For decades, space scientists have searched Mars for signs of water, the liquid generally believed to be essential for life. Now, they may well have found it.

Scientists announced Thursday that they had detected dozens of slopes across the southern hemisphere of the planet where previously undetected dark streaks come and go with the seasons. When the planet heats up, the streaks appear and expand downhill. When it gets cold, the streaks disappear.

The best explanation they have so far is that those dark, fingerlike streaks are a kind of salty water that is running on or just below the Martian surface. At one location — Newton Crater — they have counted as many as 1,000 of these possible streams flowing down the slopes and into a basin.

It’s a discovery that, if confirmed, would fundamentally change the understanding of Mars and would strongly support the widely held theory that the planet was once far more wet and warm. And scientists say the discovery of water would provide the best target yet for finding possible life beyond Earth.

“We haven’t found any good way to explain what we’re seeing without water,” said Alfred Mc­Ewan of the University of Arizona’s Lunar and Planetary Laboratory. McEwan is the lead author on a paper about the discovery in the journal Science.

“And if we confirm that it is a salty water, then we have the best idea yet about where to go to try to find extant life on Mars,” McEwan said.

The dark streaks were initially noticed by a student at the school in images sent back by the Mars Reconnaissance Orbiter (MRO). The pixelated images were taken as far back as 2007, but with so much data coming in from space missions, they had remained unstudied. McEwan suggested that the student — geophysics junior Lujendra Ojha — examine over time the locations with streaks, and Ojha found that the streaks changed dramatically by season.

“None of these images by themselves are particularly revealing,” McEwan said. “But when you put them together and see what happens over time, then you can clearly see something important is happening.”

Gradually, a team of researchers determined that the changes came with increasing and decreasing temperatures. They began scouring the MRO images for other similar sequences and so far have found seven confirmed locations and possibly 32 more. In all cases, the flows appear to go around, rather than over, obstacles such as rocks, and sometimes they peter out before they reach flat ground. They are generally between about two feet and 15 feet wide, and occur during the Martian summer, when temperatures range from 10 degrees below zero to 80 degrees Fahrenheit.

At a NASA news conference Thursday, McEwan and others associated with the MRO, Mars science and astrobiology hailed the finding as a potential turning point. Philip R. Christensen, a geophysicist at Arizona State University, said it constituted the best evidence for the possible existence of water on the Martian surface. Indiana University biogeochemist and astrobiologist Lisa Pratt said it also offered the most promising habitat discovered for current Martian life and speculated that the conditions could be similar to Siberian permafrost, where life exists. All of the speakers, however, said the finding was, at this point, circumstantial rather than proven.

Possible signs of water on Mars have been reported before but later discounted. In 2006, for instance, scientists reported what appeared to be significant changes in several crater gullies, and water flowing from underground — perhaps in the form of “flash floods” — was offered as an explanation of what was happening.

Subsequent research has cast substantial doubt on that explanation, especially since it was supposed to have taken place during the Martian winter, when temperatures are far too cold to allow for water to be liquid. The current view is that frozen carbon dioxide may have been what was flooding.

Nonetheless, it is well accepted that there is, or has been, water in various forms on Mars. NASA’s Phoenix lander uncovered frozen water just below the Martian surface in 2008, and some contend that drops of liquid water were photographed on the legs of the Phoenix after it landed. Beyond that, the many dry gullies and deltas on Mars have convinced most planetary scientists that water once ran across the Martian surface.

Because the nature of the streaks remains unconfirmed, the researchers didn’t name them streams but rather “recurring slope lineae,” or lines. They put forward other possible explanations — that the streaks are rock falls or the result of high winds or other dynamics of the Martian atmosphere — but concluded that some kind of water is most likely.

The caution also reflects the fact that a spectrometer on the orbiter did not read the presence of water from molecular data it had collected. McEwan said that the flows may well be too small for the spectrometers to pick up, and he said he is convinced that equipment in the future will be able to determine whether it is water.

If the streaks do contain water, they would have to be heavily saturated with salts, which lower the point at which water freezes. The Phoenix mission found the potent compound perchlorate on Mars, a salt that has an especially strong effect on lowering the freezing point of water. The salt level needed to allow Martian ice to sometimes melt is about the same as the salt in the Earth’s oceans, the paper reports.

NASA announced last month that its next mission to Mars — featuring a rover named Curiosity, scheduled to land a year from now — is going to the Gale Crater. McEwan said that his team has not found any of the possible water steams at that location.