<p>PASADENA, Calif. — The prospect of life on Mars has charged the public imagination for more than a century, ever since astronomers first spied what they thought were canals dug to irrigate the planet's ruddy surface.</p>
<p>But after spacecraft and Earth-based telescopes began taking a closer look at the planet, evidence of the canals, and the Martians who presumably created them, quickly vanished.</p>
