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The northern plainsof Mars include some of the flattest and smoothest real estate in thesolar system. According to planetary scientists, the plains also have alot of water locked up in the form of ice at shallow depths in theground. This false-color THEMIS mosaic, which focuses on a small partof the northern plains in Utopia, captures both aspects.
The mosaic combines visible-wavelength images with ones takenat night using heat-sensitive infrared light. Nighttime temperaturesare shown by color - bluer is cooler, redder is warmer - givingscientists clues about the surface's physical properties.
Ground temperature differences arise because at night, a dustysurface 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 thickercoatings of dust and fine-grain material, while warmer temperaturesmark where the ground is harder and rockier.
On A Pedestal
Impact craters in the martian high latitudes often show differencesfrom those nearer the equator - and this nameless crater is noexception. About 4.2 kilometers (2.6 miles) wide and about 200 meters(660 feet) deep, the crater has a flat, relatively featureless floorcovered by dust. In places, reddish tints around the rim show whererocky debris pokes into view.
Yet what sets this crater apart is the raised platform, orpedestal, surrounding the rim crest. It stands 60 to 70 meters (200 to300 feet) above the surrounding plains.
Scientists think the pedestal was born when the force ofimpact threw an apron of ejecta around the new crater. Textures on thepedestal's surface, plus its sawtooth outline reflect turbulence in thesurging debris during the moments after impact.
When the debris came to rest, it contained mostly rock, mixedperhaps with some water from the subsurface. Extending outward to aboutone crater-radius, the apron armored the ground against erosion.
As years passed, scientists argue, the topmost layers of theplains eroded due to winds or climate change or other factors. Thislowered the ground around the crater, except where the ejecta coveredit. Perhaps in response, crevasses developed on the pedestal's outeredge, where the protected zone drops to meet the plains. In this model,smaller craters may lack pedestals because they formed after the plainshad already eroded.
SecondaryBombardment
The impact of a large meteorite blasts a crater and sends debris flyingfar and wide. Away from the crater, this debris slams down, usuallywith enough force to make secondary craters on its own.
This crater cluster, the largest of which is about 460 meters(1,500feet) wide, is probably a group of secondary impacts. Their shape,spacing, and graduated sizes suggest the primary, or source, craterlies to the southwest of the group, out of the field of view.
Secondary debris that lands close to the primary crater oftenarrives on low trajectories. Striking the ground at shallow angles andlow speeds, it tends to make elongated craters and chevron patternspointing back toward the primary crater. However, these secondarieslook circular, suggesting the impactors fell at a steep angle andstruck at high speed, typical for debris from a relatively distantsource.
Likewise, the crater group's diagonalalignment suggests theyarrived from the lower left or the upper right. The placement of thelargest crater at the southwest end of the cluster offers a clue thatthis is the direction toward the source crater, since big debris oftenlands closer to the primary crater than smaller debris.
Planetary scientists use craters to date surfaces, at least inarelative sense: older surfaces have on average more craters. But theexistence of secondaries complicates the matter, because where theymasquerade as primary craters, they can make a surface appear olderthan it actually is.
Lines inLava
Mars' northern plains are built fromuncountable numbers of thin lava flows, such as those seen here.
Merging in a complex tangle of intersecting channels, faults,andlow ridges, the flows have buried essentially all trace of any earliersurface. Laser altimeter measurements show that, aside from thecraters, the plain typically varies in height from place to place by nomore than 50 meters (160 feet), and often by less.
A hodgepodge pattern now dominates. Shallow grooves and broadhollows (seen in bluer tints) have collected dust and are separated bylow, wind-swept rises. These are capped by tougher surfaces (greens andyellows) that expose more cohesive material. The scene is one of awide, gently rolling landscape under an enormous sky.
