, "these new views of Earth give us an important perspective of the true global nature of our spaceship Earth."

Indeed, I'm reminded of an excellent book I read several years ago by Robert Poole. In Earthrise: How Man First Saw The Earth, Poole tells the story of how images of Earth—such as the Blue marble and the equally famous Apollo 'Earthrise'—taken during the dawn of the space age, played a huge role in the birth of the now-popular environmental and conservation movements.

'Earthrise' photograph taken by astronaut Bill Anders during the Apollo 8 mission, on 24 December 1968. Credit: NASA

It is another aspect of these images of our blue Earth, however, that strikes me most. It is the human capacity for intelligence and creativity that enables space exploration and capturing of Earth-selfies from afar. Yet we do not see evidence of our presence in these pictures. In many ways, we are invisible to the universe. It is not life that makes Earth special. It is the blue oceans, the green forests, and the white wispy clouds in our lovely oxygen-rich atmosphere that make our world habitable. So for this postcard to our hypothetical alien planetary geologists, I want to send a snapshot of our whole world. Let them see the Earth and all its systems intertwined.

The uniqueness of Earth, however, might be under threat if a new discovery from the Kepler space telescope is anything to go by. On 23 July 2014, scientists working on the Kepler mission announced that they have found the most Earth-like extrasolar planet yet. The new planet—known as Kepler-452b—is located about 1,400 light years away, and is a similar size to Earth. In addition, Kepler-452b orbits a Sun-like star at a distance that is similar to that of Earth around the Sun. The planet is being hailed as "the first possibly rocky, habitable planet around a solar-type star". And it will thus, likely, become the focus of an intense search for extraterrestrial life. Perhaps we'll even find those alien planetary geologists there waiting for us.

Artist's concept of Kepler-452b in orbit around its parent star. Credit: NASA Ames/JPL-Caltech/T.Pyle

At a time when humanity seems to be as fractured as ever, perhaps we need a wake-up call like these ones from NASA. We need to be reminded every once in a while that we are all one family, stuck together here on our little spaceship Earth. We should do our utmost to look after it—and each other.

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

A peace of Earth offering

In recent weeks this interesting science story has caught my attention. A group of scientists has been tasked with deciding whether we are in the midst of a new geologic epoch, and if so, what historical event can be used to mark its beginning. The potential new epoch is known as the anthropocene—the 'human epoch'—and is defined as the period in which human activities have had a significant impact on the Earth and its ecosystems.

Several sensible options have been put forward as possible start points for the anthropocene. These include the invention and rise of agriculture about 10,000 years ago, or the start of the Industrial Revolution in the 18th Century. It seems, however, that these choices have lost out to the might and power of an atomic option.

A group of researchers, led by Jan Zalasiewicz at the University of Leicester, have proposed that the new epoch began with the dawn of the atomic age. They suggest that the first nuclear bomb test, on 16th July 1945 in the Jornada del Muerto desert of New Mexico, marks the moment in time when humankind left its first permanent and global imprint on Earth.

Rising fireball and forming mushroom cloud, nine seconds after the world's first atomic bomb was detonated on 16th July 1945 in New Mexico. Credit: U.S. Department of Defense

Earth's geologic timescale is divided into units of various lengths that stretch back to the planet's formation about 4.54 billion years ago. Any given eon, era, period, or epoch consists of characteristic rock strata that can be distinguished from those that are directly above (younger) and below (older) it. Often, these stratigraphic boundaries are marked by major compositional or paleontological changes, such as mass extinctions. If the anthropocene is to be added to the top of Earth's stratigraphic column, it therefore makes sense to identify its beginning with a similarly global signature.

The geologic timescale. Credit: Science Education Resource Center at Carleton College

Luckily, as Zalasiewicz and colleagues point out, the nuclear tests that took place between 1945 and 1963 caused an unambiguous and detectable change in the Earth's atmosphere. The nuclear explosions were the primary source of man-made radionuclides (i.e., radioactive isotopes) found in the atmosphere. The most abundant of these nuclides—caesium-137—has no natural sources and is the product of nuclear fission processes. The first pronounced increase in the atmospheric concentration of Cs-137 occurred in 1954, and there was an additional peak in 1963. Man-made radionuclides therefore represent the best chronological markers for the atomically-heralded anthropocene.

Fallout of anthropogenic radionuclides (including caesium-137). Credit: Hancock et al., 2014, Geol. Soc. London

I think it is pretty obvious that a piece of the anthropocene Earth should be sent into the cosmos as our next geological postcard. This period, in which humans have begun to leave an indelible mark on our planet, is uniquely representative of Earth's capacity to support intelligent life. Caesium-137, however, with a half-life of only 30 years, is not a great choice. It will not be able to journey very far into the vast expanse of the universe before it decays away to almost nothing. So instead, I'd like to send an actual rock that is a direct product of the first nuclear test.

After the bomb detonation on 16 July 1945, a glassy residue was left on the desert floor at the 'Trinity' test site. The glassy deposit is composed mainly of feldspar and quartz-rich sand grains that were melted during the blast. This rock—known as trinitite, after its type locality—is usually light green in colour, and is mildly radioactive.

A trinitite hand specimen, produced by the first nuclear bomb detonation. Credit: Paul M. Schumacher

 

I like to think that our hypothetical alien planetary geologists are a friendly race and would extend a hand of peace if we ever encounter them. As evidenced by the anthropocene, we are now able to alter the workings of our own planet. So let those aliens beware the destruction we would unleash—Independence Day style—if they turn out to be not quite that welcoming. A piece of trinitite should therefore serve as a suitable example of the awesome and destructive power humankind can wield when we put our mind to it.