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Today we can hardly imagine that rivers once flowed on Mars. But the barren, red planet looked very different a long time ago. Astronomers have now determined how long it took for this water to form the valleys of Mars.
“Mars is now one big desert, but its surface is full of evidence that running water once existed. For example, some river beds can be seen. “How long it took for these valleys to form speaks to the viability of early Mars, as long periods of stable liquid water support life,” said Mars researcher Alexander Morgan Institute of Planetary Sciences in Tucson, Arizona.
The craters as proof
The valleys on Mars formed more than 3 billion years ago and have long been the strongest evidence that liquid water once existed on the Red Planet. Previous studies showed that it took at least tens of thousands of years of erosion for such things as valleys to exist, but how often water flowed through them, and therefore the total time it took for the riverbeds to form, has never been determined.
To find out, Morgan examined the craters on the Red Planet. “I used craters that formed before and after the valleys to constrain the time period over which these systems formed. Therefore, these new results provide an upper limit on the time scale over which the valleys of Mars were active,” explains the scientist. It reaches a period of hundreds of millions of years. “Based on what we know about erosion on early Mars, longer time scales suggest that the conditions enabling flows only existed at long intervals, with long dry periods alternating with short episodes of flowing water.”
A network of valleys on Mars with red and blue circles around the craters. The blue craters date from before the valleys were formed, the red ones from after. Image: MOLA MEGDR, NASA/USGS; THEMIS mosaic, ASU/NASA/USGS; CTX, NASA/MSSS.
Two camps
Scientists studying early Mars don’t all agree. There are two camps: one group assumes that it was warm and wet with an ocean at the time. Others say it was cold and icy with large ice caps. “We have discovered over the last decade that these descriptions are far too general. “It makes no sense to summarize hundreds of millions of years of climate history in two words,” Morgan said.
Just as ice ages alternated with warmer periods on the early Earth, the climate on Mars was not always the same. “Just like our planet, Mars used to be complex and have diverse conditions that allowed surface water to form. The Earth has experienced enormous climate changes throughout its history. Twenty thousand years ago, what is now Chicago was covered by half a kilometer of ice. “This is probably how the conditions for flowing water on Mars arose and developed,” explains the planetary scientist.
Needs some warmth
The results suggest that rivers on Mars eroded very slowly, similar to parts of the Atacama Desert in Chile. One explanation for this is that erosion may have been stopped by large stones in the riverbed that could not be further mined. Another explanation is that the rivers flowed very irregularly, perhaps only 0.001 percent of the time. The rivers on Mars would have been bone dry most of the time, but became more active during volcanic activity or when the planet was closer to the sun, making it warmer. We are also experiencing such long-term changes on Earth. Here they are called Milankovitch cycles. For example, they are responsible for the last ice ages on our planet.
“In the short term, the flow of rivers is determined by precipitation or melting snow. “But the world’s rivers are also affected by climate change over longer periods of time,” explains the scientist. “For example, 20,000 years ago there were large lakes and rivers in what is now Nevada. Martian rivers would have existed in the same way, with short-term fluctuations due to storms or precipitation and long-term fluctuations due to changes in the planet’s rotation and changes in its orbit around the Sun.”
What are Milankovitch cycles?
Milankovitch cycles are periodic changes in the climate as Earth’s orbit and position relative to the Sun change slightly each year. This means that more or less sunlight reaches the earth, which affects the temperature.
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