Scientists who thought they understood the weather of Mars after American spacecraft landed there in the 1970s are learning that the red planet’s climate is not so predictable.

New photos from the Hubble Space Telescope released this week show Mars’ atmosphere is clearer and cooler than when the Viking spacecraft orbited the planet and dropped a lander to the surface in 1976.

Dust storms raged during the first year of the Viking visit, leaving the Martian atmosphere choked with dust particles. But the Hubble photos suggest that is not the constant condition on Mars.

“Hubble is showing that our early understanding based on these visits is wrong,” said Steven Lee of the University of Colorado in Boulder. “We just happened to visit Mars when it was dusty, and now the dust has settled out.”