Habitability and Biology - page 5
The Miller-Urey Experiment
In 1953, two researchers at the University of Chicago conducted what is considered to be the classic experiment on the origin of life. Stanley L. Miller and Harold C. Urey based their experiment on hypotheses suggesting that conditions on early Earth triggered chemical reactions, which created organic compounds from inorganic matter. So, they decided to simulate in a laboratory the hypothetical conditions that would have existed on early Earth.
The experiment used water, methane, ammonia, and hydrogen, all of which were sealed inside test tubes and flasks connected in a loop system. The water was heated to produce evaporation, lightning was simulated, and then the atmosphere was allowed to cool so that water could condense and flow back into the flask in a continuous cycle. After one week, Miller and Urey observed that 10 to 15% of the system's carbon was in the form of organic compounds, and 2% of the carbon had formed amino acids - considered to be the building blocks of life. Although only simple organic molecules were produced, the results of the experiment established that the hypothetical processes of primitive Earth could produce the building blocks of life, without requiring life to synthesize them first.
The experiment initially had its detractors, some of whom claimed that atmospheric components on early Earth may have been different than the ones tested. However, various follow-up experiments, which used different mixes of gases thought to better represent Earth's early atmosphere, have now produced all of the amino acids that are found in most living organisms.
The creation of amino acids from Earth's raw materials may well have been the first step of evolution. Because the proposed atmosphere of early Earth was based on elements common throughout the universe, it also opens the possibility that similar atoms and amino acids could have formed elsewhere.
A Controversial Meteorite
Antarctica is prime real estate for finding meteorites. In fact, many of the meteorites we now have were found in Antarctica by members of scientific expeditions. Because very few terrestrial rocks end up on the Antarctic ice, a meteorite there stands out like a cockroach on a polar bear.
A Martian meteorite found in Antarctica, cataloged as ALH84001, displays features that many scientists believe to be fossils of microscopic bacteria. (Image credit: NASA)
In 1984, a team of scientists in Antarctica's Allan Hills region found a potato-shaped meteorite, which came to be known as ALH84001. At first, they didn't have any reason to believe there was anything special about this meteorite, and it underwent the typical cataloging process that all such meteorites go through. Ten years later, a curious researcher used radiometric dating on ALH84001 and discovered that it was a Martian meteorite that had formed 4.5 billion years ago.
Because of its old age, which meant that it existed on Mars when liquid water probably flowed, scientists decided to examine the rock more closely. Observing the meteorite through a special scanning electron microscope that provides high-resolution images of a sample surface, they found chains of rod-shaped structures that looked like the fossils of ancient Martian bacteria-like organisms. In addition, carbonate deposits in the rock revealed that liquid water had flowed through it, indicating that one of the main requirements for life had been present in the rock at some time.
But some scientists questioned the evidence, claiming that non-biological mechanisms could have produced the structures. Others believed that the observed structures were the result of contamination of the meteorite during its 13,000 years on Earth.
The jury is still out on ALH84001's fossil-like structures, and researchers continue to conduct more testing on it as new technologies are developed. But determining whether life existed on Mars only by examining meteorites will be difficult. For example, most rocks on Earth do not contain fossils, yet we know that life has existed here for a long time. If an impact event were to launch a rock from Earth into space, and the rock was found by another civilization in 4 billion years, they would be wrong if they assumed that the lack of fossils in the rock ruled out life on Earth.
Life in a Subsurface Ocean: Europa
Considering the unique requirements of life-bearing planets, only a handful of celestial neighborhoods in our solar system qualify as likely candidates for life. One such location is Europa, one of Jupiter's four large moons.
The image on the left, acquired by the Galileo spacecraft, shows a region of Europa's crust that is made up of blocks thought to have broken apart and "rafted" into new positions. These features are the best geologic evidence to date that Europa may have had a subsurface ocean at some time in its past. (Image credit: NASA/JPL-Caltech)
In 1979, images from the Voyager spacecrafts revealed a network of cracks and ridges on Europa's surface, and later investigations confirmed that Europa is covered by a shell of ice. Most planets and moons without an atmosphere are covered with impact craters, yet Europa has surprisingly few of them - leading researchers to believe that recent geologic activity has essentially erased the craters from view.
Disconnected islands of ice on Europa's surface are visible in this enhanced color image taken by the Galileo spacecraft. (Image credit: Galileo Project/JPL/NASA)
Many scientists now believe that a vast ocean of liquid water could lie beneath Europa's surface ice layer, and that subsurface water from the ocean may occasionally break through and repave Europa's surface. In the 1990s, instruments onboard the Galileo mission detected a magnetic field on Europa - highly unusual for a moon. The magnetic field is believed to be caused by currents from an electrical conductor, and a salty ocean could serve as such a conductor.
Oceans on Earth are overflowing with life, and many biologists believe that life on Earth got its jump-start in deep-sea vents. If Europa has an ocean with similar deep ocean vents, they could provide the necessary energy source for chemical reactions and, thus, the formation of complex organic molecules.
Future missions to Europa could probe the surface with radar to determine the thickness of the ice and also penetrate the top layers of ice to see exactly what lies beneath.