By Dr David Whitehouse
BBC News Online science editor
A series of dust storms could threaten the success of three space probes heading for Mars, astronomers warn.
Mars on 13 December as the dust storm was spreading
The scientists say some small storms are combining to obscure a large part of the planet's northern hemisphere.
If they build into a global storm, which can happen, it could interfere with the solar panels the probes use to generate power for their instruments.
On 25 December, Europe's Beagle 2 will land, followed by two US rovers, Spirit and Opportunity, on 3 and 24 January.
Dust storms are not unusual on Mars. Two years ago one of the largest dust storms seen in several decades raged on the planet for many weeks, making it impossible to see surface features.
Observers noted the current storms starting to churn on Mars in the first week of December.
Since then, they have grown in size, and have been joined by smaller storms on their fringes.
Bruce Jakosky, of the University of Colorado, helped choose the US probes' landing sites. He said that it was too early to know if this storm pattern was something to worry about.
"We don't know what is going to happen to this one," he said.
If they continue to develop to encompass the whole planet, the three spacecraft on their way to the Martian surface could experience operational difficulties when they get down.
Beagle's solar panels may be affected
Dust in the atmosphere could be deposited on their solar panels, reducing the amount of energy they generate throughout their missions. But it is not just the landing phase that can be hindered.
Sunlight heats airborne dust grains, warming the atmosphere, which then expands.
When this happens it can cause problems for orbiting spacecraft, or for spacecraft entering the atmosphere en route to a touchdown.
US space agency scientists say they are monitoring the situation closely.