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Gully landforms, like in this photo, are thought to have been caused by geologically-recent seepage and runoff of liquid water on Mars.
Nearly all gullies form on slopes that face away from sunlight. The channels start wide and get narrower downslope.
This image covers an area approximately 2.8 km (1.7 mi) wide by 2.1 km (1.3 mi) high. Sunlight illuminates the image from the upper left.
Streamlined islands, like these in the photo below, are an indication that water once flowed on Mars. These teardrop-shaped islands were possibley formed by water during a catastrophic flood inside Ares Valles.
Impact craters are resistant to water erosion and create a barrier around which the water flows.
The parallel grooves on the channel floor is also an indication that water once flowed here. Fast flowing, turbulent water often carve such grooves.
The small martian gullies discovered in Mars Orbiter Camera (MOC) pictures appear to be geologically young. This means that, on the scale of Mars that is 4.5 billion years old, the gullies may be only a few million, or less, years old.
Some evidence suggests that the gullies may, in many cases, be much younger than a few million years - in fact, some might be actively seeping water in modern times.
First picture: Apron Covering Dunes: Shows a deep, prominent martian gully in a south-facing wall in Nirgal Vallis.
The evenly-spaced, almost parallel ridges at the bottom of this photo are dunes created by windblown sand. Since the apron covers this dunes it must be more recent than the sand dunes - the apron is the fanlike deposit at the lower end of the deep channel.
Second picture: Apron on Polygons. Shows aprons deposited at the base of this slope in an impact crater in Noachis Terra ( map of Mars ). (Terra means heavily cratered highland).
Polygonal patterns like these on Mars are also found on Earth. These patterns on Earth are usually only several to tens of thousands of years old, at most.
This means that the gully in this picture may be no more than a few tens of thousands of years old, and could be much, much younger.
Third picture: Fresh, Dust-free Surfaces. Shows small, dark channels eroded into one of the gully alcoves found in the 'Aerobraking Crater'.
Two aspects of this picture indicate that these gullies were formed in the near-recent past. In this case, near-recent could mean 'within a few days of when the picture was taken' to 'within a few years of when the picture was taken.'
The one aspect is the sharp contrast between dark-toned and light-toned surfaces. Typically on Mars dust is forever settling everwhere and this will quickly cover up such sharply constrasted areas.
The other aspect is the large number of boulders and their sharp, crisp relief. This indicates that they have not yet broken into finer debris, nor have they been covered up by sand or dust.
These pictures show the difference between slopes eroded by water and slopes not eroded by water.
Picture taken by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC).
This photo is from Elysium Planitia ( map of Mars ) . The high resolution view (A, on the left) shows part of a 10 kilometer in diameter impact crater. The overall crater area is shown in the photo B in the middle.
On the slopes in shadows a series of gullies - defined by their erosional alcoves, deep channels, and apron deposits - are seen.
On the sunlit slope the lower slopes show streaks of dry debris that has slid down the slope. This forms deposits that are distinctly different from the gully aprons.
The photo C has been rotated so that the two slopes - one with gullies (right) and one without (left)--can be more easily compared.
This picture shows several examples of martian gullies that all start in a specific layer roughly a hundred meters beneath the surface of Mars. These gullies are located on the south-facing wall of a trough in the Gorgonum Chaos ( map of Mars ) region. This area has many examples of gullies proposed to have formed by seepage and runoff of liquid water in recent martian times.
The larger gullies have formed an alcove (an area above the overhang from which debris has collapsed to leave a dark-toned scar). The small, bright, parallel features at the base of the cliff at the center-right of the picture is a series of large windblown ripples.
There is no way to know how recent this activity was, but educated guesses center between a few to tens of years. It is possible that the area shown in this image has water seeping out of the ground today.
Picture by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC).
This picture covers an area 3 kilometers wide by 2.6 km high.
It shows a series of troughs and layered mesas in the Gorgonum Chaos region on Mars.
Gullies proposed to have been formed by seeping ground water emanate from a specific layer near the tops of trough walls, particularly on south-facing slopes. (South is at the bottom of this picture).
Photo of gullies eroded into the wall of an impact crater in Noachis Terra ( map of Mars ) . It covers an area approximately 3 kilometers wide by 6.7 km high.
Since there are no small craters superimposed on these channels and apron deposits this means that these features are geologically young. It is possible that these gullies indicate that liquid water is present below the martian surface today.
The color in this photo is only a crude approximation (color added to a black and white picture) of martian colors and should only be considered representative of Mars. It is likely that this area on Mars would not look like this to a human observer at Mars.
All images on this webpage are credited to NASA / JPL / Malin Space Science Systems
