Exploring Caves From 30 Feet in the Air
It was Thursday morning, just after dawn. A group of about 20 of us had been driving through the Mojave Desert since 4:30 am, in the dark, and we had finally arrived at our destination: a large flat parking area near the top of a volcanic cinder cone, a short ways up a makeshift road from an abandoned mine. Before us stretched a vast black lava field, home to more than 300 lava-tube caves.
We were there as part of Spaceward Bound, a NASA program that brings elementary-, middle- and high-school teachers together with scientists to do field research focused on the search for life on Mars.
It was the caves that had brought us there, and it was their entrances that J. Judson "Jut" Wynne was hoping to detect – from a hot-air balloon, tethered to a quartet of SUVs and hovering 30 feet off the ground.
Wynne, who is with the USGS-Southwest Biological Science Center in Flagstaff, AZ, and a PhD candidate in the Department of Biological Sciences at Northern Arizona University, has a grant from NASA’s Exobiology Program to find out whether it is possible to detect caves by studying their thermal signatures, the amount of heat they radiate. If he can reliably pinpoint caves here on Earth, Wynne believes, it should be possible to do it on Mars as well.
Martian caves interest NASA because they are not subject to the intense radiation, wide temperature swings, hurricane-force winds and bombardment by micrometeorites that plague the martian surface. This makes them good environments for preserving evidence of past life on Mars. It also makes them good places for humans to call home. An inflatable habitat set up inside a cave would shelter earthlings from harsh surface conditions.
A poster presented at the 38th Lunar and Planetary Science Conference (LPSC) in League City, Texas, reported on the observation by THEMIS, a visible and near-infrared camera on board NASA’s Mars Odyssey orbiter, of seven openings on the martian surface that may be cave entrances.
Interpretation of the THEMIS data suggests that it may be possible to spot caves on Mars from orbit, but the camera’s resolution is not high enough to make a definitive identification. Wynne, a co-author on the LPSC paper, is honing thermal-cave-detection techniques here on Earth in hopes of some day sending a more-sensitive instrument into orbit around Mars, and ultimately perhaps flying a thermal imager across the martian landscape attached to a hot-air balloon.
He is using a commercially available Flir B20 ThermaCam, which measures the temperature of whatever it is pointed at, and displays a 4-by-6-inch LCD image, in hues of red, yellow, green and blue, that provides a visual indication of where in the scene the hot and cold spots are. Thermal imaging cameras like this one have many common uses – fighting forest fires, checking the thermal efficiency of homes, finding weaknesses in power transformers – although most people probably know about them from watching CSI on television.
Cave entrances have distinctive thermal signatures, which vary from cave to cave, depending on the internal configuration of the cave, its rock type, the season and the time of day.
Deep inside a cave, in what is known as its "dark zone" because sunlight never reaches it, the temperature of its walls remains constant, day and night, all year round. The rocky surface surrounding a cave’s entrance, however, undergoes broad temperature swings as it alternately gets baked by the sun during the day and then releases its heat at night. The temperature of the rock just at the entrance to the cave goes through similar changes as the surrounding surface rock, but these changes are dampened by interaction with the internal cave air, which is at a near-constant temperature.
What Wynne is particularly interested in this morning is finding warm (red) spots – these are the cave entrances, which have retained some of the internal heat of the cave throughout the night – surrounded by cooler (blue) surface terrain, which has lost its heat overnight and has not yet been warmed by the sun.
Long-term, Wynne hopes to tease out the patterns of these temperature differences; he wants to know when the contrast between the temperature of the surface rock and the cave-entrance rock is greatest. That’s when the opening will show up most distinctly in a thermal image.
He has spent the last year and a half crawling in and out of caves throughout the southwestern U.S., first to place temperature sensors that record hourly temperature readings, and then on repeated return visits to download data and replace worn batteries. In each cave, he has placed three sensors: one in the dark zone, one at the cave entrance, and one on the surface near the entrance. He also collected data during a two-week period last summer at a pair of caves in Chile’s Atacama Desert.
From all this data, he has begun to see clear patterns emerge. Some caves are easiest to detect during midday, others at night. It depends on the internal structure of the cave. But he is also learning there is seasonal variation. Some caves, for example, can be detected more easily in the winter than in the summer. He presented preliminary results of this work earlier this year in a second LPSC poster; this work has also been submitted for publication in a peer-reviewed journal. His conclusion is that it is feasible to search for cave openings by looking for their thermal signatures.
"We’ve proven the concept," he says. "We’ve shown that there is indeed a significant temperature contrast during certain times of day, certain times of year, between the entrance and the surface. We have hourly data that shows that, and we have thermograms that clearly show that as well."
The next step is to correlate the hourly data with thermal images taken from an aerial platform. That’s why he’s here in the Mojave, waiting for a balloon to take him aloft. Although he’s collected copious data from within caves, he’s had only one previous opportunity to search for them from the air.
"Standing in front of the entrance of a cave and taking pictures," he says, "and seeing that … there are differences" between the temperature on the surface and at the cave entrance, "we’ve done that till the cows come home. But actually looking at it from an aerial platform and being able to say, ‘Yes, that’s a cave’ – well, that hasn’t been done yet."
Wynne did a 3,000-foot flyover of some of his cave sites in the southwest with NASA-Goddard in December 2006, using NASA’s QWIP thermal camera, and saw indications in the thermal imagery that caves were present. But the flight was at night, so he couldn’t take actual photographs that would allow him to correlate visible features of the landscape with the thermal images. And the GPS unit he was using malfunctioned. "As a result," he said, when he looks at the thermal images, "we don’t know where the heck we are." He is currently working with engineers at Goddard to resolve this issue, and hopes to begin analyzing these images in coming months.
The balloon flight in the Mojave provides an important opportunity to compare what he can see with the thermal camera with what he can see visually. Meanwhile, as the balloon crew is making preparations, Wynne has clambered up a steep hillside of volcanic scree, about a 100-foot gain in elevation, his thermal camera, tripod, notebook and walkie-talkie in hand, to the top of the cinder cone that rises above the parking area. From here he has an excellent vantage point out over the lava field. So good, in fact, that he’s reluctant to come back down the hill when the balloon is ready to launch. So after about 15 minutes in the air, he lands, and scrambles back up the cinder cone. It’s not that the view from the balloon is bad; it’s just better from the hilltop.
Finding cave openings with a thermal camera is still as much art as it is science, however, so Wynne is using human assistants – their body heat, actually – as a visual aid. He radios to his colleagues a request to walk around and tracks their movements, which show up as tiny red blips. They walk to a cave opening, which they have verified by close-up visual inspection, and Wynne checks the camera to see if the expected thermal contrast registers on its screen. It does. Within an hour, he is able definitively to identify six cave openings – a first.
What remains to be done, in phase two of his project, is to develop a set of computer models that delineate the thermal behavior of a wide variety of cave types on Earth; and then to tweak those models for the environmental conditions on Mars, where temperatures, solar radiation and atmospheric density are quite different. Eventually, he hopes to be able to take a sequence of thermal measurements from a target surface feature on Mars, and then to determine whether it behaves thermally like his models tell him a martian cave should.
"Then we can say, ‘Okay, is it just a little divot that goes back 10 feet and that’s it, or does it possibly lead to a major passage?’ Obviously," he says, "the bigger the cave, the greater the interest. The deeper the cave, the more protected the environment. Caves with considerable passages will be of greatest interest for exploration."
And some day, if everything checks out – who knows? – NASA just might begin making plans to move in.