On Monday August 6 our latest robotic explorer will arrive at Mars, and, this being an American mission, they’re doing it bigger and better than ever before. The Mars Science Laboratory, or the Curiosity rover, will slam into the upper Martian atmosphere at more than 20,000 km/h, shedding its interplanetary velocity as an impressive shooting star. A few minutes later, she will pop out a drogue parachute, cutting loose again after just 100 seconds, before finally being winched the last few metres to the ground beneath a hovering rocket-powered platform, known as the sky crane. This is the first time that such a complicated and risky descent system has been attempted, but it’s absolutely necessary to land such a large rover mission.
Curiosity is a beast of a machine. It has a mass of nearly a tonne and its camera mast stands taller than a grown man. Nasa is trying to land this Jeep-sized rover onto the rusty soil — a six-wheeled, nuclear-powered vehicle loaded with scientific instruments and experiments, and wielding a high-powered laser to zap rocks for remote analyses. The investigative capability of this miniaturised lab-on-wheels is unparalleled, and planetary scientists worldwide are waiting with acute anticipation for the results to start beaming back. In particular, after decades of speculation and too many movies to count, they want an answer to the big one: was Mars ever suitable for life?
What excites astrobiologists like me most about Curiosity’s landing site in Gale Crater is the mountain rising 5.5km above the crater floor. This is believed to be a sedimentary feature, offering a long-term record of the profound environmental changes that Mars has experienced. By examining exposed layers and rocks strewn down the side of this mound, Curiosity will operate like a time machine, tracing back through billions of years of environmental history. With any luck the probe will teach us an enormous amount about the story of the liquid water of Mars and whether it has ever been a world habitable for life. The rover also has an instrument that could discover the first traces of organic molecules in the surface rocks, the building blocks of all life as we know it.
The more we learn about extremophile (the hardiest) life on Earth, the more plausible life on other worlds seems. But what exactly were the ancient environmental conditions on Mars? Even if Mars never has developed life of its own, we’d still expect to find organic molecules in the ground. We know that simple compounds, such as amino acids, sugars and even the components of DNA, are produced extraterrestrially by “astrochemistry”, because we find them in certain kinds of organic-rich meteorites. And Mars should have received a substantial amount of these space-synthesised molecules, delivered aboard comets and meteorites raining onto its surface over billions of years.
Curiosity is equipped to find this stuff, but not microbial life itself. For that, astrobiologists have their hopes pinned on the next European Space Agency rover, ExoMars.
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ExoMars is being specifically designed as a life-detection mission. It’s slated for launch in 2018, and will carry several scientific instruments able to pick out the tell-tale signatures of microbial life, even if that life has long-since fallen extinct. This rover will also be the first Mars probe packing a drill, able to delve two metres underground. The Martian topsoil is bathed in radiation from space and laced with destructive oxidising chemicals. So, for the best possible chances of success, we want to grab some samples from deeper underground.
One experiment, the Life Marker Chip, uses the same principle as a home pregnancy kit to try to pick up trace amounts of biological molecules extracted from soil samples. Another, which I’m involved with, is the Raman Laser Spectrometer. This will take soil samples brought up by the drill and interrogate them with a laser beam of a precise wavelength. By measuring how this wavelength is shifted in the light scattering back out again, the Raman instrument is able to read not only the minerals present, but also a whole range of key biological molecules. Compounds such as chlorophyll, which underpins all photosynthesis on Earth, show up well. So do pigments that microbes use to protect themselves, and Raman has proven itself time and time again in tests in some of the most inhospitable Mars-like environments on Earth, including the Dry Valleys in Antarctica.
For iron-clad evidence of Martian life, however, we’re going to want to go one step beyond anything we’ve attempted so far. Rather than trying to miniaturise laboratory equipment to launch to Mars on a probe, why not, instead, bring pieces of Mars home to scrutinise with cutting-edge analysis technology back here on Earth? This is the rationale behind a Mars Sample Return mission, which could be launched sometime in the 2020s.
Meanwhile, many astrobiologists are already looking further afield — to Jupiter and its giant icy moon, Europa. In the late 1990s the Galileo spacecraft discovered several lines of evidence that Europa, although frozen hard on the surface, hides a global saltwater ocean beneath this icy shell. Similar sealed-in watery pockets exist on Earth, such as lakes lying deep beneath polar ice sheets, and have been found to support life. So the hope is that Europa, too, might host microbial communities drifting through its deep, dark waters.
If Europa’s core is hot and active enough, the moon may also drive hydrothermal vents on its sea floor, like the black smokers that power flourishing ecosystems in the inky black depths of Earth’s own oceans. One of the fiercest debates in current planetary science, though, is just how thick the ice shell overlaying the ocean actually is — the data beamed back by Galileo wasn’t substantial enough to be conclusive. We know there’s liquid water down there, we just don’t know how deep it starts, or if it ever comes into contact with the surface ice. And these details are absolutely critical to the habitability of the ocean for life, and for our prospects for drilling down to the ocean one day to search for biology with robotic submersibles.
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Astrobiologists cheered quietly in May, when the European Space Agency announced that its next flagship space mission will be going to the Jovian system. Fighting off stiff competition from an X-ray space telescope and an array of spacecraft to study gravity waves, the Jupiter Icy Moons Explorer, or Juice, was selected for launch in 2022, to arrive at the gas giant eight years later. The probe is scheduled for two close fly-bys of Europa over the course of its looping tour round the gas-giant planet and its satellites before settling into orbit around Ganymede to investigate its magnetic field. Juice will use a powerful radar system to peer through Europa’s ice shell, determining its minimum thickness and probing for recently active regions — where, for example, the opening and closing of tidal cracks or melt-through events may have brought fresh seawater to the surface. The hope is that we’ll finally resolve many of the mysteries surrounding Europa and its potential for bearing alien life.
Lewis Dartnell is an astrobiology research fellow at University College London, and the author of Life in the Universe: A Beginner’s Guide (Oneworld, £9.99)
Extremophiles
One of the most astounding discoveries of recent years has been just how tough life on Earth is — and how similar our most extreme ecosystems may be to those of other planets. This gives astrobiologists hope.
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“Extremophiles” thrive in physical extremes of temperature, pressure and radiation, and chemical extremes of salinity, acidity and limited availability of water.
“Thermophiles” are specialists within that group, found basking around the scalding vents of geothermal springs. The current record for hot life on Earth is 121C, held by “Strain 121”, a single-celled microbe discovered in 2003 near a hydrothermal vent two miles beneath the surface of the Pacific off the coast of Washington State. Such vents matter particularly to astrobiology because they could provide energy sources for ecosystems elsewhere in the solar system, such as the watery ocean beneath the surface of Europa.
“Acidophiles” thrive in very acidic solutions at a pH below three. One of the most acidic environments on Earth is Rio Tinto in southern Spain, a drainage system with a pH of 2.3 which supports a large ecosystem of microbes, most of which power themselves by oxidising high concentrations of iron in the water. Rio Tinto is thought to closely resemble the iron-rich rivers and lakes that once covered the face of Mars.
Louisa Preston is a post-doctoral research associate at the Open University