A new animation by NASA scientists illustrates what Mars – the fourth planet from the Sun and the second smallest planet in the Solar System – may have looked like billions of years ago.

美国航空航天局科学家日前发布了一个新动画短片，推演了火星40亿年前的模样。

Today, Mars is a cold, desert world. Liquid water cannot exist pervasively on its surface due to the low atmospheric pressure and surface temperature, although there is evidence for spurts of liquid flow that perhaps consist of a briny（海水的） solution with reduced freezing temperature. Water under current conditions can be ice or sublimate directly into vapor without staying in a liquid phase.

Around 4 billions of years ago when the planet was young, it appears to have had a thick atmosphere that was warm enough to support oceans of liquid water - a critical ingredient for life.

The animation shows how the surface of the Red Planet might have appeared during this ancient clement（温和的） period, beginning with a flyover（天桥，立交桥） of a lake.

"There are characteristic dendritic structured channels that, like on Earth, are consistent with surface erosion by water flows. The interiors of some impact craters have basins suggesting crater lakes, with many showing connecting channels consistent with water flows into and out of the crater," explained Dr Joseph Grebowsky from NASA's Goddard Space Flight Center in Greenbelt, who is the project scientist for NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission.

"Small impact craters have been removed with time and larger craters show signs of erosion by water before 3.7 billion years ago. And sedimentary layering is seen on valley walls. Minerals are present on the surface that can only be produced in the presence of liquid water."

Estimates of the amount of water needed to explain these features have been made that equated to possibly as much as a planet-wide layer 1,640 feet deep or more. If liquid surface water existed in the past, then Mars' atmosphere had to have had a different climate that was warmer and a pressure near or greater than the current terrestrial atmospheric pressure at the surface.

In the animation, rapidly moving clouds are employed to suggest the passage of time, and the shift from a warm and wet to a cold and dry climate is shown as the video progresses.

The lakes dry up and freeze over, while the atmosphere gradually transitions from Earthlike blue skies to the dusty pink and tan hues seen on Mars today.

It's unknown if the habitable climate lasted long enough for life to emerge on Mars.

"The only direct evidence for life early in the history of a planet's evolution is that on Earth," Dr Grebowsky said.

"The earliest evidence for terrestrial life is the organic chemical structure of a rock found on the surface in Greenland. The surface was thought to be from an ancient sea floor sediment. The age of the rock was estimated to be 3.8 billion years, 700 million years from the Earth’s creation. No fossil evidence of life has yet been found from this period. The oldest claimed micro-fossils date to 3.5 billion years ago. The existence of a potential life-nurturing climate on Mars ended near these times. A comparison between the two planet's life histories must be done with caution, due to the different chemical compositions of the surfaces and different volcanic and meteoroid impact histories. Also, the histories of life on either planet may not have been continuous. Catastrophic events could have killed off all life at one time only to have it start anew."

The animation ends with an illustration of NASA's MAVEN mission in orbit around present-day Mars.

MAVEN will investigate how Mars lost its atmosphere. Scheduled to be launched in November, it will arrive at Mars in September 2014.