Phobos’s monolith

The monolith and its shadow · 12 September 1998 — NASA/JPL/Malin Space Science Systems; PDS decoding, crop and linear brightness stretch: VectorPoint
Processed image. Mars Global Surveyor MOC SP2-55103, acquired 12 September 1998 starting 22:44:33.98 SCET. A native 700 × 700 pixel crop from the 2048 × 9600 sample strip shows the bright boulder near the centre and its elongated shadow below. Linear brightness stretch for viewing; no geometric correction or added detail. Native scan proportions are not a calibrated surface shape or height. The 6 June 2000 archive release is not the capture date.
Public domain — MOC gallery PDS IMG; new brightness-stretched crop
· Original image source · Image licence

One bright rock casts a conspicuous shadow on Phobos. Follow the independent discovery back to the original Mars Global Surveyor camera data, and explore what the silhouette can tell us.

World: Phobos
Case date or period: 12 September 1998 · MOC archive acquisition

A named feature starts with an identifiable exposure

The original Malin archive identifies SP2-55103 as the narrow-angle record of Phobos’s fourth encounter. Acquisition began on 12 September 1998 at 22:44:33.98 spacecraft event time. The archive’s public release followed on 6 June 2000. Keeping both dates prevents a later webpage or crop from being mistaken for a new observation. The source is a long image strip, 2048 pixels across and 9600 lines down, rather than a purpose-made close-up of a structure selected by an investigator. Its frame identity provides a route back to the underlying camera data.

Palermo describes his discovery

Efrain Palermo’s own account begins with a composite of SP255103 frames A through O. A protrusion and its shadow caught his attention; he enlarged that part in Photoshop and interpreted its appearance as cylindrical with a sloping top. He describes contacting Mark Carlotto and then Lan Fleming, who shared his interest in the feature. These are the discoverer’s reports of a correspondence and image interpretation. They preserve who pursued the question, but do not independently establish dimensions, geology or a manufactured origin. The nickname “monolith” should remain a visual description rather than a conclusion.

An adjusted illustration has a different purpose

Palermo also describes changing the image geometry until a reference crater appeared circular. That adjustment changes the surrounding scene too. His enlargement, animation and physical mock-up help communicate his interpretation, but none is another camera exposure. The original scene and the edited illustration therefore answer different questions: what did the spacecraft record, and what shape did an investigator infer from it? Repeating the edited crop on later community pages adds circulation, not independent confirmation. A faithful investigation keeps that processing history attached to the apparent form.

The geometry belongs to the camera record

The archive supplies a 3.5 millisecond line-integration time and a pixel aspect ratio of 0.60. It describes the processed browse image as not map-projected. These details warn against treating square display pixels as a ready-made surface survey. Its derived centre coordinates describe the image, not a separately located boulder. A target-specific position would need identification and geometric calculation in the actual data. The generic Mars target label in the data header also requires the accompanying Phobos encounter description; a label read in isolation can misidentify the scene.

Explore the rock and its surroundings

The photograph here comes from the archived camera file, not from Palermo’s altered illustration. A native 700-pixel crop keeps the bright rock, its elongated dark shadow and nearby craters together. Brightness has been stretched linearly for viewing; the scan geometry has not been reshaped. The apparent form can therefore be explored without treating it as a calibrated height measurement. Compare the compact bright patch with the much longer shadow and the adjacent crater rims. Those are clues to illumination and relief, while the original strip remains available for checking the selection.

Relief is supported; height and origin need more

NASA’s broader 1998 Phobos imagery documents a boulder-rich surface, including blocks interpreted as impact ejecta. That is geological context, not proof that a particular labelled block is Palermo’s exact target. The monolith question should be investigated with the identified exposure and its illumination geometry, while keeping the larger landscape visible. A conspicuous shadow can support relief without resolving a precise height or explaining how the rock arrived there. Neither a striking silhouette nor a discoverer’s conviction supplies evidence of construction.