Dark slope streak

Did You Know?

• Slope streaks modify less than 0.1% of the martian surface but transport several global storm equivalents of dust per Mars year.
• New streaks were first identified by comparing images from the Viking Orbiters of the 1970s to images taken by the MGS Mars Orbiter Camera in the late 1990s.
• A statistical treatment using overlapping MOC images suggested that slope streaks may form at a rate of about 70 per day.

Nature of streaks on Mars

Dark slope streaks are a type of albedo feature, meaning they show up as a difference in brightness between the streak itself and the lighter slope behind it. In most images, there is no visible change in the slope’s shape or height that sets the streak apart, though the very highest resolution pictures can reveal some relief. Often, the slope’s original surface texture remains intact and continuous across the streak, as if the formation process left it untouched.

The overall look is much like a partial shadow cast down a hillside. These clues suggest that whatever creates the streaks only alters the very topmost layer of the surface. The streaks are only about 10 percent darker than the surrounding area, but they often appear black in photos because the contrast has been artificially boosted. Albedo features on Mars come in many sizes and are caused by different amounts of dust covering the ground.

Martian dust is a bright reddish ochre, while the bedrock and soil are dark gray. Most of these features form when wind either clears dust away, leaving darker rock behind, or deposits dust to create brighter patches. Dark slope streaks are smaller and differ from larger albedo features because gravity, not wind, drives their formation, though wind might play a role in starting them. The exact reason for their dark color is unknown.

The particles involved are thought to be very fine—sand, silt, and clay-sized—and no visible rock fragments appear. The underlying bedrock is never exposed, meaning dust is sliding over more dust. Other optical, mechanical, or chemical factors likely cause the darker tone.

Morphology and occurrence

Dark slope streaks are long, narrow features that run downhill along crater rims and steep cliffs. When viewed from a distance, they look like thin, parallel lines. While many are straight, some curve or take on an S-like shape. At higher resolution, these streaks appear more like elongated fans.

Their width ranges from roughly 20 to 200 meters, and their length typically falls between a few hundred meters and just over a kilometer. Streaks longer than two kilometers are rare, and almost all stop at the slope rather than continuing onto flat ground. Each streak usually begins at a single point high on the slope, often near a small ridge, knob, or a locally steeper area. From that starting point, the streak widens in a triangular shape as it moves downhill, with the widest part usually occurring before the halfway point.

A streak may split into two branches to go around an obstacle or form a braided pattern. At the downhill end, multiple finger-like extensions are common. High-resolution images from the HiRISE camera on the Mars Reconnaissance Orbiter reveal that many streaks have a slight depression, with the streaked surface sitting about one meter lower than the surrounding terrain. These features are most frequent in Mars’ equatorial regions, especially in Tharsis, Arabia Terra, and Amazonis Planitia, and are found between 39 degrees north and 28 degrees south.

In their northernmost range, they appear more often on warmer, south-facing slopes. Interestingly, the streaks also tend to occur where surface temperatures reach 275 Kelvin, close to the triple point of water, which has led some scientists to suspect liquid water may play a role in their formation.

Formation mechanism

Scientists have suggested several ways dark slope streaks might form. The leading idea is that they come from dust avalanches, similar to loose snow avalanches on Earth, caused by dry granular material sliding on overly steep slopes. Other explanations involve water, such as spring flows, wet debris slides, or seasonal movement of salty brines. Data from the Mars Odyssey neutron spectrometer showed that streaks in the Schiaparelli basin appear where the ground holds 7.0 to 9.0 percent water-equivalent hydrogen, while typical areas have less than 4 percent.

This hints at a link, but processes needing large amounts of liquid water are considered unlikely because water is unstable on Mars. Another proposal is that streaks form from ground-hugging dust flows lubricated by carbon dioxide gas. A small slump could release CO2 trapped in subsurface grains, creating a thin, gas-supported current that moves downhill, possibly explaining very long streaks. Observations also suggest impacts can trigger them.

Images from CTX in 2007 and 2010 revealed a new streak in the Olympus Mons aureole, and a later HiRISE picture showed a fresh crater at its top. A similar impact-linked streak was found in the Arabia quadrangle.