A French Inventor Once Proposed a Giant Mirror to Burn a Message on Mars

perhaps the mirror doesn't need to be a monolithic one...If I were to do that, I would install many smaller mirrors around the earth -- then the burning process could go on around the clock 24 hours non-stop, and it would have higher availability (as the probability for all the area in the day time zone to be covered by clouds and storms is expected to be small)...

Some extra advantages for this distributed approach is modularity (you just need to design and create one mold and production of many would be easy), scalability (in case you find the current mirrors are not powerful enough), as well as re-configurability (in case you want to have a long conversation with the presumable Martians, and not just a single message)...

Then on the Martian side, if they could see our mirrors, then this might just look like a LCD monitor from far away...