Free Astronomy Magazine March-April 2021

MARCH-APRIL 2021 MARS ROVERS L eft and below, close-up views of Hottah reveal more details of the out- crop. Broken surfaces of the outcrop have rounded, gravel clasts, such as the one circled in white, which is about 1.2 inches (3 centimeters) across. Erosion of the outcrop results in gravel clasts that protrude from the outcrop and ulti- mately fall onto the ground, creating the gravel pile at left. [NASA/JPL-Caltech/ MSSS] been able to move and shape peb- bles that the images show, which reach four cm in size − a non-negli- gible size amount- ing to a non-ne- gligible Martian weight. Not even the smallest gravel can be deposited through mecha- nisms other than those dominated by water. For example, it is unthinkable that the Martian wind smoothed and transported all of that material. In short, there is no doubt that Curios- ity has identified the first dry stream on the sur- face of Mars, con- firming once and for all the exis- tence of free water on the sur- face in very re- mote times. About a year after this pivotal dis- covery, Curiosity’s exploration also proved with certainty that an area of Gale Crater called Yellowknife Bay housed a lake and that this lake lasted long enough to house ele-