Mars, known for its dry, red soil and dusty sky, is affectionately named the "Red Planet." But billions of years ago, the fourth planet was more like its sister Earth: warm, wet, and blue.
According to a new study in the journal Nature Geoscience, Mars likely supported a vast sea that covered one-third of the planet's surface.
The ancient ocean existed around 3.5 billion years ago and blanketed the whole northern low-lying region of the planet, Gaetano Di Achille and Brian Hynek of the University of Colorado reported.
Di Achille and Hynek examined the distribution of 52 Martian deltas and various river valley networks in the Northern Hemisphere.
They discovered that the deltas occurred along a similar elevation, suggesting the rivers emptied into a common body of water. If the researchers are right, the deltas mark out a global sea level for ancient Mars.
Determining whether or not Mars once supported an ancient ocean in the northern lowlands is one of the great controversies among scientists studying the Red Planet. In the past three billion years, modification and erosion of the Martian surface have wiped away most evidence of an ocean.