Early air points to cold, dry Mars

Scientists have worked out the characteristics of the air on Mars 3.6 billion years ago. The work by a team of US researchers suggests that the thin atmosphere would have led to cold, dry conditions on the planet. The results are important because they shed light on how habitable Mars was billions of years ago, and how long any surface water persisted.
Details have been published in the journal Nature Geoscience. The scientists calculated the early Martian atmospheric pressure using patterns of ancient meteor craters and dry river beds seen on its surface today. They used new Mars orbiter data to test out an idea first proposed by Carl Sagan more than 20 years ago, that atmospheric pressure is recorded by the size of the smallest impact craters.
The ancient Martian climate is inferred from the landscape of water-sculpted lakes and river beds seen today. They show that liquid water must have existed on its surface early in the planet’s history. The new result, however, implies that Mars was not a permanently warm wet world and that periods of arid, sub-zero conditions existed.
River channels at Aeolis Dorsa, near Gale Crater on Mars, interweave with impact craters, and it is the smallest of these craters that are key to the new findings.