Clouds that cast shadows on another planet
Processed image. Voyager 2 colour observation from the August 1989 encounter, two hours before closest approach, at a range of about 157,000 kilometres. Sunlight comes from the lower left. Bright cloud streaks cast shadows on a deeper cloud deck; scattering makes shadows less distinct at shorter wavelengths. The publisher reports 11 kilometres per pixel and estimates separation of about 50 kilometres between the cloud layers, not the height above a solid surface.
NASA/JPL image-use policy · credited mission imagery, resized to WebP
· Original image source · Image licenceBright clouds and darker layers reveal atmospheric depth.
- World
- Neptune
- Case date or period
- Camera observations · 1989–2020
Look closer
Voyager’s Neptune images show bright cloud structures near the Great Dark Spot and elsewhere. In favourable geometry, shadows provide clues to their height above lower clouds.
Follow the evidence
The images include processed colour views and different encounter frames. Compare local shadows and textures before using overall brightness as a measure of cloud thickness.
A ground-based record extends the brief encounter
Erandi Chavez, Imke de Pater and colleagues measured cloud coverage and brightness in Keck, Lick and Hubble observations spanning 1994–2022. Near-infrared brightness changes were dominated by discrete clouds rather than the background haze. Cloud activity correlated with solar ultraviolet output, supporting a photochemical influence, while individual storms and seasonal processes complicate the picture. This long record complements Voyager’s cloud-shadow geometry: one measures changing cloud abundance, the other constrains vertical separation. Correlation across several solar cycles supports a hypothesis rather than uniquely proving how sunlight produces each cloud.
Watch clouds change across hours and decades
The Voyager plate samples the same broad storm neighbourhood roughly one rotation apart. Follow the bright clouds around the dark oval rather than treating the three panels as a smooth film. The Hubble plate stretches the comparison across selected years, with its own filters, processing and viewing conditions. It links the short encounter to a long observing programme. Neither visual comparison alone establishes a solar cause: calibrated measurements and competing atmospheric processes remain part of the investigation.
Sources & further reading
- Neptune Clouds Showing Vertical Relief — NASA Image and Video Library / JPL
- Neptune Shadows — NASA Image and Video Library / JPL
- Neptune - Great Dark Spot and Scooter — NASA Image and Video Library / JPL
- Evolution of Neptune at Near-Infrared Wavelengths from 1994 through 2022 — Chavez and colleagues — Original investigator research paper
- Changes in the Great Dark Spot — original Voyager cloud sequence — NASA / original mission publisher
- Neptune cloud cover over three decades — original Hubble photographic plate — NASA / ESA / Hubble original image release