New York, Dec 2 (IANS): Dramatic climate cycles on early Mars, triggered by buildup of greenhouse gases, may be the key to understanding how liquid water left its mark on the planet's surface, a study says.

Using climate models, the researchers showed warming periods -- caused when greenhouse gases reached a certain tipping point -- lasted millions of years on Mars, melting the glaciers that covered the surface of the planet, thereby creating liquid water.

Scientists have long debated how deep canyons and extensive valley networks -- like the kinds carved by running water over millions of years on Earth -- could form on Mars some 3.8 billion years ago, a time many believe the planet was frozen.

Previous studies suggested asteroid impacts might have warmed the planet, creating steam atmospheres that led to rain. But those warm periods would have much shorter durations and struggle to produce enough water, the researchers said.

"We think Mars had to be warm for millions to tens of millions of years, and the impact hypothesis can keep it warm for thousands of years," said study co-author Jim Kasting, Professor at Pennsylvania State University in the US.

"In terms of water, we need millions of meters of rainfall, and they (previous studies) can get hundreds of meters," Kasting said.

The new study suggest a glacier-covered early Mars could have experienced long warm periods, lasting up to 10 million years at a time, caused by a thick atmosphere of carbon dioxide and hydrogen.

The team, which published its findings in the journal Earth and Planetary Science Letters, found the warming cycles would have lasted long enough, and produced enough water, to create the features.

"With the cycling hypothesis, you get these long periods of warmth that give you sufficient time to form all the different Martian valley networks," Natasha Batalha, graduate student at Pennsylvania State University, explained.