Interesting, could you let us know a bit about Mattermost, is it missing any (main) features that Slack has?
The #NeuroMethods slack is currently on Slack with about 18000 users.. Would that be too much?

Edit: I realized that you were posting for Planetary scientists and we are Neuroscientists but I am just generally interested in any slack alternatives (not necessarily in invading your space! 😅)