12020.1.11172. Was There Water on Ancient Mars?Despite Mars's bleak,cold climate today,there is abundant geologic evidence that liquid water once flowed on the Martian surface (and even recent evidence that some water still periodically flows today).The evidence of past flows includes river channels,now dry,that were once carved by powerful foods,valley networks of branching streams and tributaries indicating precipitation falling over wide areas,river deltas and layered sediments suggesting flow into standing bodes of liquid water,and salt and other minerals that dissolved in liquid water and were left behind when water evaporated. When that ancient wet period existed and when it ended is difficult to say. The best method for dating a surface uses radioactivity,the process by which certain elements decay over time.One collects rocks from the surface and measures their radioactive elements to see how much of the decay products have accumulated in the rock since it formed.Transferring this age to the age of the surface can be tricky unless one knows where the rock came from.And one needs to collect the samples and bring them to a well-equipped laboratory. The other way to date a planetary surface,which is used when rock samples are unavailable,Is by counting craters.The longer a surface has been exposed to meteorite bombardment,the more craters it will have.This gives the relative age,and allows one to determine which surfaces are older and which surfaces are younger. To get the actual age one needs a standard.This was established for the Moon when lunar samples were returned to Earth as part of the United States Apollo program.The crater counts of the areas of the Moon from which the samples came were carefully noted.The samples were dated in terrestrial laboratories using radioactive decay,and the relation between absolute age in years and crater density--numbers per unit area-was determined.This relation is valid for the Moon,and it is possible that it is valid for Mars as well. Then counting craters on a Martian surface should yield its actual age. It is possible,however,that the number of impacting objects is different for Mars.The problem has been carefully studied,and adjustments have been made,but the general conclusion is that a wet period occurred early in Mars's 4.5-billon year history. This causes a problem for Mars climate science,because the Sun was supposed to be fainter.perhaps by 20 or 30 percent,during the Mars wet period.Yet a watery Mars arguably would have required relatively warm temperatures.The power output of the Sun-its luminosity over time-is based on models of its interior and evolution. The models are well tested and agree with the luminosities of stars like the Sun whose ages are known.The same problem exists for Earth,which appears to have been warmer billions of years ago than it is today. Geologists...
