Curiosity, a trilogy

This was a piece I wrote in August for the recently published November/December 2012 issue of the Popular Astronomy Magazine.

Each planet has its own story to tell, a history of how it became what it is today. The Grand Canyon, a 2km deep crevasse dug out by the Colorado River, tells a 3 billion year old tale of how changing environments and atmospheres on Earth affected the local geology. Curiosity, the Martian rover that is carrying the Mars Science Laboratory (MSL), will be looking for a similar geological timeline on Mars.

MSL was developed as part of the NASA Exploration Program to study our nearest neighbour, the Red Planet, and it has travelled a perilous 560 million km to its final destination. But its journey is far from over. There are three chapters to Curiosity’s adventure: its travel to Mars and hair-raising entry, the required system checks, and finally the chance to explore a new world.

The first chapter in Curiosity’s story ended with a spectacular landing using retro-rockets and a sky-crane on August 6th 2012, which will hopefully set the tone for an exciting adventure on Mars.

Curiosity has been designed to house 10 experiments, each with a different scientific goal, but until scientists were sure that the rover would rove, there was no point in going any further. So, Curiosity took its wheels out for their first test drive on August 22nd, confirming that they were fully functioning, and all systems were ready to go.

Before Curiosity can set off and start the third chapter in its adventure, exploring the Martian surface, it has to ensure all instruments are fully functioning. Since the landing, Curiosity has been running hundreds of checks to make sure the instruments, of which there are three groups: the survey instruments, the contact instruments and the laboratories, are ready.

To find a suitable place to stop and study, the survey instruments scope out the local environment and look for interesting features. These instruments include black-and-white cameras to take 3D images, a colour camera to take small high-resolution images and the laser instrument, Chemistry and Camera (ChemCam). ChemCam fires a laser into rocks to turn them into a plasma, before analysing their elemental composition with an on-board spectrograph.

Once a destination has been decided, the contact instruments take over. One is the Alpha Particle X-ray Spectrometer, which is placed up against rocks to give an even more accurate picture of the chemical composition. Then there is the Mars Hand Lens Imager, which takes close-up images of individual mineral grains inside rocks. These tools give geologists a lot more information than a picture taken from afar. By getting up close to the sand, it is possible to derive whether the grains are angular or rounded. This could provide information about how long those grains have been travelling around and if they have been frozen.

Finally the third set of instruments, the big analytical laboratories, are put into action. A drill and a scoop on a robotic arm will deliver samples of rock and soil to Sample Analysis at Mars, a mass spectrometer used to detect organic molecules, and Chemistry and Mineralogy, which will look at the mineralogy of the rocks.

Following these rigorous checks, Curiosity will start its third and final chapter – exploring the Martian surface. Curiosity’s journey could first take it to Triple Junction, a spot only 400m from its landing site, before climbing Mount Sharp. Triple Junction is a point that joins three different local terrains that have been mapped out by geologists. Curiosity landed on one, another contains an ancient and completely dry channel, and the third is home to many craters.

The channel in the second terrain could provide insights into the composition of the Martian surface. The entire southern part of this channel, as seen from the Mars Reconnaissance Orbiter, has a high thermal inertia (the resistance of a surface to temperature changes). Rocky materials are known to have high thermal inertia, which means that this area could be made of rock or that minerals have cemented soil together and made it more compact.

The craters in the third terrain could, according to JPL scientist Dr Ashwin Vasavada – deputy project leader for MSL, be a sign that this is a much older piece of Mars. “The number of craters can be related to the age of that surface. An older surface has more time to collect meteors. This is interesting because there is a surface nearby which appears much older than the one we are on now; we’d like to figure out why.”

After spending a possible few weeks at Triple Junction, Curiosity will go on to Mount Sharp, the 5km high peak sitting in the Gale Crater. Mount Sharp is made up of layers of different geological structures. “From orbit we see that the bottom-most layers tend to have clay minerals in them,” says Vasavada, “the layers just above contain sulphate minerals, and then almost the entire upper half doesn’t show any mineral signatures.” Vasavada hopes that the rover will only have to climb up to 500m in elevation to be able to study the clay minerals and the sulphate rocks and reveal a significant part of Martian history.

Dr Peter Grindrod, a planetary scientist from University College London, thinks that these different layers could tell us more about the way in which the Martian environment has changed over the last few billion years. “The oldest rocks, from the Noachian era, contain phyllosilicates, which are minerals found in clay, and they tend to occur when water with a neutral pH balance interacts with basalt. In the next layer of rocks you find sulphate minerals that form in water with an acidic pH balance. This would have occurred in the Hesperian era. The final Amazonian era witnessed a very harsh, oxidising landscape, when all the water disappeared, and it has been much the same since.”

The change in pH balance of the water could be a sign of the thinning Martian atmosphere. One theory, according to Grindrod, is that over time the core dynamo that gave Mars its magnetic field must have died out, and so Mars lost its magnetic field. “This means that the atmosphere could then be stripped away by the solar winds, making it thinner, and so water isn’t stable on the surface anymore. Some is lost to space, but most of it goes under ground in the form of ice. As the atmosphere was thinning, all the massive volcanoes were erupting, pumping huge amounts of volcanic gases into the atmosphere. These gases reacted with what little water was left over, turning it acidic and creating sulphate minerals.”

At the same time, the Martian surface would also have been bombarded by cosmic rays, so essentially the transition from the Noachian to the Hesperian eras was bad news for life. “The layers in Mount Sharp could show a transition in the habitability on Mars” says Grindrod.

So potentially Mars is no longer an exotic, science-fiction planet; instead it could be a much closer relative of Earth. This begs the question, why doesn’t it have life like Earth does? This is one of the many things that Curiosity will look for, and hopefully it will bring an exciting ending to an already thrilling tale.

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