Last updated: December 17, 2015
Alien Life in Martian Caves?
Mars is anotoriously ruthless place. Its daily temperatures fluctuate wildly;evenduring its warmest summer month, the average high is 36 degrees Fahrenheit, butwhen a tepid summer's day turns to night, it plummets to -105 degrees. UnlikeEarth, which sees transient yet localized storm systems, Mars can experiencemonths-long dust storms that spread over the entire planet.
It is no surprise,then, that NASA's curious robotic explorers have not stumbled upon any Martianlife. In fact, it seems likely that the most sensible place to look is not onthe surface at all, but beneath it –in caves.
In 1992, astrobiologist Penny Boston proposed a speculative idea –that the subsurface was "the last best place" to look for potential life on Mars. At the time, there was littleconvincing evidence to support this notion. Then, NASA began to look closer.
Over the last 20 years a series of orbiting satellites have unveiled at least 300 clearly identifiable subsurface features on Mars, notably lava tubes and pit craters. These places are isolated from the severe temperature fluctuations, solar radiation, and suffocating dust storms present on the Martian surface. In short, the subsurface is dramatically more stable – a consistency not unlike the climate inside Carlsbad Caverns, which stays around 56 degrees year round.
Dr. Boston asserts that if cave life can thrive on Earth, it may also do so on Mars. In the mid 1980's, a labyrinth in the cave system was discovered in Carlsbad Caverns National Park, Lechuguilla Cave. Long ago, its entrance was sealed off by boulders and debris, isolating Lechuguilla from the surface world. Although still on Earth, any life here would have to adapt to a highly unusual environment – a placethat is always without sunlight and has negligible contact, if any, with the land above.
As it turns out, life in this subsurface worldflourishes.
Dr. Boston found that the microorganisms there live in thick mats upon the cave walls, but each patch contains species that are considerably different than the next. "The biodiversity down there is extreme – it's super biodiverse –one patch of bacteria in Lechuguilla might have just a 20 percent overlap with the patch next to it."
The amount of life here may be immeasurable, but what is equally compelling is how these creatures stay alive. Unlike life upon Earth's surface, these organisms have little or no organic material to eat – so some eat the rock.
This might sound incredible, but Dr. Boston has proved it in a laboratory setting. Given nothing to consume but manganese and iron compounds, Lechuguilla microorganisms chemically transform this material, and in doing so, reap the energy rewards from the reactions. Below the red, inhospitable Martian bedrock, microbial life may be doing the same thing right now –consuming the inorganic in the dark, and staying alive in places we cannot so easily see.
In the past decade, the potential of alien life inextra-planetary caves has been overshadowed by the excitement of icy worlds inour solar system, notably the moons Europa and Enceladus. Each are believed to contain massive oceans beneath their icy shells, and as we know on Earth,oceans harbor an abundance of life, and many suspect that life first arose in this great stew of organic material.
Dr. Boston, however, is skeptical. "It's difficult to imagine that life on Earth began in the ocean. Anything on the surface of early Earth would have experienced extremely inhospitable conditions."
Our moon demonstrates this well –its barren surface is covered in impact craters, one of which is 1,600 miles across. These telltale scars give us insight into the violent conditions present in our developing solar system. The Earth was certainly not immune, its surface constantly met with dramatic collisions and catastrophic upheaval. In such circumstances, only the subsurface could provide a hospitable place for life to be kindled and then sheltered from the chaos above.
Perhaps, though, alien life exists both in Martian caves and swims in Europa's oceans. Dr. Boston proposes that these ocean-encompassing moons should instead be thought of as planetary scale water-filled caves, whose bedrock is composed of thick sheets of impenetrable ice.
If so, Europa might better fit the cave paradigm –It is an enclosed system, not an open ocean – which could provide enough hospitality to invite life. This is not too dissimilar from Lechuguilla Cave in Carlsbad Caverns National Park.
Or caves on Mars.
Photo: A pit crater is a steep walled and deep vertical shaft that could potentially lead to caves on Mars. Credit: NASA JPL University of Arizona