Interview to Nadine Barlow

For the smaller of the two craters (11x9 km), latitude is -12, longitude is 243. The crater has a sharp rim, a central peak, and a slightly asymmetric ejecta blanket. It is to the lower right of a larger, rimless crater.

-- This crater is only very slightly asymmetric (11x9 km in size), but the asymmetric ejecta blanket indicates it did come in at an angle less than about 15 degrees to the surface (or, I should say, the meteorite which produced the crater came in at less than about 15 degrees to the horizon). According to the studies of how material gets ejected off Mars, near-vertical impacts producing craters larger than 100-km-diameter or low angle (<15 degrees to the horizon) impacts creating craters greater than about 10 km are the only ones that meet the necessary criteria. The other two smaller craters look somewhat elliptical but I think that is due to this image not being entirely corrected for the Viking Orbiter's oblique viewing angle.

The second crater (23x15 km) is at latitude -14 longitude 343.5. It is superposed on the rim of a larger, rimless crater. It has a nice asymmetric ejecta blanket that flows into the floor of the larger rimless crater.

-- The channel is called Evros Vallis. It is one of many channels formed early in martian history (within about the first billion years or so of the planet's history). The origin of these channels is debated. They are most likely formed by flowing water, but of course current atmospheric conditions on Mars prevent the existence of liquid water on the surface. One idea to explain the channels is that atmospheric conditions were different in the past on Mars, that the atmosphere was thicker and thus water could exist on the surface. Some speculate that perhaps Mars even had rainfall during this time. So this theory says that the channels were formed by flowing water across teh surface of Mars, similar to the way terrestrial river channels form. Another idea is that the channels are formed by flowing water, but the water is underground. As the underground rivers dry up, the overlying surface material loses the underlying support and the material collapses, creating a surface expression of the underground river's course.

-- Maybe, although without any channels crossing the crater rim, the source of the water is speculative. Another possibility is that the flat floor is simply due to infilling by wind-blown dust over the ages, since this crater (based on its degraded state) is likely about 3.5 to 4.0 billion (10^9) years old.

-- The dark area appears to be a deposit of darker material. Since it crosses the same area of the three in-line craters, it may be a subsurface deposit excavated by all three. However, since all three of those craters have infilled floors, I would suspect that any such deposit would have been covered up by the later sediments. Thus, I suspect that it may be a dark deposit emplaced over that region at a later time, perhaps by wind- blown processes. Interiors of craters are generally protected by their walls from later wind erosion, so a deposit emplaced over the area could be eroded away from areas outside the craters but left in place inside the craters.

-- Laboratory studies indicate that in low-angle impacts (<15 degrees to the surface) most of the ejected material ends up being ejected to the side of the crater due to the process known as jetting. Thus an asymmetric ejecta blanket is always a clue to a low-angle impact, regardless of whether the crater is elliptical (which occurs for impacts at less than about 10 degrees to the surface) or not (such as you see for Crater 1).

-- Coincidence is always a possibility. Alternately we now know that many asteroids are actually double (two objects of about the same size orbiting around a common center of gravity) or have small moons accompanying them. It's possible that the meteorite that created the large elliptical crater had such a companion that impacted nearby, creating the smaller elliptical crater. Unfortunately without any age data for either of these craters, that idea cannot be tested at this time.