Being the most Earthlike planet, Mars has long inspired speculation as to whether life could have ever existed on worlds other than Earth. Telescopic observations of “canali” on Mars by observers such as Schiaparelli and Lowell promoted the idea of dying Martian civilizations. These dreams were quashed by the first images of Mars’s surface obtained by the Mariner 4 spacecraft in 1965. The Mariner images revealed a desolate, cratered, and desert-like world that appeared to be hostile for life.
The twin Viking landers arrived on Mars in 1976. Tests of soil samples did not discover any known forms of life. In 1996, after a 20-year exploration hiatus, NASA scientists held a press conference and claimed that evidence of Martian fossil life had been found in a Martian meteorite named ALH84001 that had been recovered in Antarctica. Microscope images revealed apparently biogenic, wiggly, worm-like structures. Other scientists were quick to condemn this conclusion, and subsequent research proved that the anomalous structures could have resulted from non-living mineral growths.
The strategy of looking for microbes on Mars had not been conclusive, so mission planners next employed the strategy of looking of water. On Earth, water is the most important requirement for life, and Mars imagery displays many features that resemble fluvial erosion. Mars also has white seasonal polar ice caps which consist mainly of frozen carbon dioxide (dry ice), but which also possess frozen water.
In 2004, NASA deployed the twin roving spacecraft Spirit and Opportunity to look for evidence of past water along the Martian equator. The rovers are driven remotely from Earth, and they are equipped with a variety of instruments for examining Martian rock and soil. Opportunity immediately sighted Martian bedrock, which appeared to be layered like sedimentary rocks on Earth. The rock also displayed dark, spherical objects christened “blueberries”. Tests of the soil chemistry revealed toxic levels of salt, which would likely contribute to a lifeless environment.
The Mars Phoenix lander was sent in 2007 to study Mars’s polar region. It was reasoned that liquid water might be found associated with the Martian polar ice caps. The Phoenix was able to directly examine ice in the Martian soil, and it became the first space mission to directly touch water on another planet. Soil tests indicated pH levels ideal for life, yet containing a troubling abundance of perchlorate, a chemical toxic for most life. Despite this discovery, some bacteria on Earth are able to subsist on perchlorate, the case for life on Mars has still not been settled.
Around 3 billion years ago, Mars and Earth both had environments favorable for life (termed “Second Genesis” in the video), and life might have appeared independently on each world, or it could have arisen separately on one planet and been introduced to the other via delivery by meteorite. Eventually, Mars became an apparently lifeless world unlike the Earth, where life is found nearly everywhere in great abundance, from harsh deserts to the sunless ocean depths. It is possible that a large meteor impact might have disrupted the early Martian environment. Despite this, periodic shifts in the Martian axis may have created warm periods when liquid water could exist. Currently, Mars’s axis of rotation is tilted by 25°, not much different from Earth’s 23.5° tilt. Evidence found in the remote sensing of rock layers provides evidence that the Martian tilt might periodically reach 45°, which would create enhanced summer weather.
The video ends with an introduction to the next scheduled Mars mission, the Mars Science Laboratory (MSL), and its rover Curiosity, which landed on Mars in 2012. The Curiosity rover is nearly the size of a small automobile, and is currently the most sophisticated rover ever deployed to Mars.
The question of life on Mars
has not been conclusively settled. If life appeared independently on both Mars and Earth, than life is likely common throughout the universe. Or life could have be a lucky, one-off development on Earth alone. Either way, the question of the origin of life leads to profound insights into the status of humanity in the universe.
The PDF contains a two-sided video worksheet consisting of 54 multiple choice and true-or-false questions, along with an answer key, and an MS Word download link. You will need to obtain a DVD of the video, use the PBS Internet site, or YouTube