Head and his colleagues analyzed two proposed coastlines that were suggested by geologist Tim Parker in papers published in 1989 and 1993. Parker, who works at NASA's Jet Propulsion Laboratory, noticed in images from the Viking orbiter that certain features that appeared to encircle Mars' northern basin resembled lakeshores on Earth. He detected linear features that appeared to alternate between beaches and sea cliffs he said. After detailed study, he mapped two lines that he believes were shores of the ancient ocean at two different levels. The diagram above shows a topographical map of Mars' northern hemisphere on the right.
Cool colors represent low elevations.
The highest land features are shown in red. The outlines, designated Contact 1 and Contact 2, represent two coastlines proposed by Parker and colleagues. Because any coastline would stand at the same elevation along its entire length, Head and his team measured Parker's proposed coastlines, which were named Contact 1 and Contact 2. One of those coastlines appears to have too much variation in elevation to be a coastline, but the other was surprisingly level, Head said. Over the entire several thousand miles of proposed coastline, the elevation varies by only a few hundred meters, on average.