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'.

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

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.