Floods and Craters in Maja Valles

The Martian outflow channel Maja Valles runs northward for 1,600 kilometers (1,000 miles) from its source in Juventae Chasma. This is a depression measuring 180 x 250 km (110 x 155 mi) that lies on the northern side of the giant rift Valles Marineris. 

Between Maja and its even longer companion valley to the west, Kasei Valles, stretches Lunae Planum, a broad, relatively uncratered plain. Both Maja and Kasei empty into Chryse Planitia, where NASA's path-breaking lander Viking 1 touched down 30 years ago.
Maja has a width that varies from 50 to 150 km (30 to 90 mi). Scientists studying Maja have found its floor shows giant ripple marks, streamlined islands (such as here), and other features that point unmistakably to erosion by catastrophic floods.
The floods were colossal - and fast. Scientists estimate their peak discharge (which may have been very brief) released as much water as 100 million cubic meters per second, roughly like 500 Amazon Rivers put together. While the valley shows evidence for separate smaller flooding episodes over weeks, months, or years, large floods could have raced through the entire Maja system - Juventae to Chryse - in about 40 hours.
As the floods slowed and finally ended, Maja stopped eroding and began to age. Stretches of the valley nearer to Juventae show fewer craters than the downstream end, suggesting that the last floods made it only partway down the channel before petering out. Impacts big and small scattered craters across the valley floor, destroying evidence of flow features. Finally, dust and sand drifted in on the wind to cover the valley's features, a process still at work today.
This scene comes from the Thermal Emission Imaging System (THEMIS), a camera aboard NASA's Mars Odyssey orbiter. THEMIS takes images at 5 visual and 10 infrared wavelengths, giving Mars scientists an integrated view of the planet.
High Watermark
Geological processes have left their mark on the streamlined island at the right side of the main image. (In this scene, the waters flowed from the bottom of the image to the top.) Maja Valles is carved into the vast basalt flows of Lunae Planum, part of the huge volcanic province of Tharsis. And volcanism put its signature on the earliest geologic process revealed here.
Cutting across the channel and continuing up onto the raised island is a wrinkle ridge. These landforms occur when volcanic basalt flows are compressed. The brittle lava buckles at the surface, raising a geological welt. Such features are widespread on Mars (and on the Moon, where lava flows are also common).
In this case, the Maja floods have eroded the portion of the wrinkle ridge that lies in the channel bed. The water hurtling down the channel planed off most of the wrinkle ridge's height, leaving only a flattened remnant. On the streamlined island, however, the ridge preserves intact the structure produced by compression.
The impact crater at the head of the streamlined island spans 5.6 km (3.5 mi) and is about 600 m (1,900 ft) deep. When it formed, the impact ejected a blanket of debris whose remnant still surrounds the crater.
As the waters surged down Maja, they washed away the outer part of the ejecta, leaving only the thickest and rockiest part near the crater's rim. In the slacker water downstream from the crater, the floods deposited debris carried in suspension, building up an island about 100 m (330 ft) higher than the channel bed. Similar streamlined islands are visible in channels elsewhere on Mars, including Ares Vallis, another large outflow channel.
Such ejecta blankets typically act as a kind of armor for the surface material, helping them survive a flood (provided the flood isn't too great). This is why the streamlined islands often have an impact crater at their head.
Finally, the tip of the island shows several terraces. These may be beach lines created by steadily weakening floods. Or, perhaps more likely, they are layers in the basalt flows of Lunae Planum that have been exposed by the floods.
Secondary Sights
The big crater in the middle of the main image has almost the same size as the two that lie at the heads of islands, but it formed some time after the floods had stopped. About 6.5 km (4.0 mi) in diameter and 650 m (2,100 ft) deep, the crater is surrounded by an ejecta blanket that is both intact and has an upraised outer edge.
For scientists, the upraised edge is a telltale sign that the ground was saturated with water (or ice) when the impacting meteorite struck. Similar "rampart craters" are common in latitudes of 60 degrees north or south on Mars, where the ground is full of water even today.
As the debris flies from the impact, groundwater is caught up in the explosion and lubricates the flow of ejecta across the surface. When the water escapes, the ejecta flow stops abruptly, piling up a raised outer edge. Here the rampart stands about 80 meters (260 ft) higher than the floor of Maja, and the outer lip is 8 to 10 m (25 to 33 ft) higher than the ejecta blanket closer to the crater rim.
The crater is also surrounded by numerous secondary craters partly covered by wind-blown dust and sand. The secondaries were produced when debris flying from the primary impact struck the ground. Scientists suspect they are seeing secondaries whenever they find small craters around a large one arrayed in clusters and strings.
New Impressions
Some features on the floor of Maja are relatively fresh, for example the tiny black crater near the upper end of the main image.
Clean and dark, this crater, 170 m (560 feet) across, counts as recent, although it may yet be millions of years old. The dark tone shows that it is still undusted by debris - but until scientists learn how fast dust accumulates in this part of Mars, the crater's age in years will remain unknown.
A similarly sized crater lies at about the 10 o'clock position relative to the middle crater in the channel. It appears slightly more dust-covered, thus older. And zooming in on the main image reveals the floor of Maja has a great many small craters.
Crater counting is the time-honored way of dating planetary surfaces seen from afar. The principle is that older surfaces show more, and larger, craters. Secondary craters, however, can skew ages if they are not identified and removed from whatever sample scientists are studying.
In the field of planetary science right now, scientists are vigorously debating the role and importance of secondary craters. And this debate will likely sharpen still more as space probes reveal Martian craters down to ever-smaller sizes.
So how old is Maja? Scientists estimate the valley is late Hesperian to early Amazonian in age, a relative dating based on the number of craters it shows. Translating this into years involves a lot of guesswork, but Maja likely ranges from 2 to 3.5 billion years old.
Landscapes on Mars are often startlingly old in comparison to ones on Earth. The reasons are many, but perhaps the biggest difference is that on Mars, for a long, long time there has been no rain to wash away dust and sand into rivers, lakes, and oceans. (Plus, of course, no Martian oceans, lakes, or rivers.) This means debris that settles on a Martian landscape may sit in place essentially unchanged for millions of years. 


Vital Statistics

11.2°N, 301°E
Image Size: 

28.8 x 43.7 km
17.9 x 27.2 mi
1601 x 2430 pixels

18m (59 ft)