Cloudburst rains needed to make Mars valley networks

In many regions, Mars has branching networks of valleys and channels that were carved by flowing water. A great many previous studies have tried to date when they were active and for how long, while others have looked in detail at the form and shape of the channels.

channel in Hawaii

HAWAIIAN MARS CHANNEL. A channel cut into the tephra of the Kau Desert shows, in small scale, features seen in Mars channels. When a flood is progress, water flow is toward the camera. Gully headwalls form where a tougher layer makes a caprock (arrow, A) over which the water drops and digs out a plunge pool. (Red scale bar is 5 cm/2 inches long.) Downstream (B), headwalls and pools grow in size and height. (Image taken from Figure 4 in the paper.)

A new study by a team of geologists led by Robert Craddock (National Air and Space Museum), tackles the question of how the rocky nature of the Martian surface affected the channels’ formation and growth. A conclusion the team arrives at is that heavy rainfalls were likely necessary to kickstart the networks forming and probably also to keep them going. Their report appears in the Journal of Geophysical Research (August 22, 2012).

The geologists examined an area known as the Kau Desert, part of Kilaeua volcano on the Big Island of Hawaii. The ground surface there is a geologic unit called the Keanakakoi tephra. This is a basaltic, pyroclastic deposit that occurs mainly in the Kilauea summit area and in the Kau Desert.

Tephra is volcanic material that erupted violently enough to fly into the air and blow downwind. It is commonly porous and made of fine-grain angular fragments. Besides being found on Earth, tephra is also expected on Mars, a heavily volcanic planet. Water tends to soak into tephra rather than run off across its surface — unless the flow is fast and heavy and it encounters a harder, less permeable layer at a relatively shallow depth.

“The Keanakakoi tephra is up to about 10 meters [33 feet] thick and largely devoid of vegetation,” say the researchers. “This makes it a good analog for the Martian surface.”

On Mars, valley systems typically cover an area many times larger than this one area on Kilauea. Yet despite the difference in scales, the researchers note, “the drainage networks that have incised the Keanakakoi tephra share many of the same morphologic characteristics as Martian valley networks.”

These similarities include amphitheater-shaped headwalls and knickpoints along the channels, variable channel widths downslope, and channel floors that have little relief and are generally flat. Previous researchers have suggested that undermining (or sapping) by groundwater caused these features. Instead, the team says, the soft nature of the surface materials coupled with strong floods caused by brief, heavy rains better explains how the channels form.

The Kau Desert, they note, gets about 130 centimeters (50 inches) of rain a year, but most of its erosion occurs during winter storms, which can dump more than a meter of rain in 24 hours. “Recent climatic models indicate that similar cyclones may have formed early in Martian history if an ocean in the northern hemisphere was present,” they note.

“The morphology of many Martian valley networks may be strongly influenced by local lithology,” the scientists conclude. “And large, slow-moving storms capable of delivering precipitation at rates of tens to hundreds of centimeters a day may have been necessary to generate the runoff necessary to carve the valley networks.”

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