HOW IS A MEXICAN SINKHOLE LIKE A JOVIAN MOON?

I recently caught up with David Wettergreen, a research professor at Carnegie Mellon's famed Robotics Institute. Wettergreen must have one of the coolest jobs for a tech-minded person: he designs and builds autonomous robots and then sets them loose in extreme environments. A year and a half ago, I followed him and his crew around Chile's Atacama Desert as they field-tested a next-generation planetary rover called Zoe (see "Halfway to Mars").

This winter, Wettergreen has been spending time on a ranch outside Tampico, in northeastern Mexico, putting another bot through its paces. This machine, designed by Bill Stone of Stone Aerospace, is an autonomous underwater vehicle—a smart submersible that operates without tethers, cables, or any real-time guidance from humans. Stone and his crew have been using the robot to explore deep, water-filled sinkholes, or cenotes, of which Mexico has thousands; some go very deep indeed and connect up to underwater caves.

Photo: DEPTHX PROJECT

DEEP DIVER: The DEPTHX smart submersible uses an array of 56 sonar sensors to map uncharted territory.

The oval-shaped, bright orange craft looks kind of like a giant M&M and is known as DEPTHX, for Deep Phreatic Thermal Explorer (the geo-jargon term in the middle derives from the Greek for "spring water"). During the bot's first outing in January, one of its main jobs was figuring out where it was. Sounds simple, but if you were plunked down in the middle of nowhere, without a map or a compass or even a cellphone, would you know what to do? DEPTHX does: it takes a look around and then draws its own map, using an array of 56 sonar sensors.

Here's a video of the sonar system mapping a 115-meter-deep cenote known as La Pilita. The video shows a 1-hour mission compressed to about 30 seconds. The conical flashes are the pencil-beam sonar modules firing away—340 000 times—as DEPTHX descends to the bottom of La Pilita and builds its map in real time. (The mapping software, by the way, was developed by Ph.D. student Nathaniel Fairfield.) Later on, DEPTHX swam around the cenote for about 3 hours, covering some 2200 meters, and returned to within 15 centimeters of its starting point. Pretty amazing.

On the robot's second outing, which began this past weekend, it will attempt an actual scientific mission at La Pilita. DEPTHX is equipped with various sampling tools, so the researchers will send it off to collect soil and other interesting goop from the cenote, and then bring the samples back up to the surface for further study.

The team's third and final tour of duty this season will take place in May, at a much larger cenote called El Zacatón. Though it's been popular with scuba divers for decades, nobody knows just how deep Zacatón goes or how far back its caves extend. So if all goes according to plan, DEPTHX could produce the first definitive map of Zacatón. (I'll be heading down to Mexico myself to cover that part of the project, so look for my report in a future issue of Spectrum.)

Which brings me back to my earlier question: Just what does a Mexican sinkhole have to do with a Jovian moon such as Europa? As Stone explained to me last summer, paddling around cenotes is all well and good, but the ultimate goal is to build an exoplanetary bot suitable for scoping out other watery worlds—like Europa! One of Jupiter's larger moons, Europa is believed to have a vast ocean beneath its icy surface. Assuming it could melt through that frozen expanse, a robot like DEPTHX could then, on its own, have a look around and then resurface and report back to Earth, just as it's now doing in Mexico.

For field reports on DEPTHX's excellent Mexican adventure, go to: Stone Aerospace and the DEPTHX Project.

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