Martian Bunny Chase
Recognizing a fossil using the criterion of shape alone poses some challenges, particularly without actually being on the surface of Mars. An example found in one evocative image illustrates one case to consider the broader issues of how to hunt for promising paleontology, just using color, size, position, and shape alone.

Brewing Sulfur with Martian Water
If the very high sulfur content found at the Opportunity landing site points to its aqueous history, then what speculative biology could take advantage of brewing sulfur with water. According to one Mars' veteran, there are fascinating extreme microbes that can make good use of these chemical combinations.

Could Opportunity Find Life on Mars?
Andrew Knoll is a member of the Mars Exploration Rover science team and Fisher Professor of Natural History at Harvard University. His research focuses on ancient rocks on Earth; he studies how well they preserve evidence of ancient terrestrial life. Shortly after Opportunity landed on Mars, Astrobiology Magazine's managing editor, Henry Bortman, spoke with Knoll about the scientific potential of the Opportunity landing site. In a previous interview segment, Knoll discussed how iron deposits near the Rio Tinto in Spain could help scientists understand the history of the hematite deposits on Mars. In this segment, Knoll considers the possibility that Opportunity could find signs of fossil life on Mars.

Dry Limit of Life
Among the triad of biological limits to life on Mars--cold, thin air and dryness--a new study in the driest place on Earth reveals a remarkably sterile crucible for testing instruments that might one day answer questions about microbial life on other planets. The Atacama desert in Chile, when probed with some of the same techniques used during the 1976 Viking mission, found no life on Earth, a finding that may help scientists understand the dry limits to life and the potential importance of site selection.

Viking Dust
The soft-landing Viking missions to Mars offered a challenging set of experiments to test for biological activity in 1976. As biology has progressed in the ensuing quarter-century, one of the principal investigators continues to mull over what that mission sought to test. In preparation for the three planned missions in the next month and half, those results are revisited.

Chomping on Nano-Nuggets
Eight years ago, nanometer-sized features resembling bacteria were discovered in the Martian meteorite ALH84001. Although some scientists think nanometer-sized life can’t exist, others contend that nanobacteria are the new frontier in life science. A recent study published in the journal Geology suggests that nanometer-sized structures are proof of life, although they are not life itself.

Martian Hot Spots
One of the most intriguing questions for scientists who study Mars is where is the best place to search for evidence of hospitable conditions. NASA's Mars missions have adopted the credo to 'follow the water'. As new infrared images are becoming available from orbit, however, some sites have sufficient temperature anomalies, or hot spots, to suggest not just ice-water as exploration candidates. One such site is located in the Hellas impact basin, among so-called ice towers which owe their name to analogies to Antarctic steaming vents.

Life Pinned on Viking Horns?
The 1976 Viking mission sampled the strongly rusting soil on Mars. Retrospectively, its three biology experiments give insight into the challenges of working remotely, where one can never have too much data for further analysis.

Hitchhikers May Have Thumbed A Ride to Mars
Could dormant forms of bacteria called endospores potentially travel from Earth to Mars aboard spacecraft? If so, new experiments suggest that even a dry and cold Mars might not prove so inhospitable, despite the possibility of self-sterilizing and oxidizing martian soil.

Mars: Tilting towards Life?
Where is the best place on Mars to look for evidence of life? At the poles, says one scientist. Although frozen solid today, in past eras, when Mars was more highly tilted, the poles were warm enough for liquid water to form.