MIAMI (AFP) - Mars may once have supported life but is now a cold, dry planet, and scientists said on Thursday that a stormy sun likely accelerated the loss of its atmosphere.

In fact, researchers believe the thick, protective atmosphere that allowed ancient Mars to be warm and wet billions of years ago may have disappeared far earlier in its history than previously thought.

Data from Maven, an unmanned spacecraft that has been circling Mars for the past year, was published in scientific studies, including four in the journal Science and 44 more in Geophysical Research Letters.

Instruments aboard the spacecraft are measuring ions in the upper atmosphere of Mars and are also keeping track of solar wind to help understand what influences the escape of gas to space.

"The key findings that we are publishing this week all relate to how escape occurs and how much escape is occurring today," lead researcher Bruce Jakosky said in a podcast interview with Science magazine.

"What we are finding is that the rate of loss out the top is relatively slow today, maybe only about 100 grams per second globally, but over time that can be a significant loss," he said.

"And we think that that is the tip of the iceberg so to speak, that early in history the loss rates were much greater and that this mechanism could account for the loss of a very thick early atmosphere."

Maven, which stands for Mars Atmosphere and Volatile Evolution, launched in 2013 and began orbiting the Red Planet in September 2014.

Its goal is to help scientists understand one of the solar system's biggest mysteries - what happened to the water on Mars and the carbon dioxide in its atmosphere several billion years ago?

Mars today has a very thin atmosphere, less than 1 per cent as thick as Earth's.

Previous space missions using robotic rovers and orbiters have shown plenty of geological and geochemical evidence that climate change occurred on Mars.

But knowing more about what drove these changes could shed light on the potential for life on Mars, said Jakosky.

"We think that Mars had a very thick CO2 atmosphere and we are trying to understand where that atmosphere went," he said.

Researchers believe that the loss of ions to space was a "significant player" in changing Mars' climate, he added.

Data from Maven helped show a large solar storm in March and indicated that this loss "increases dramatically during a solar storm event, when a coronal mass ejection hits Mars," he said.

"The variability we are seeing suggests that the escape must have been much greater in the past when the sun was more active and more intense." The Earth is generally protected from solar flares by its magnetic field.

Mars, which is further from the sun than Earth, may have had a similar magnetic field once, but it was diminished when the planet's core froze.

Another report based on Maven data in the journal Science found that auroras are generated on Mars after solar storms, much as they are on Earth in a colourful phenomenon often called the Northern Lights.

"However, where this type of aurora on Earth is driven by magnetism of the poles, the authors suspect that Mars' aurora may be driven by the remnant magnetic field of the crust, creating a more even and diffuse aurora," the study said.