Scientists using the Mars Reconnaissance Orbiter applied its radar in an innovative new way to learn about flood channels buried beneath the surface.
Using a technique never applied at Mars, Mars Reconnaissance Orbiter’s Shallow Radar (SHARAD) instrument was used to profile a large flood channel system that has been filled with lava flows. The flood channels are a part of the Marte Vallis channel system, which sits just north of the Martian equator in a region known as Elysium Planitia. Elysium Planitia is a fairly young volcanic feature on Mars, and lava flows from this area have obliterated most signs of Marte Vallis, at least from the surface. Only the lower reaches of Marte Vallis, where the system drains into the northern lowlands, remain exposed at the surface. The remnants of Marte Vallis suggest there was once a great river system in the area, but how great has been a matter of speculation.
The new data from the SHARAD system answers the question: Marte Vallis was much larger than anyone had guessed. Radar systems work by sending a pulse of radio energy at the ground and timing how long it takes that pulse to return. Usually most of that energy is returned by the surface, while the rest is absorbed by the surface. However, in certain cases flat surfaces buried underground can reflect enough of this absorbed energy to be picked up by the receiver. The technique has been used on Earth before, most famously to image buried river channels in the Sahara, but this is the first time the technique has been applied on Mars. “Our findings show the scale of erosion that created the channels previously was underestimated and the channel depth was at least twice that of previous approximations. This work demonstrates the importance of orbital sounding radar in understanding how water has shaped the surface of Mars,” said Gareth Taylor (National Air and Space Museum Center of Earth and Planetary Studies), a lead author on the paper.
From the source to its mouth, Marte Vallis is 1000km (620 miles) long, and up to 60km (37 miles) wide. Individual channels may be scoured down more than 130m (420 feet) into the surrounding terrain. That size puts it on par with other areas of Mars known to be large flood channels, such as those found in the Chryse Planitia region. The new data shows that Marte Vallis has its source in the Cerberus Fossae fracture system, which has also been partially buried by Elysium Planitia lava flows.
What the images show are a series of braided channels with islands that have been eroded into a streamlined shape by the floodwaters. A second pulse of floodwater carved an even deeper channel nearby, leaving the older channels perched on a feature known as a fluvial terrace. The channels were probably created during catastrophic flood events, which occur when large reservoirs are suddenly released. Perhaps the best example on Earth is the Channeled Scablands in Washington. There, glaciers dammed a major drainage, forming a massive lake that suddenly drained when the ice dams failed.
So what caused the flood channels of Marte Vallis? No one really knows the answer. The feature is believed to have formed within the last 500 million years, and all evidence indicates that Mars has been cold and dry for the entire time. One possibility is that water collected underground in Cerberus Fossae and was released by seismic or volcanic activity. Or, the source could originate in from some other feature not yet imaged by radar. Regardless of the source, the amount of water released had to come from somewhere, raising questions as to how water cycles operated in the ancient Martian deserts. Mars shows signs of having large underground reservoirs in the past, so what happened to them? Are there still some in existence, or have they been long lost to the vacuum of space?
Those questions will prove extremely difficult to answer short of beginning a widespread drilling program on Mars. However, the search for the reservoirs might lead to design modifications for future sentinels at the Red Planet. NASA’s goal has been “follow the water”, and with new evidence showing that large aquifers could exist in the arid conditions on Mars (at least in the ancient past) mission planners may be tempted to design a spacecraft or rover to look for them.
Image: Profile of the channels as seen from SHARAD data. Credit: NASA/JPL-Caltech/Sapienza University of Rome/Smithsonian Institution/USGS