Evidence for Liquid Surface Water on Mars

Dust Devil in Solis Planum

Eight successive images from the Viking mission are shown in the foreground. The object is located in northern Solis Planum and can be found on two Viking Orbiter images of 1978-08-01, 775A10 and 775A11, separated by 4.5 seconds (ENA 16°, INA 65°, LST 15.37, 12.7 m/pix). The object is at the white cross at 16.7° S 79.2° W.

It is visible now below a crater as a bright dot. The site was neither imaged by MOC nor Themis visual but a Themis thermal image covered it. The left image strip is Themis visual I06681044 (100 m/pix), the right one is Themis visual I06656022, the middle one is from the map above and the blinked in image is Themis thermal I06226010 (LST 5.44, INA 97°). The area shows an active past: Chaotic terrain in the upper left and an elliptical volcano pit lower left of center. Most craters and the pit are thermal dark, perhaps cooled by evaporated water during the night. A notable exception is the lower right crater with its hot interior. It is almost certainly an impact crater but perhaps connected to a vulcanic water vent. The site of the object shows no thermal activity.

The work of Di Pietro and Molenaar presented considerable support that the object is a water eruption. The plume is very bright, which only few dust devils are. There is only a shadow, but no ground track, which dust devils usualy create. The fast shape change of the shadow is very unusual for dust devils. The change from narrow pointed to wide is much what is expected from a violent liquid eruption. The cloud expands due to enclosed and dissolved gases. The bright halo and its widening in the second image further support this interpretation. It was never challenged in any publication until now:

Here the two VO images are adjusted and matched together in a red/green stereo anaglyph of the object and its immediate surroundings. The large crater with the ramp (probably an evaporite) has a diameter of about 800 m. The base of the plume is like a dark conical column of 100 m diameter and about 1000 m high (ENA 16°) and the bright cloud even higher. The shadow of the object seems two-parted. One flat on the ground and one leaving the ground. That would mean it is no shadow but a dark upward column of water or dust.

Here the two images are matched to a fast blink. The topographic background of the object is well fixed but the object moves from the left to the right. It is notable that the whole object with all of its shadow moves. The movement of the shadow could not be related to a stereo effect, the topography is too flat. The movement of the whole object strongly points to a dust devil. In a water eruption only the upper plume could drift by wind, never the lower part because surface winds are too slow and the ejection momentum at the speed of an expected vent much too high. The conclusion of a dust devil is further supported by the partial transparency of the object. That is typical for dust devils but not for water eruptions. Water droplets are of much larger size than dust and therefore always opaque like terrestrial clouds.

What in 1980 was thought to be a shadow is actually the column of the tornado-like dust devil. The column with about the same shape is visible in both images. But in the second, a dark veil on the right gives the impression of a widening shadow. This veil could be some dust in the upper part of the dust devil far above ground in the line of sight. Just by chance it corresponds to the visible geometry of the column to support the impression of a shadow. Albedo changes probably due to shadow movement are visible to about 1.9 km (150 pix) from the column base. At INA 65° that corresponds to a column height of about 4 km, not unusual for a Martian dust devil. MSSS reported one of 8 km height in a MOC wide angle image in 1999. Most shadows on MOC narrow angle images leave the frame. The end of this shadow is perhaps not visible in the double-image. Without movement in the single image part of 775A11 it is not discernible from surface colorations. Note the transmission error artifacts and the black dot reference marks in this blink. It is not cleaned (like the ones above) to allow better analysis.

One wonders why NASA published the discovery of the object in 1980 but did no further research on it. Investigators from outside like Di Pietro and Molenaar back in 1981 did not have the equipment to do the processing above. But JPL had both the equipment and the physicists for the object analysis. So the discovery of the Martian dust devils was delayed to the late 1980s.