On the night of November 28, 1659, a Dutch astronomer named Christiaan Huygens aimed toward the sky a 22-foot telescope of his own invention, peered through its compound eyepiece, and drew the first known illustration—the first map, really—of Mars. His sketch, though crude, captured a dark, distinctive surface feature. Today, astronomers know it as Syrtis Major.

And they're about to get to know it a lot better.

At Syrtis Major's northeastern edge you’ll find one of the most intriguing crops of geology ever observed on another planet. Its terrain—sandwiched between a large volcano and one of the biggest, oldest craters on Mars—preserves a chapter of the planet’s early history marked by warm, watery environments where microbial life might have flourished. Now, 358 years after Hyugens first described Syrtis Major’s outlines, planetary geologists have charted its fascinating northeasterly province at higher resolution, and in finer geological detail, than ever before.

“People have explored the mineralogy and geology of the larger area before, but nobody has put down the magnifying glass and looked at this one region up close,” says Michael Bramble, the planetary geologist at Brown University who led the mapping effort.

His team’s map, which appears in the latest issue of the planetary science journal Icarus, recounts the history of Northeast Syrtis. “It’s a big step for the planetary science community," says UT Austin geoscientist Tim Goudge. "It helps us understand what happened here, why it’s unique, why it’s so mineralogically diverse.” That’s a big endorsement: Not only is Goudge unaffiliated with Bramble’s project, he’s something of a rival.

See, Northeast Syrtis is one of the two most promising landing sites currently under consideration for NASA’s Mars 2020 rover. The competing landing site is Jezero crater, home to an ancient lake whose sediments might now carry traces of past life—and Goudge is its lead mapper.

The locations of the three most popular sites of exploration, as proposed by attendees at the most recent landing site workshop. NASA

Planetary scientists have been deliberating over where to land NASA’s rover for several years now. And with good reason: The site's composition will have a major impact on the agency's research. NASA’s next rover, which is slated to launch in 2020, will investigate Mars’ geological history, evaluate the planet’s past habitability, and hunt for signs of ancient life. Crucially, it will also be the first rover to cache samples of Martian soil and rock—samples which NASA hopes to retrieve on a future mission and analyze here on Earth.

NASA is getting closer to a verdict. Northeast Syrtis and Jezero rose to the top of the pack just in February, when some 200 planetary scientists convened at a workshop in Monrovia, California to trim the list of recommended candidates from eight to three. (Columbia Hills, a site previously explored by NASA’s Spirit Rover, also made the cut, though the other sites seem more promising).

You can think of Bramble’s new imagery as a treasure map: The black and white image depicts the Martian terrain NASA’s rover would have to traverse to access samples of the region’s geology, while the colored overlay describes the geological layers the rover can expect to find throughout its travels. The blue indicates the oldest, lowest layers in the region—a “basement,” Bramble says, created by an impact event nearly 4-billion years ago. The clay minerals in this region suggest the presence of two one-time habitable, aqueous environments. The green overlay demarcates the next geographic layer, a terrain where the mineral olivine has weathered to become carbonate. That reaction liberates hydrogen—a known energy source for microbial communities here on Earth. The upper layers of the region, indicated in peach, are rich in sulfates thought to have formed when water percolated through these layers. An Earthly analog to this unit is the Rio Tinto in Spain, a river whose acidic waters are known to host microbial life. Bramble et al.

Though only one bears its name, both sites reside in the northeasterly reaches of Syrtis Major. (They look close on a map, but to NASA’s next rover, which, on a good day, might travel a couple hundred meters, they might as well be a million miles apart.) Jezero was once home to a river delta that every scientist I spoke with described as either beautiful, spectacular, or both—oh, and its minerals may once have supported microbial life. “To the extent that ancient lakes and deltas were habitable environments, and that we believe them to now preserve traces of ancient life, Jezero is absolutely the best choice among the remaining sites,” says John Mustard, a planetary geologist at Brown. (Mustard is the bridge between the two sites; he is a coauthor on Bramble’s Northeast Syrtis paper, but once served as Goudge’s thesis advisor—small world, no?)

Northeast Syrtis on the other hand, is more likely to provide NASA’s rover easy access to lots of geologic environments—something the new map of the region confirms. Of particular interest are the clay minerals in the bottommost geological layers, sulfate-bearing terrains in the uppermost strata, and carbonated olivine minerals in between—all of which hint at one-time habitable, aqueous environments. What’s more, they’re all readily accessible. “The regions of interest are more clustered in Northeast Syrtis,” Goudge says. That means NASA’s rover could conceivably start doing science there as soon as it lands, drilling and caching samples from a range of geologic periods in a relatively small window of time.

A geologic map of Jezero crater, its watershed, and the surrounding area. The crater basin itself appears at the bottom righthand corner of the image, and is outlined in white. The thick black lines outline the two adjacent watersheds, from which water would have flowed into the impact crater. Goudge et al.

That clustering will almost certainly factor into NASA’s final decision. The agency’s Jet Propulsion Lab has already used the map to run thousands of potential landing and exploration scenarios throughout the region. “You know, if you dropped the rover at this latitude and longitude where would it go, what route would it traverse, what obstacles would it have to avoid,” Mustard says. Goudge has produced similar maps of Jezero, though at slightly larger scales and lower resolution. He says he and his colleagues will be collecting more detailed imagery of the crater in the months ahead, which NASA will also use to model landing and traverse scenarios.

NASA will choose a landing site for its rover in the next couple of years, based largely on the research and guidance of planetary geologists like Bramble, Mustard, and Goudge. Barring any surprises, it will very likely be one of the sites in Syrtis Major.

Which is pretty poetic, if you think about it. Christiaan Huygens surely knew, as he was drawing his rudimentary map more than three centuries ago, that future generations would go on to chart the surface of Mars and other planets in increasingly fine detail. He might even have guessed we’d seek them out in search of life. (Like many of his contemporaries, Huygens was a big believer in extraterrestrials.) But what he couldn't have known that November night was that more than 350 years later, astronomers would direct their attentions back to Syrtis Major—to the fringes of the dark mark he so carefully described for the first time.