Last week’s blog talked about the rapid traverse of Perseverance to the Delta. This weeks blog entry will talk about the Delta itself, and why it is something worth rapidly traversing towards!

The prospect of the delta for me is that every day will be full of excitement and could bring anything. Let me explain that a bit further. On a space mission like M2020 you get used an exciting timeline of activities, but for a geologist, the excitement of the delta comes from not knowing what is coming next. Every image that is returned by the rover of the delta rocks will be in a very real sense unique.

Let’s explore that line of thinking a bit.

A delta forms when a sediment laden river runs into a body of standing water, and as it does so, slows and can no longer hold the sediment, so it drops the rocks, gravel and soil into the water body, which gently sinks to the bottom and forms a delta. Over time, the delta becomes a layered repository, like an book with pages, which one can turn over each day to learn more about the history of Mars.

How does it achieve this? Well, the rocks and sediments had to come from somewhere. They were sourced in a region called the “watershed” of the delta. This is a much bigger area than Jezero crater (itself about 45km across), and the rocks we see in the delta will inform us about a wide range of Martian process, and some rocks may even be *older* than the Jezero crater itself (about 3.9 billion years old).

This might happen, for example, if a very old rock is preserved in the watershed, and then is broken off by water, and then carried by that fluid into the river, and finally into Jezero crater. If we do get access to these super-old rocks, then this would be a very interesting thing for the rover to sample for eventual return to Earth.

Another mind bending possibility is that we may find fossilized traces of ancient Martian life in these delta rocks. In one scenario, life might have got started in the early Noachian period (about 4 billion years old) when Mars was probably more friendly to life, and was preserved in the watershed until one fateful day when they were washed into the river system, and then the crater.

So these interesting rocks will arrive somewhat randomly to us as we explore the delta, one can appreciate that everyday of the year-long “Delta campaign” will be exciting for every scientist on the team, because every day could be the day we hit it big. Real big.