Enlarge /. Perseverance's two Mastcam Z imagers (in the gray boxes) are part of the rover's remote sensing mast.
The seven minutes of terror are over. The parachute unfolds; The Skycrane missiles fired. Robot truck goes ping! Perseverance, a rover built by humans to do science 128 million miles away has come down on Mars. Phew
Percy has now opened his many eyes and looked around.
The rover is littered with a couple of dozen cameras – 25 if you count the two in the drone helicopter. Most of them help the vehicle to drive safely. Some look closely and intensely at old Martian rocks and sands, looking for signs that something once lived there. Some of the cameras see colors and textures almost exactly as the people who built them do. But you also see more. And less. The rover's cameras envision colors beyond the colors human eyes and brains can have. Yet the human brain has yet to understand the images they send home.
To find clues about life one has to go to a place that was probably habitable in the past. In this case it is the Jezero crater. Three or four billion years ago it was a shallow lake with sediment flowing down its walls. Today these are 150 foot cliffs, streaked and multicolored from the sediments that spread and dry up in the ancient delta.
These colors are a geological infographic. They represent the time determined in layers, layer by layer, epoch by epoch. And they represent chemistry. NASA scientists pointing cameras at them – the right kind of cameras – can tell what minerals they are looking at and whether small Martian beasts once called these sediments home. "If there are sedimentary rocks on Mars that hold evidence of an ancient biosphere, we will find them here," says Jim Bell, planetary scientist at Arizona State University and principal researcher on one of the rover sets of eyes. "This is where they should be."
This is what they are looking for. But they won't see that. Because some of the most interesting colors are in this real life, 50 meter infographics are invisible. At least they would be on earth for you and me. Colors are what happens when light reflects off or around or through something and then hits an eye. But the light on Mars is a little different from the light on Earth. And Perseverance's eyes can see light that humans cannot – light from reflected X-rays or infrared or ultraviolet. The physics are the same; the perception is not.
Bell's team operates Mastcam-Z, super science binoculars mounted on the Perseverance tower. (The Z is for the zoom.) “We designed Mastcam-Z for a rover going to an unselected point on Mars. So we had to design it with all possibilities in mind – the optimal pair of eyes. Capture the geology of any point on Mars, ”says Melissa Rice, planetary scientist at Western Washington University and coinvestigator at Mastcam-Z.
In close-up, Mastcam-Z can see details around 1 millimeter in diameter. A function only 4 centimeters wide is recorded from a distance of 100 meters. This is better than you and me. It also sees color better – or rather “multispectral” – and captures the visible broadband spectrum that people are used to, but also around a dozen narrow-band, not entirely colored colors. (Rice co-wrote a very good geek-out on all of these things.)
The two cameras provide this super-vision experience with off-the-shelf image sensors from Kodak, charge-coupled devices like those in your phone. The filters make them special. In front of the CCD there is a pixel layer that picks up red, green and blue. Imagine a square grid – the top squares are blue and green, the bottom squares are green and red. Now spread that out in a repeating mosaic. This is called the Bayer pattern, a silicon version of the three color-sensitive photoreceptors in your eye.
Emily Lakdawalla | MSSS
Mars and Earth bathe in the same sunlight – the same jumble of light at every wavelength. But there is less of it on Mars because the planet is further away. And while Earth has a thick atmosphere full of water vapor to reflect and refract all that light, Mars has only a small atmosphere and is full of reddish dust.
On Mars that means a lot of red and brown. But when you see them on Mars, a completely different filter of perception is added. “We're talking about showing an approximate, true color image that is essentially close to a raw color image that we capture with very minimal processing. It's a version of what Mars would look like to a human eye, ”says Rice. “But the human eye evolved to see landscapes under terrestrial lighting. If we want to reproduce what Mars would look like to a human eye, we should simulate the Earth's lighting conditions on these Martian landscapes. "
On the one hand, the image processing team working on Perseverance's raw food can adapt the Mars colors to earthly colors. Or the team can simulate the spectra of Martian light hitting objects on Mars. That would look a little different. No less true, but perhaps more like what a person on Mars would actually see. (There is no telling what a Martian would see, because if he had eyes, those eyes would have evolved to see color under this sky, and their brains would be alien.)
Enlarge /. NASA's Mars Perseverance Rover captured this image with its left Mastcam-Z camera.
NASA | JPL-Caltech | ASU
But Rice somehow doesn't care. "For me, the result is in a sense not even visual. The result I'm interested in is quantitative," she says. Reis is looking for how much light at a certain wavelength is reflected or absorbed by the material in the rocks. This “reflectance value” can tell scientists exactly what they are seeing. The Bayer filter is transparent to light with a wavelength greater than 840 nanometers, which is infrared. In front of this layer is a wheel with another set of filters; block those colors of light visible to humans, and you have an infrared camera, choose narrower sets of wavelengths and you can use them to identify certain types of rock and distinguish how they reflect different wavelengths of infrared light.
