Mars stream-bed evidence.
Restored from the Empower Network archive (2011–2017), lightly edited to meet our current advertising standards. Views are the original author’s.
How do we know that Mars had a river at some point in the past?
First some Geology before the Marsology. Rocks contain an exact history of their formation within themselves. The particular ratios of chemicals, the crystal sizes and shapes, structures and forces affecting its appearance all combine in their own way to produce the rock you may hold in your hand. You may recall that sedimentary rocks are formed through the deposition of material and some binding agent. When we see smaller rocks present in the sample we look to see the size and shape of the included rocks. If they are angular then it is called a breccia (brek-e-uh). If the pebbles are rounded then it is called a conglomerate. The significance of the distinction is that the rounded pebbles have been formed by bouncing along in a stream for many years. The amount of force presented by the moving water at any one location tends to sort the rocks it carries according to size. Any particular conglomerate will have predominantly pebbles of similar size. Look at the side-by-side comparison of the Mars rover rock and a conglomerate from Earth.
NASA / JPL-Caltech / MSSS / PSI This set of images compares the Link outcrop of rocks on Mars (left) with similar rocks seen on Earth (right). The image of Link, obtained by NASA’s Curiosity rover, shows rounded gravel fragments, or clasts, up to a couple of inches (few centimeters) wide, within the rock outcrop.
The sizes of the pebbles are fairly uniform and the pieces are rounded. Any geologist would feel comfortable (well, as comfortable as can be from only a photo) identifying both photos as conglomerates. That being the case, the presence sometime in the past of a river is easy to infer. Naturally a scientist is going to question the underlying assumption that geologic processes that are known and understood here on Earth are the same on Mars. The burden of proof in this case lies with any who suggest that different processes are in effect on Mars compared with the Earth. The physics and chemistry of rock formation is well known and there is little disagreement about that end of things.
O.K. there was a river on Mars, what does that mean?
There are a lot of implications to the presence of flowing water on Mars. Some have to do with climatological concerns for the Earth’s future, some have to do with theories of planet formation. The one of interest to many people right now is the question of life on Mars. Whether ancient and extinct and “only” microbial, is not the important thing. The certain presence of life off of our planet is the riveting question. Life “as we know it” revolves around the presence of water, for a variety of reasons I’ll leave to another blog entry. Suffice it to say that unfrozen water is considered essential. So a river in existence long enough to form conglomerate rock suggests that the conditions for life might have been present at some time in the history of Mars. That is one of the reasons that this finding of the Mars Curiosity rover is so interesting. Really, we just need one sure example of life “not on Earth” to open the possibility of life in a lot of “elsewheres”.