Humans have never been limited by their curiosity of the world around them. Rather, they have only been constrained by the technology enabling them to extend their explorations further and further. We are still discovering and exploring the nooks and crannies of Earth. For instance, it has only been within the last few decades that we have had the technology to venture into the greatest depths of the oceans. In fact, we were able to set foot on the Moon before exploring parts of our own planet! In our explorations, we have discovered unusual and mind-boggling environments. And many of these environments, despite their often extreme conditions, have also been identified as harboring living organisms. On Earth, we have started creating lists of these extreme environments. Ranging from the dry, cold Antarctic Valleys to the boiling, acidic hot springs of Yellowstone National Park, Earth certainly lays claim to its own fair share of extreme environments. Astrobiologists strive to understand these environments in hopes that they will help us to better understand other extreme environments within the Solar System.
Since we define environments as extreme with respect to a human's ability to survive, it shouldn't come as a surprise that our Solar System has many extreme environments. A handful of these environments are of interest as possibly yielding insight into the origins and evolution of life on Earth. For instance, the environment on Saturn's moon Titan may at first seem to be so strange as to be completely unrelated to anything ever seen on Earth. With a thick atmosphere, frigid temperatures, and liquid methane lakes, the scenery on Titan is certainly like nothing beheld by human eyes. However, scientists have long contended that Titan could be a frozen analog of early Earth. Consequently, scientists were elated when the Cassini spacecraft successfully captured images of the surprisingly dynamic moon. With sufficient evidence of tectonism and erosion, Titan's extreme environment may reveal secrets about our own planet's childhood.
Another moon within our solar system represents another interesting extreme environment. Europa, one of Jupiter's moons, is covered by a thick, water ice crust that is thought to conceal a global ocean. With only a very thin oxygen atmosphere, this cold moon is highly vulnerable to the ionizing radiation emitted from Jupiter. Life on the surface of Europa would find it extremely difficult to survive. However, beneath the icy crust life might be able to seek haven from radiation and find geothermal energy sources at the bottom of the ocean. Nearly absent of impact craters, Europa is thought to have a very active surface. Explorations of our own planet might help us plan future missions to Europa. Lake Vostok in Antarctica is sometimes thought of as a miniature analog of Europa. Its surface is completely frozen over; the water below has not been exposed to air for millions of years. Learning how to drill through the frozen layer without contaminating the water below may provide hints for future exploration of Europan oceans.
One of our closest neighbors, Mars, has been of the utmost interest to scientists. Its proximity to Earth has made it possible to send numerous spacecraft and satellites to the red planet. What has all this instrumentation revealed? Mars is a cold planet with a thin atmosphere consisting primarily of carbon dioxide. Bombarded with ultraviolet radiation from our Sun, the surface of Mars is highly oxidized. However, recent Mars missions have revealed frozen subsurface water in several regions, including a possible frozen sea near the equator. Frozen water is of the utmost interest to astrobiologists as most agree that liquid water is the one prerequisite for life. Despite the harsh conditions on the planet's surface, the possibility for pockets of subsurface liquid water is intriguing. Mars represents one extreme environment in the Solar System from which we are able to collect a significant amount of data.
Extreme environments in our Solar System are the rule rather than the exception. For instance, one look at Jupiter's four largest moons reveals four very distinct and extreme environments. Venus, our sister planet, has extremely high temperatures and acidic conditions. Mercury is boiling on the side facing our Sun and freezing on the other. The challenge is not identifying extreme environments. No, the challenge will be to identify life in these extreme environments if it does indeed exist there.