Twenty-five years ago last month, the Viking 1 space probe landed on Mars and spent the next 61/2 years sending back data about the Red Planet. Viking 1 stopped transmitting in 1982 while its sister probe, Viking 2, which landed in September 1976, fell silent after three years.

These nearly forgotten space probes are worth remembering for two reasons; one is the opportunity their anniversary gives us to review what had been learned about Mars in the past quarter-century, the other to note the fact that data from Viking 1, shelved for 25 years, are now at the center of the continuing controversy about whether life has ever existed on the only other planet in the solar system remotely able to support it.

John Wilford, in his book “Mars Beckons,” tells how the Romans made Mars the god of war because of its distinctive red color. Oddly enough, it fell to a latter-day astronomer, a wealthy Bostonian named Percival Lowell, to excite the popular imagination as to whether life might exist on Mars. Lowell read the work of an Italian astronomer named Giovanni Schiaparelli who reported the presence of canals on Mars in 1877. This inspired Lowell, says Wilford, to build an observatory near Flagstaff, Ariz., and devote years to studying Mars. Between 1895 and 1908, he published three books and innumerable articles about the intricate network of canals and even theorized that they were used to carry water from the poles to cities lying near the equator.

Most professional astronomers dismissed Lowell’s sightings but he sparked an intense interest in Mars among everyday readers. Edgar Rice Burroughs, the author of “Tarzan,” wrote 11 books about Mars while H.G. Wells’ dramatic radio presentation of “The War of the Worlds” threw the nation into a panic in 1938.

Astronomers dismiss the possibility that life beyond rudimentary microorganisms could exist on Mars. But Mars has been the target of many interplanetary probes searching for the possibility that water, and hence the conditions for primitive life, once may have existed on the arid, wind-swept surface.

Mariner 4 started Martian exploration with a flyby in 1965, and the first U.S. landing on Mars was Viking 1 on July 20, 1976. Viking 1 conducted a series of experiments searching for signs of life that, at the time, were labeled inconclusive, but now it may be this judgment was premature.

NASA’s fortunes took a nosedive in 1993 when its $1 billion Mars Observer mysteriously disappeared just as it was due to enter Martian orbit, but revived in 1997 when Pathfinder went into orbit and deposited a small sniffing robot, Sojourner, on the Martian surface. Since then, two major probes, the Mars Climate Observer and the Mars Polar Lander, have been lost. NASA hopes its fortunes will revive again with the Mars Surveyor and Mars Lander due to land in 2001 and 2002 respectively.

The Viking 1 lander carried four experiments that were designed to look for signs of primitive life in the Martian subsoil. A robotic arm scooped up a sample of soil, mixed it with radioactive nutrients, and gently warmed the mixture. The idea was that any microbes present would consume the nutrients and release radioactive gases. This, in fact, appeared to happen but critics quickly pointed out that the results could be explained equally well by inorganic chemical reactions.

Interest in Martian microbes revived in 1996 when NASA researchers reported traces of fossilized bacteriallike organisms in a meteorite of Martian origin. Again critics offered alternative explanations and Richard Kerr, in the Nov. 20, 1998, issue of Science, wrote “that few scientists now believe the claim.”

Now a July 27 Reuters report by Kevin Krolicki says that the 25-year-old Viking 1 data may have been correct. Joseph Miller, a USC neurobiologist, retrieved Viking 1 data stored in NASA archives and found that the radioactive gases released from Martian soil followed the same circadian rhythms as do Earth-bound organisms in their feeding and respiration cycles. “I think, basically, it’s bugs,” says Miller, who is trying to access the complete Viking data computer tapes. This may prove impossible, he says, because they are coded in a format so old that the programmers who knew it have died.

The key to life is water and there is increasing evidence that Mars once was a wet planet. The July 12 issue of Nature carries several articles reviewing current knowledge about the ancient Martian surface. In one, Victor Baker shows pictures of what he says are glacial plains, channels cut by running water, and a distinctive pattern of polygons caused by repeated freezing and thawing in an active layer overlying an ice-rich permafrost region. In another, Bruce Jakosky and Roger Phillips illustrate valleys cut by runoff and discuss what the ancient Martian climate must have been like at the time of their formation.

So where is the water today? Researchers believe it is locked beneath the surface in permafrost or in the polar caps. Either way, it looks like Mars was sufficiently wet in the past that the search for microbial life, past or present, is not merely a quixotic quest by a few die-hard romantics.

Clair Wood taught chemistry and physics for more than 10 years at Eastern Maine Technical College in Bangor.