A moving shadow, a towering whirlwind

Thorofare Ridge · 30 August 2023 — NASA/JPL-Caltech
Processed image. Perseverance’s enhanced Navcam image shows the lower portion of a dust devil at Thorofare Ridge on sol 899. This is the photographic record associated with the time-lapse in Films. Only the lower 118 metres are visible; the published full-height estimate uses its shadow and an assumed vertical column.
NASA/JPL image-use policy · credited mission imagery, resized to WebP
· Original image source · Image licence

Dust makes an otherwise invisible Martian atmosphere visible.

World: Mars
Case date or period: Image archive · 2012–2023

Look closer

Orbital images catch dust columns and their shadows together. A thin trace on the ground can belong to a surprisingly tall whirlwind; the column’s shadow is part of the scale evidence.

Follow the evidence

Compare active columns with the dark tracks left behind. A track records a past passage, while a visible plume is a moment caught by the camera. These images show different locations, not successive frames of one storm.

The curved plume is not a route

HiRISE scientist Paul Geissler’s account of the February 2012 serpent image separates the airborne shape from the ground track. The whirlwind travelled southeast, leaving a bright trail; a westerly breeze roughly 250 metres above the surface bent its plume eastward. Following the curved dust tail as though it mapped the storm’s journey would therefore misread the photograph. The shadow supplied the estimate of a plume exceeding 800 metres.

A vortex need not carry visible dust

Brian Jackson’s analysis of Perseverance’s MEDA pressure and radiation records found 309 vortex encounters in the mission’s first 89 sols. About one quarter showed evidence of lifting dust, although an unfavourable passage relative to the sensors could hide dust in other encounters. These are instrument detections within one place and period, not 309 photographed whirlwinds or a planet-wide frequency. Pressure measurements and visible plumes answer related but different questions.

Watch the observation, then examine the estimate

The rover sequence follows a dust devil on 30 August 2023. Its frames were recorded four seconds apart, so the published playback compresses the observation rather than showing real-time motion. Mission researchers placed the vortex about four kilometres away and estimated a width of roughly sixty metres. Its top lies outside the frame. The approximately two-kilometre full-height estimate therefore depends on the shadow and the assumption that the column is vertical.