Periodically, wind will remove dust from the rovers' panels

The vehicles - Opportunity and Spirit - had been kept nearly inactive for six weeks to preserve the limited power being generated by their solar panels.

But Nasa says the skies have cleared sufficiently for the rovers to resume some limited driving.

Opportunity has now edged itself closer to its next major science target: the 60m-deep bowl known as Victoria Crater.

The crater has high walls with layers of exposed rock that should reveal significant new information about the planet's geological past.

Meanwhile, Spirit - which is operating at a distant site from Opportunity but also just south of the Martian equator - has rolled back a few tens of centimetres, positioning itself to take pictures of a rock it has been studying.

The electricity generated from each vehicles' solar panels has reached about 300 watt-hours. That is more than twice as much as five weeks ago, but still less than half as much as two months ago. It is enough to run a 100-watt bulb for three hours.

Opportunity will be sent into Victoria Crater to study its walls

Both rovers are now communicating with Earth on a daily basis. Opportunity had previously been conserving energy by going three or four days between communications.

Nasa says no new storms have been lifting dust into the air near either rover in the past two weeks; and skies are gradually brightening above both robots.

"The clearing could take months," added rover project scientist Bruce Banerdt. "There is a lot of very fine material suspended high in the atmosphere."

The agency says this material will settle on the rovers' solar panels as it falls out of the atmosphere and lessen the panels' capacity for converting sunlight to electricity, even while more sunlight is getting through the clearer atmosphere.

In the past, dust accumulations on the panels have been removed by Martian winds.