Erosion and Exhumation

The oldest parts of Mars, like those of the Moon, show heavy cratering from impacts by meteorites and asteroids. But while the Moon's terrain divides clearly into two ages - ancient highlands and somewhat younger lava plains - even its youngest plains are still a couple billion years old.

Mars, however, has seen geological activity during all of its 4.6-billion-year history - including abundant volcanic action. Moreover, with an atmosphere, water, and climate cycles, the Martian geologic tapestry is woven far more densely than the Moon's.
The region seen here is the south half of an ancient crater in northern Arabia Terra. The image comes from frames taken by THEMIS, the Thermal Emission Imaging System, a sophisticated camera on NASA's Mars Odyssey orbiter that "sees" in 5 visible and 10 infrared colors.
The anonymous crater, which spans a width of about 100 kilometers (62 miles), shows a heavily degraded face, a hallmark of great age on any planet or moon. The crater and its surroundings date from the Noachian period, the earliest in Martian geologic time. As currently defined, the Noachian lasted from the formation of Mars some 4.6 billion years ago until about 3.9 billion years ago.
Debris Mantle
Floors of fresh craters, especially large ones, often look chaotic. Pools of impact-melted rock mingle with shattered and upheaved blocks in a geological jumble. But as time passes, every rough crater floor grows smoother and flatter as debris and sediments collect inside and lay a mantle over the rubble.
Sediments typically include dust, silt, and sand blown in by the wind. Impacts nearby can throw debris, usually partly molten, across craters in their vicinity. Sediments also include volcanic ash from distant eruptions that falls from the atmosphere. And fine-grain silt or dust may mix with snow and ice and cover the ground, just as on Earth.
The small detail photo shows where the smooth mantle that fills the floor of this big crater is breaking up and eroding. As exhumation continues its work, the older surface below slowly emerges into view. While the pits and hollows look deep, they actually lie only about 50 m (160 ft) or so below the smooth mantle.
The mantle thrown across the big crater's floor never became thick enough to bury this small impact crater - but the hole probably looks more impressive from above than it would from the ground. It's really a dimple: while it spans 7.4 km (4.6 mi) wide, it's only 150 m (500 ft) deep.
On Mars, scientists see thick deposits of dust that may have also included snow or frost mixed in. A blanketing mantle of such material rich in dust and ice would drape craters of all sizes, giving the terrain a softened look.
But these deposits would be unstable, given that ice can survive at the surface of Mars only in the coldest areas. Currently, these lie at the north and south poles. But Mars' spin axis varies its tilt relative to the planet's orbit over cycles lasting hundreds of thousands of years.
When the axis tilts at greater angles than today's 25°, the middle latitudes (north and south) become cold enough to accumulate thick mantles of debris. Some may still be rich in ice, at least below a depth of a meter (yard) or two.
Various breaks, pits, and channels in the smooth mantle material point to its fragility. If the mantle settled as time went by, it would crack and any ice caught in the debris would melt and escape. This would accelerate during those times when the Martian climate was closer to today's regime, with warmer middle latitudes (where this crater lies) and chillier poles.
Water escaping toward the north from the pit crater carved a route through the sediments, leaving channels with raised edges and a streamlined island.
Yet the channel bed isn't just a smooth downward slope. Around the south end of the island, the channel rises about 40 meters (130 ft) before descending again to the north. Water might have flowed uphill - possible for a short distance if it's moving fast enough. But the bump more likely reflects geological activity (uneven settling, perhaps) that occurred after the channel was cut.
Ancient locations typically have the longest geological histories, and often the most complex. Here as elsewhere on Mars, scientists have just begun to decipher the record.

Vital Statistics

29.5°N, 6.6°E
Image Size: 

68.4 x 45.6 km
42.5 x 28.3 mi
3798 x 2532 pixels

18m (59 ft)