Wake up and taste the water

Comets have made the news a number of times in recent months. These interplanetary travellers, which were once thought to herald doom, are now known to be among the most primitive objects in the solar system. And of course, comets periodically make visits from the far reaches of the solar system (regions known as the Oort Cloud and the Kuiper belt) to our more local neighbourhood.

Back in December, Comet ISON—the 'comet of the century'—made a much-watched and disappointing suicide plunge into the Sun. And just this week, the European Space Agency successfully 'woke up' its Rosetta spacecraft from its two-and-a-half-year hibernation.

Currently more than 400 million miles away from home, Rosetta is journeying towards the comet 67P/Churyumov-Gerasimenko. Once it arrives, it will first orbit, and then land on the comet's surface. Together, the orbiter and lander portions of the spacecraft are equipped with over 20 scientific instruments that will make important characterizations of the comet. Scientists working on the mission hope to find out if comets such as this, which contain complex organic molecules, may have played a role in seeding life on Earth.

Artist's impression of the European Space Agency's Rosetta spacecraft orbiting the comet 67P/Churyumov-Gerasimenko. Credit: ESA, C.Carreau / ATG medialab

There has also been a long-lived debate over whether or not comets contributed significantly to the delivery, early in its history, of Earth's vast water inventory.

Because water is such a vital ingredient for the sustenance of life here on Earth, we target our search for extraterrestrial life on places where water exists. Water therefore seems like a pretty obvious choice for an Earth-material to send as one of our interplanetary postcards. But how might an alien scientist be able to tell our Earth-water apart from any other foreign H₂O? We all know that water can vary drastically in its colour, salinity, taste, etc. So is there a characteristic signature of our water that portrays its Earthly provenance?

Earth: A water world. Credit: NASA

Lucky for us, chemistry has an answer. Hydrogen (H), like all elements is defined by the number of protons it contains. 'Normal' hydrogen contains just one proton in its nucleus. However, it is possible for a stable hydrogen atom to contain one or two neutrons in addition to the lone proton. The hydrogen isotope that has one proton and one neutron is known as deuterium (D). Heavy water is enriched in molecules that contain deuterium in place of the normal hydrogen.

Planetary scientists have shown that D/H ratios in water vary throughout the solar system. Measurements for a number of Oort Cloud comets reveal that they have D/H ratios which are more than twice the value for Earth's oceans, and are thus unlikely to have been the source of water on our planet.

Range of deuterium/hydrogen (D/H) ratios in solar system objects.
Credit: A. E. Saal et al. 2013, Science

Paul Hartogh and colleagues, however, showed in 2011 that a Jupiter-family comet (103P / Hartley 2), which probably originated from the Kuiper belt, has a D/H ratio that is much more consistent with that of Earth. This means that at least some of Earth's water may have been delivered by comets. Although in more recent work, Conel Alexander et al. argue that CI chondrites (the class of meteorite whose composition most closely resembles that of the Sun) were the principal source of terrestrial water.

And so the great water debate continues.  But no matter how, and from where, the water got here; get here it did. And we wouldn't be alive without it.

The Earth giveth and the Earth taketh away

We all know the story of the great big asteroid impact that brought about the demise of the dinosaurs (and this geological event is so popularly known that I'm sure it will feature as the subject of a future postcard). However, the dinosaur-ending mass extinction pales in comparison to one particular event that occurred about 250 million years ago.

This event marks the boundary between the Permian and Triassic geologic periods. The Permian was a time when the Earth contained a single supercontinent—Pangaea—vast areas of which were covered by arid deserts. But in a relatively abrupt period a large proportion of life on Earth was lost. In all, over 90% of marine species and 70% of terrestrial vertebrates became extinct.

The Pangaea supercontinent existed during the Permian and Triassic periods.
Credit: Mikkel Juul Jensen / Bonnier Publications / SPL

Until recently, however, the cause of this 'Great Dying' has remained unclear. But new work, presented at the American Geophysical Union's December 2013 Fall Meeting in San Francisco, provides us with a smoking gun.

Accurate measurements, conducted by geologists Seth Burgess and Samuel Browning at MIT, on rocks from Meishan in the Zhejiang Province of China give a new and precise age for the actual Permian–Triassic boundary that marks this extinction. They show that the extinction began 251.941 million years ago and lasted until 251.880 million years ago, i.e., lasting for 61,000 years.

That time frame shortly follows the first eruptions of vast volcanic flows in Siberia 252.28 million years ago. These Siberian Traps cover an area of about 2 million km² (about the size of western Europe) and represent one of the largest eruptive events in Earth's history.

Part of the vast Siberian Traps. Credit: Jon Ranson

For decades scientists have questioned whether these two great, and closely spaced, markers in Earth's life could be linked and if the volcanic eruptions may have somehow caused the extinction. With the new age dating it is possible to at least show that the volcanism temporally preceded the mass extinction.  It is up to other scientists to work out what mechanism(s) link the two and could have caused so much destruction.

One recently proposed scenario sees huge amounts of sulfur dioxide from the volcanic eruptions being injected into the atmosphere. Acid rain, with the pH of lemon juice, would have then rained down on the northern hemisphere to wreak havoc on its vegetation and for the animals reliant upon it. Other materials, originating from the coal deposits through which the Siberian Traps were erupted, are likely to have been forced into the atmosphere. A resulting toxic metal-bearing layer of ash is thought to have blanketed the northern hemisphere and played a large part in the extinction event.

Although our Earth provides us with the conditions and materials to sustain life, it can also turn on its geological nastiness from time to time and try its hardest to kill us. I'd send some of these informative rocks from southern China and Siberia into space for one of our alien planetary geologists to find. So they know just how Mother Earth likes to do her 'loving'.

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.