The reappearance of clouds on Mars will help scientists measure the density of particles hanging in the Martian atmosphere and forecast dust storms on the red planet.

Astronomers have spotted a white vapor trail stretching over a long dead volcano on the Martian equator.

It looks like this is a sign of an eruption, but with the Red Planet known to be cold on the inside, scientists think they have an explanation for this, as they have seen a similar phenomenon before, Science Alert wrote.

The 1,500 kilometer (930 mile) trail of vapor billowing over Arsia Mons was captured on photographs taken by the Mars Express orbiter on September 13.

Even though the vapor looks as if it were coming from the top of volcano, researchers at the European Space Agency believe this is simply an optical illusion, because Mars hasn't seen a single eruption for millions of years.

What is it then?

Meteorologists call this phenomenon an orographic cloud, typically seen on the downwind side of mountains when dense air close to the surface flows uphill and expands as it cools, allowing moisture to condense on particles of dust.

Every few Earth years, clouds of water ice bloom as moisture-filled air moves up the volcano's side. Similar images were taken by Mars Express in 2009, 2012, and 2015, seeing this again in 2018 is no surprise.

READ MORE: Russian-Made Neutron Detector Finds Water Ice in Mars' Subequatorial Areas

The cloud's reappearance will help scientists measure the density of particles hanging in the Martian atmosphere.

A massive dust storm that happened on the red planet earlier this year, which apparently immobilized the Mars rover Opportunity, could give scientists new insight into how dust rises and settles on Mars, informing the scope of future missions.