A fiery sky over Paranal

Have you ever seen a sunset so red? Probably not, since the cause of this reddened twilight sky is something quite dramatic: a volcanic eruption. This Picture of the Week was captured at ESO’s Paranal Observatory in Chile; under the Milky Way, on the top of the dark silhouette of Cerro Paranal, ESO’s Very Large Telescope looks upwards to the sky.

On 15 January 2022, the submarine volcano Hunga Tonga–Hunga Ha‘apai erupted in the southern Pacific Ocean. This eruption created shock waves that rippled through the atmosphere, reaching places far from the volcano itself. At ESO’s Paranal and La Silla observatories in Chile, more than 10 000 kilometres away, weather stations detected these atmospheric disturbances.

The eruption also launched an ash plume 57 kilometres tall, releasing massive quantities of particles into the atmosphere, including water vapour and dust. Sunlight is scattered and reddened by these tiny dust particles, and this effect was detected in calibration images taken during twilight by several ESO telescopes. This Picture of the Week, taken 6 months after the eruption, shows that the effects of these particles were not transitory. At the time of writing, one year later, the sky has still not returned to its pre-eruption state.