Editor's Note: James Wray is an assistant professor of Earth and atmospheric sciences at the Georgia Institute of Technology in Atlanta. He is a collaborator on the Curiosity science team, affiliated with the Sample Analysis at Mars investigation. His research explores the chemistry, mineralogy and geology of Martian rocks as records of environmental conditions throughout the planet’s history.
NASA’s newest adventure to Mars has begun!
The Mars Science Laboratory mission delivered the Curiosity rover to Gale Crater, and the Internet and Twitterverse are abuzz. But NASA has landed on Mars six times before and has returned more information from the red planet than from all other planets beyond Earth combined.
So what is all the fuss about? Everyone who has learned a little about Curiosity is excited for different reasons; here are a few of my own.
1. The landing system. The rover landed in a complicated process NASA has called “seven minutes of terror.” Even the engineers who planned this unprecedented sequence of maneuvers for reaching the surface admit that "it looks crazy," but it worked! And this is innovation with a purpose.
It started in the upper atmosphere, where the heat shield first began to slow the entering spacecraft. During this phase of their landings, Spirit and Opportunity were just along for the ride, but Curiosity actually steered her way through the upper atmosphere, firing thrusters to adjust course. This allowed much more precise targeting of a landing area only 4 by 12 miles (7 by 20 kilometers) across, roughly one-fifth the size of prior landing ellipses.
Without this landing mechanism, we could not have safely landed in Gale Crater, between its bowl-shaped crater walls and Mount Sharp rising from its center. Guided entry will be critical for future landings in other scientifically rich — but small - areas of Mars.
Another major innovation was the sky crane system for surface delivery. It’s a big change from the airbags that have cushioned the landings of past rovers, but Curiosity is just too heavy for airbags. The sky crane allowed this rover — and, hopefully, future missions — to carry some big, complex science instruments, including those described below.
2. The laser on its head. So, we landed in Gale Crater … now what?
Our Mars orbiters have shown us that some sedimentary layers in Gale have interacted with water in the past, a good first clue in Curiosity’s hunt for habitable environments. But how do we choose which particular rocks to approach on the surface?
Cameras will help, but to find the salt- and clay mineral-rich rocks that directed us to Gale, we need a way to survey composition from a distance. Past missions have shown that many Mars rocks are coated with dust, hiding their true compositions.
The laser on Curiosity’s ChemCam instrument is the perfect tool for blasting through this dust layer to reveal the chemistry of any rock within 25 feet (7 meters) of the rover. Some areas in Gale appear to have thicker dust cover than the alternative sites considered for Curiosity, so ChemCam is especially well-suited for exploring Gale.
3. Definitive mineralogy. Don’t confuse ChemCam with CheMin, another instrument that Curiosity is carrying to Mars for the very first time. While ChemCam will provide a first look at the chemical elements in a rock or soil, CheMin will show how those elements are arranged into minerals. It uses X-ray diffraction, a favorite technique of laboratory mineralogists.
Orbital remote sensing has shown us that some layers in Gale contain clays, but what else do they contain? Are they 50% clay or 5% clay? The answers, which CheMin can deliver, have major implications for the style and duration of water activity that formed the clays. Ditto for the sulfate salts detected from orbit in other layers of Mount Sharp.
4. The search for organic molecules. CheMin is one of two instruments that will analyze samples scooped from the soil or drilled from the rocks of Gale Crater; the other is SAM, short for Sample Analysis at Mars. SAM is a gas chromatograph/mass spectrometer, the first one sent to Mars since the 1976 Viking landers, and its highest-profile job is to search for organic molecules.
Viking didn’t find any (although this conclusion has recently been questioned), but SAM will heat samples to twice as high a temperature, allowing detection of even the most “stubborn” organics that Viking might have missed. SAM even has some unprecedented “wet chemistry” experiments that could detect still other types of organics. Life on Earth is built almost entirely of organic molecules, but they also rain into planetary atmospheres constantly aboard meteorites and comets.
So if SAM finds no organics, it would imply that something on Mars actively destroys them (or at least has done so sometime since Gale’s sediments were deposited up to 3.6 billion years ago). If organics are found, then studying their properties with SAM may be our first step in moving from “was Mars ever habitable” to “did Mars actually host life?”
5. Settling the methane question. SAM has another component bundled with its gas chromatograph/mass spectrometer: a tunable laser spectrometer. This instrument fills a tube with gas from the Martian atmosphere (or boiled off from drill samples) and bounces a laser beam through it dozens of times, then looks to see how much light the gas has absorbed from the beam.
There are two lasers, and they can be “tuned” to different wavelengths, allowing for different components of the gas to be studied. Methane is one of these. In case you’ve “tuned” out for the past nine years, methane has been reported in the atmosphere of Mars by several research groups, but the claims have all been controversial. It matters because on Earth, roughly 98% of our atmospheric methane is ultimately due to life … and even if Mars had produced it through a non-life mechanism (e.g., volcanic activity), it shouldn’t survive there for more than a few centuries.
So modern methane would imply an active Mars today, exciting no matter what its cause. While it’s possible that Mars’ methane emerges only from places far from Gale Crater, the winds should ultimately blow some of it Curiosity’s way, and SAM’s exquisite sensitivity should allow us to catch a whiff.