The white or bluish-white features are clouds.
Clouds are common over the larger Tharsis volcanoes in mid-afternoon.
Viewed from Earth by telescope before any spacecraft had visited the planet, astronomers often described a 'W-shaped white cloud over the Tharsis region. This 'W' was actually the result of seeing the combined effects of bright clouds hanging over the Ascraeus, Pavonis, Arsia, and Olympus volcanoes. The clouds result when warm air containg water vapor rises up the slopes of each volcano, cools at the higher altitude, and causes the water vapor to freeze and form a cloud of ice crystals. Pavonis Mons lies on the martian equator, north is up, and sunlight is illuminating the scene from the left. The picture is a mosaic of red and blue filter images taken on three consecutive orbits.