In Hawaii, as on Mars, Lava Tubes Hide Secrets Beneath the Surface

Most of our time on this simulated Mars mission is spent inside a geodesic dome. We conduct research, make and document meals for our food study, do chores, and fill out psychological and behavioral surveys. It’s no surprise, then, that adventure is hard to come by. But on Wednesday of this week, three of us gave it our best shot.

We donned our green spacesuit simulators and took a hike. For two and a half hours, we clambered over the shifty and crumbling lava rocks just east of our habitat. We walked to the edges of pits and peered over steep drop-offs. And we investigated a nearby lava tube cave, hollowed out years ago by an immense column of molten lava.

A walkabout such as this is called an EVA, or extra-vehicular activity. Wednesday’s EVA had a particular objective: to explore the caves and cavities near our habitat. Technically called lava tubes and skylights, these structures interest our crew geologist, Oleg Abramov.

Back on Earth, Oleg is a research space scientist with the USGS astrogeology branch in Flagstaff, AZ. Here on simulated Mars, he plans and leads geological EVAs. On this EVA, our longest and most ambitious to date, Oleg’s intent was to collect samples. He wanted to get better acquainted with the geology of the region, including the composition of lavas and white coatings we’ve seen on many rocks in the area.

In addition, he wanted to ground-truth satellite images for a number of skylights he spotted on Google Earth/Simulated Mars. His hope was that some of these holes in the ground would provide access to lava tube caves below.

Thanks to satellite imagery, we now know that both Mars and the moon also have lava tubes and skylights. These caves and holes likely formed the same way they do on Earth. As a channel of molten lava flows, its top layer, exposed to air, cools and forms a crust. Below, the hotter lava continues to course until it empties out, leaving behind a tube-like cave. Skylights form when parts of the lava tube ceiling collapse.

Sometimes these ceilings crumble and completely block access to the cave. Other times, they fall away clean, leaving pits with dangerous, potentially unstable overhangs. But once in a while, the rocks fall in such a way to give unfettered access to a lava-carved tunnel.

Caves and skylights on Mars and the moon have recently attracted the attention of researchers keen on finding the best places to built extra-terrestrial habitats (as well as those looking for extra-terrestrial — likely microbial — life). Temperatures underground are less variable than on the surface. Moreover, caves protect against radiation; a below-ground habitat wouldn’t need heavy, expensive radiation shielding.

But on our EVA this week, we weren’t looking for a new place to live. We just wanted to explore and collect some geological samples. The first pit we approached was an enormous gouge in the ground. Inside, a cave mouth gaped roughly 20 meters high. Unfortunately, the edges of the skylight were simply too dangerous to descend in our bulky spacesuit simulators and without rappelling gear.

The next skylight was smaller, about five meters across, but its perimeter seemed unstable. We couldn’t see a lava tube entrance and could only get close enough to get the sense that the drop would be a doozy. We affectionately called this one the Pit of Death.

After a few more inaccessible skylights, we came upon a fairly shallow one that sported a lava tube opening with surprisingly easy access. Oleg, Yajaira Sierra-Sastre (our science officer), and I made our way with caution. Last September, archaeologists discovered what appeared to be an ancient Hawaiian burial site in a lava tube in an area adjacent to our habitat.

Human remains were found, along with a hearth and kukui nut shells. Sensitive to the possibility of disturbing such a site, we stayed near the entrance and looked for indicators of prior human activity. There were none, so Oleg collected samples of white coating on the rocks while Yajaira and I took pictures and video to document the structure.

On the way back to our habitat, we came upon a few more skylights and another death pit. We didn’t dawdle or get too close. Our return journey was slow and mostly consisted of walking on solidified pāhoehoe lava flows. These are the kind that can take on a variety of looks: like heavy fabric that’s bunched and frozen in place, like thick petrified ropes or like piles of shattered dinner plates.

We also made a difficult crossing of an aʻā lava flow, the type that’s sharp, jagged and brittle. And the final challenge between us and our dome was a steep up-and-over on a cinder cone ridge made of small, gravel-like stones.

Two steps forward, one step back. After what seemed an eternity on uneven, ankle-breaking terrain, we finally made it home. We arrived safe, sound, and grateful for the chance to explore.