In a small saucepan, combine the water, sugar and salt over medium heat until the sugar has dissolved. Pour into a large bowl and combine with the grapefruit juice, blood orange juice, Campari, mezcal, and sweet vermouth. Cover and refrigerate until thoroughly chilled.
Place the mixture in a 9” x 13” pan and freeze, uncovered, for one hour, until the sides and top are just starting to get solid. Using a fork, scrape the mixture, breaking up any larger chunks. Repeat this process of freezing and scraping, every 30 minutes, for about 4 hours. Once you’re happy with the texture (light and fluffy like snow) place in an airtight container and keep frozen until ready to serve.