WHEN SCIENTISTS ANNOUNCED last summer that they had potential evidence of an-¦ cient life on Mars, no one was more excited than Carl Sagan. For decades he had scoured the heavens for any credible sign that life existed elsewhere in the vastness of the universe. Along the way he became America's best known popularizer of science, an astronomer with a touch of the poet and a voice that seemed to have a separate control for "awe." In his hugely popular 1980 PBS series, Cosmos, Sagan guided millions of people through the galaxies. "I like to explain science because it's like being in love," he once said. "When you're in love you want to tell the world."

And he did. When he died on Dec. 20 at 62, Sagan had written 22 books and won the Pulitzer Prize for The Dragons of Eden: Speculations on the Evolution of Human Intelligence, a 1978 nonfiction work. His 1984 sci-fi novel, Contact, is set to become a film with Jodie Foster.

Sagan died at the Fred Hutchinson Cancer Research Center in Seattle from a rare blood disease, myelodysplasia, that devastates the immune system and opens the way to illnesses like leukemia and pneumonia, which finally killed him. In hope of arresting the disease, Sagan had a bone marrow transplant in April 1995, followed by weeks of chemotherapy. But after a few months, the disease returned last summer.

Sagan grew up in Brooklyn, "one step out of poverty," as he once put it, the son of a homemaker and a garment cutter. Earning undergraduate and graduate degrees at the University of Chicago, he established his credentials early with research that helped prove that the surface of Venus, despite its cloud cover, was burningly hot. He also distinguished himself as a teacher. "He had a wonderful enthusiasm and ability to communicate," recalls NASA space director David Morrison, who in the early '60s was one of Sagan's first graduate students. By 1969, Sagan had risen to the post he would occupy for the rest of his life, professor of astronomy and director of the Laboratory of Planetary Studies at Cornell University. He drew wide attention with his work for Pioneer 10, the 1972 NASA probe designed to depart the solar system and press on into interstellar space. Fixed to its exterior is a gold anodized plate that bears a greeting, in maps and symbols, to any extraterrestrials who might see it (it's now beyond the solar system). Sagan later took on crusades against nuclear weapons and environmental threats.

Even as he saw the end approaching, Sagan, who wrote the entry on "Life" in the Encyclopedia Britannica, held an unflinching notion of its eternity. He expected that his matter and energy would be dispersed after death throughout the cosmos. Given all that he was in life, that would be a lot of energy.