Before Perseverance left, the Mastcam-Z team had to learn exactly how the cameras saw these differences. They created a "Geo Board", a design brainstorming session that included reference color samples and also square stone slices. "We put it together with slabs of rock made of all kinds of material that we knew were on Mars, things we hoped to find on Mars," says Rice. For example? On this board were pieces of the minerals basanite and gypsum. "In normal color, they both look like bright white stones," says Rice. Both are mainly composed of calcium and sulfur, but there are more water molecules mixed in with plaster of paris, and water reflects more at some wavelengths of the IR than at others. “When we create a false color image with longer Mastcam Z wavelengths, it becomes clear which day is which,” says Rice.
With all of its multispectral multitasking, Mastcam-Z has its limits. The resolution is great for textures – more on that in a moment – but the field of view is only about 15 degrees, and the sluggish upload bandwidth would make your home router giggle. Despite all of the wonderful pictures Perseverance will send home, it really doesn't see that much. At least not all of them at once. All of these views are constrained by technology and distance. "Dude, our job is triage," says Bell. "We're using color as a proxy for" Hey, that's interesting. Maybe there's something chemical going on there, maybe there's a different mineral, a different texture. "Color is a substitute for something else."
The rover's narrow field of view means that, by definition, scientists cannot see everything they could hope for. Bell and his team learned about these limits during their simulations of the camera and robotic experience in the southern California desert. "As a kind of joke, but also as an object lesson, my colleagues once placed a dinosaur bone directly along the rover's path in one of these field tests," he says. "We drove right past it."
Mike Kaplinger | Melissa Rice | NASA | JPL | MSSS
To identify actual elements, and most importantly, to find out if they once had life, you will need even more colors. Some of these colors are even more invisible. This is where X-ray spectroscopy comes into play.
In particular, the team that operates one of the sensors on Perseverance's arm – the planetary instrument for X-ray lithochemistry or PIXL – wants to combine the elementary recipe for minerals with fine-grained textures. Here's how to find stromatolites, layers of sediment with tiny domes and cones that can only come from mats of living microbes. Stromatolites on Earth provide some of the evidence of the earliest living things here; Perseverance scientists hope they will do the same on Mars.
The PIXL team leader, astrobiologist and field geologist at the Jet Propulsion Laboratory named Abigail Allwood, has done this before. She used this technology in conjunction with high-resolution images of sediments to find signs of the earliest known life on earth in Australia – and to determine that similar sediments in Greenland were not evidence of ancient life there. It's not easy in Greenland. It will be even more difficult on Mars.
X-rays are part of the same electromagnetic spectrum as the light humans see, but at a much lower wavelength – even ultra-violet. It's ionizing radiation, only one color if you're a Kryptonian. X-rays cause different types of atoms to fluoresce in characteristic ways and emit light. "We generate x-rays to bathe the rocks and then capture that signal to study elemental chemistry," says Allwood. And PIXL and the arm also have a bright white flashlight at the end. "The lighting on the front was originally just a way to make the rocks more visible and to tie the chemistry to visible textures, something that hadn't happened on Mars before," says Allwood. The color was a bit annoying at first; Heat and cold affected the lightbulbs. "We tried white LEDs initially, but the temperature changes didn't produce the same shade of white," she says. "So the people in Denmark who delivered the camera supplied us with colored LEDs." They were red, green and blue – and ultraviolet. This combination of colors adds up to create a better and more even white light.
This combination could be able to find Martian stromatolites. After finding likely targets – possibly thanks to Mastcam-Z pans over the crater – the rover pushes up and extends its arm, and PIXL starts pinging. The smallest features, grains and veins, can tell whether the rock is igneous or sedimentary, melted together like a stew or layered like a sandwich. Colors of layers on top of other features give an indication of the age of each feature. Ideally, the map of visible colors and textures will match the invisible numbers-only map that the X-ray results produce. When the right structures match with the right minerals, Allwood can tell if she has Australia-type signs of life or a Greenland-type bust. "What we found really interesting at PIXL is that it shows you things that chemistry doesn't make you see," says Allwood. "That would be the key."
Allwood hopes PIXL's tiny scans will produce huge results – a derivative map of 6,000 individual points on the instrument's field of view, the size of a postage stamp, each with multiple spectral results. She calls this a "hyperspectral data cube".
Of course, Perseverance has different cameras and instruments, different scanners that look for different clues of meaning in boulders and regolith. Adjacent to PIXL is a device that looks at stones in a completely different way and shoots a laser at them to vibrate their molecules – that's Raman spectroscopy. The data Perseverance collects is hyperspectral, but also diverse – almost philosophical. This is what happens when you send a robot to another planet. A human mission or stones sent home via sample return would provide the best basic truth data, an exoplanet researcher told me. A little behind that are X-ray and Raman spectroscopy, then rover cameras and then orbiter cameras. And of course, all of these things work together on Mars.
"Finding life on Mars will not be:" Such and such an instrument sees something. "It will be," All instruments have seen this, that and the other, and the interpretation makes life sane, "says Allwood." There is no smoking weapon. It's an intricate tapestry. "And like a good tapestry, the entire picture is created from just a chain and a shot of color that are carefully threaded together.
This story originally appeared on wired.com.