Pedestal Craters in Utopia
The northern plains of Mars include some of the flattest and smoothest real estate in the solar system. According to planetary scientists, the plains also have a lot of water locked up in the form of ice at shallow depths in the ground. This false-color THEMIS mosaic, which focuses on a small part of the northern plains in Utopia, captures both aspects.
The mosaic combines visible-wavelength images with ones taken at night using heat-sensitive infrared light. Nighttime temperatures are shown by color - bluer is cooler, redder is warmer - giving scientists clues about the surface's physical properties.
Ground temperature differences arise because at night, a dusty surface gives up daytime heat faster than rocks do. (In the same way, bare dirt feels cooler underfoot late at night than pavement does.) Thus cool temperatures spotted by THEMIS point to patches with thicker coatings of dust and fine-grain material, while warmer temperatures mark where the ground is harder and rockier.
On A Pedestal
Impact craters in the martian high latitudes often show differences from those nearer the equator - and this nameless crater is no exception. About 4.2 kilometers (2.6 miles) wide and about 200 meters (660 feet) deep, the crater has a flat, relatively featureless floor covered by dust. In places, reddish tints around the rim show where rocky debris pokes into view.
Yet what sets this crater apart is the raised platform, or pedestal, surrounding the rim crest. It stands 60 to 70 meters (200 to 300 feet) above the surrounding plains.
Scientists think the pedestal was born when the force of impact threw an apron of ejecta around the new crater. Textures on the pedestal's surface, plus its sawtooth outline reflect turbulence in the surging debris during the moments after impact.
When the debris came to rest, it contained mostly rock, mixed perhaps with some water from the subsurface. Extending outward to about one crater-radius, the apron armored the ground against erosion.
As years passed, scientists argue, the topmost layers of the plains eroded due to winds or climate change or other factors. This lowered the ground around the crater, except where the ejecta covered it. Perhaps in response, crevasses developed on the pedestal's outer edge, where the protected zone drops to meet the plains. In this model, smaller craters may lack pedestals because they formed after the plains had already eroded.
Secondary Bombardment
The impact of a large meteorite blasts a crater and sends debris flying far and wide. Away from the crater, this debris slams down, usually with enough force to make secondary craters on its own.
This crater cluster, the largest of which is about 460 meters (1,500 feet) wide, is probably a group of secondary impacts. Their shape, spacing, and graduated sizes suggest the primary, or source, crater lies to the southwest of the group, out of the field of view.
Secondary debris that lands close to the primary crater often arrives on low trajectories. Striking the ground at shallow angles and low speeds, it tends to make elongated craters and chevron patterns pointing back toward the primary crater. However, these secondaries look circular, suggesting the impactors fell at a steep angle and struck at high speed, typical for debris from a relatively distant source.
Likewise, the crater group's diagonal alignment suggests they arrived from the lower left or the upper right. The placement of the largest crater at the southwest end of the cluster offers a clue that this is the direction toward the source crater, since big debris often lands closer to the primary crater than smaller debris.
Planetary scientists use craters to date surfaces, at least in a relative sense: older surfaces have on average more craters. But the existence of secondaries complicates the matter, because where they masquerade as primary craters, they can make a surface appear older than it actually is.
Lines In Lava
Mars' northern plains are built from uncountable numbers of thin lava flows, such as those seen here.
Merging in a complex tangle of intersecting channels, faults, and low ridges, the flows have buried essentially all trace of any earlier surface. Laser altimeter measurements show that, aside from the craters, the plain typically varies in height from place to place by no more than 50 meters (160 feet), and often by less.
A hodgepodge pattern now dominates. Shallow grooves and broad hollows (seen in bluer tints) have collected dust and are separated by low, wind-swept rises. These are capped by tougher surfaces (greens and yellows) that expose more cohesive material.
The scene is one of a wide, gently rolling landscape under an enormous sky.