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

Good morning Earthshine

When I first mentioned the overall idea for this blog to a friend and colleague, he immediately found an interesting way to slightly reframe the question. Instead of considering how alien planetary geologists might recognize rock specimens representative of Earth, he wondered how Earth might look to an alien astronomer observing us with a faraway telescope.

This question is actually a pretty obvious one, especially given the popular pursuit of extrasolar planets in current astronomical research. The first confirmed detection of an 'exoplanet'—a planet that orbits a star other than our own Sun—was not made until 1992, but this field of research has now, almost literally, exploded. More than 1800 exoplanets have since been discovered, and this has largely been possible because of NASA's Kepler mission. The aim for this space observatory was to discover Earth-like exoplanets that are located in, or near, the 'habitable zone' of their parent star. This habitable—or Goldilocks—zone is the region around a star where planets (with sufficient atmospheric pressure) can support liquid water at their surface.

The habitable zones—where liquid water can exist on the surface of a planet—of different size and temperature stars. Credit: NASA Kepler Mission

This is all part of humankind's everlasting desire to discover life—potentially sentient—elsewhere in the universe. As our home continues to be the sole 'datapoint' for life, it is natural that we use Earth-like planets as a base for our search. And it is the special feature of liquid water at the surface that makes our planet so hospitable. The modern-day search for extraterrestrial life is therefore often focused on the hunt for this precious H₂O. A discovery of a planet hosting water at its surface within another star's habitable zone, however, does not equal the discovery of life elsewhere in the cosmos.

To be more certain of a water-hosting planet's potential to harbor life, an additional telltale detection is required. The spectra from the observed exoplanets must include signs of life—biosignatures—along with the sign of water. But what would an astronomical biosignature look like, and would we even recognize such a signal from an exoplanet? To begin to answer this question, we first need to understand how Earth's atmosphere looks from afar and which of its properties hint at the rich biosphere that lies beneath. This information can then be used as a reliable baseline with which to compare exoplanet detections.

 

Spectra for Venus, Earth, and Mars illustrate Earth's unique biosignatures. All three planets have a strong atmospheric absorption caused by carbon dioxide (CO₂), but only Earth's atmosphere has signals due to water (H₂O) and ozone (O₃) that can be representative of life. Credit: Mark Elowitz

As detailed in a 1993 study by Carl Sagan and colleagues, observations of Earth's atmosphere from space—in this case from the Galileo spacecraft—reveal several biosignatures. These include abundances of molecular oxygen and methane that are far from chemical equilibrium, as well as a sharp increase in albedo at wavelengths longer than 700 nm, which is caused by vegetation. It is also known that as light passes through Earth's atmosphere it can be polarized due to scattering by aersols and cloud particles, and reflected at variable amounts by oceans and land. In a more recent paper, Michael Sterzik et al. use a technique known as spectropolarimetry to make a detailed analysis of Earth's atmospheric properties.

Instead of using space-based measurements of Earth, Sterzik and co-workers made observations of 'Earthshine' by pointing their telescopes at the Moon. This rather romantic sounding light originates from the Sun before being reflected by the Earth onto the Moon, and then back to Earth again. It is the reason you can sometimes to see the 'dark' part of a non-full Moon.

Earthshine illuminates the 'dark' portion of the Moon. Credit: Will Gater

Sterzik et al. used a technique known as spectropolarimetry (a combination of spectroscopy and photopolarimetry) to conduct a detailed investigation of Earth's atmosphere. This methodology is better than standard spectroscopy for characterizing exoplanet atmospheres. The Earthshine observations could be used to determine the fractional contribution of cloud and ocean contributions within the reflecting surface, and were sensitive to relatively small areas of vegetation.

It is measurements such as these, using the Moon as a handy mirror, that can be used as a benchmark for diagnosing the atmospheric composition and surfaces of potential life-bearing exoplanets. They also serve as a clue to what an alien astronomer might see when they glance in our direction. I hope they too can recognize how special our Earth is.