四年多之后，美国宇航局的双胞胎机器人探险器终于在火星上着陆，这个效率是远远低于了预期。在这一路上，他们经历了有记录以来最严重的火星风暴和几乎致命的软件故障，方向盘损坏，令人恐惧的降落在陡峭的斜坡上。2008年5月25日， 他们在这红色星球上有个一个新的同伴：美国航天局的凤凰号探测器，它戏剧性地在7月“品尝”了这个星球上的water ice（专业术语？不懂...）。 本集中展示了最新的科学成果，提供了外星球第一现场的化学测试。随着火星探测器的调查结果公布，凤凰号有望揭示新的线索关于在这个诱人的红色星球寻找水和生命。
With unique access to the NASA Phoenix and Mars Exploration Rover missions, NOVA shows scientists and engineers in action, directing the operations of spacecraft millions of miles away, as the robotic explorers drill into rock, claw into soil, analyze samples, and trundle across the rock-strewn landscape in search of signs that Mars once or maybe even still harbors some form of life.
They may not be on Mars themselves, but there is drama enough for the Earth-bound operators, as they must deal with harsh martian conditions and equipment emergencies in pursuit of their science objectives. NOVA captures the payoff in stunning surprises that bring cheers to the science operations centers. (See Behind the Scenes and Mars Up Close for more on the rovers' exploits.)
So far, there is good news and bad news on Mars. The good news is that the planet has water today in the form of subsurface ice, and there is evidence that water once flowed in abundance across the planet billions of years ago. (For more, see Life's Little Essential and Mars From Afar.) The martian soil is also conducive to life in some locations, although neither Phoenix nor the twin rovers were equipped to look for life directly. (To see what the rovers are equipped for, go to Anatomy of a Rover.)
The bad news is that for all its superficial resemblance to Earth, Mars and our planet clearly took different paths billions of years ago. Mars today is a cold, dry, hostile world with an ultra-thin atmosphere that won't support liquid water at the surface.
What happened? Through interviews with scientists and stunning computer animations of the changing martian environment, NOVA pieces together a picture of a world rocked by cataclysms. One relic of an ancient catastrophe is a mysterious global difference between the terrain in the northern and southern hemispheres.
To explain this asymmetry, a recent theory proposes that Mars once captured an asteroid into orbit around the planet, generating a strong magnetic field in the process and resulting in a thicker atmosphere and surface water. The object later crashed into Mars, creating the dramatic contrast between northern and southern terrain, and also rendering the planet the arid place it is today.
Whether there was ever life on this wetter, more Earth-like Mars is harder to say. NOVA interviews paleontologist Andrew Knoll of Harvard University and planetary scientist Christopher McKay of the NASA Ames Research Center. Both are on the science teams for Mars landing missions.
Knoll contends that the possibility for life on Mars—past or present—is looking less and less likely as the data come in. By contrast, McKay fervently believes that definitive signs of life will eventually turn up.
With the issue still up in the air, both sides feel all the more compelled to get to the bottom of this momentous question, which can only be answered by more missions and new discoveries on this enigmatic world. (For the latest updates on the twin rovers, Phoenix, and the upcoming Mars Science Laboratory, see "Live" From Mars.)