In what could boost the recent discoveries about the presence of water on Mars, a team of US scientists have discovered that the formation of clay on the Martian surface is not restricted to the most ancient time period on Mars.
Most of that alteration is thought to have happened during the earliest part of Martian history more than 3.7 billion years ago.
But the new study shows that later alteration -- within the last two billion years or so -- may be more common than many scientists had thought.
The assumption is particularly true of clay deposits found in crater central peaks.
Central peaks are formed when, in the aftermath of an impact, rocks from within the crust rebound upward, bringing layers to the surface that had been buried many kilometers deep.
"Because central peaks contain rocks uplifted from depth, some previous studies have assumed the clays found within central peak regions are uplifted too," said geologist Ralph Milliken from Brown University.
Recent orbital and rover missions to Mars have turned up ample evidence of clay and other hydrated minerals formed when rocks are altered by the presence of water.
The lion's share of the clay deposits found on Mars have turned up in terrains that date back to the earliest Martian epoch, known as the Noachian period.
Milliken and colleagues performed a survey of 633 crater central peaks distributed across the Martian surface.
Of those 633 peaks, Milliken and co-author Vivian Sun found 265 that have evidence of hydrated minerals, the majority of which were consistent with clays.
They found that in about 65 percent of cases the clay minerals were indeed associated with uplifted bedrock.
"That is a majority but it still leaves a substantial number of craters -- 35 percent -- where these minerals are present and not clearly associated with uplift," Milliken noted.
Within those 35 percent, Milliken and co-author Vivian Sun found examples where clays exist in dunes, unconsolidated soil or other formations not associated with bedrock.
"You do apparently have a lot of local environments in these crater settings where you can still form clays, and it may have occurred more often than many people had thought," the authors pointed out.
So far, much of the surface exploration by rovers has focused on ancient terrains and whether or not the environments they record were habitable.
"But if we wanted to look at an environment that was more recent, we've identified craters that might be possible candidates," said Sun.
The research is forthcoming in the Journal of Geophysical Research: Planets.
