Like he needed the cash, Tesla and SpaceX head honcho Elon Musk just signed a $3.5 million book deal with Penguin.

Details are scarce at this point, but apparently it’s a book about Earth and Mars, “half about the issues facing us on Earth — sustainability issues in particular. The second half will be about the idea of a multiplanetary existence — about what’s possible, about the adventure of experience,” according to Business Insider.

Several publishers were pitched on the project and reportedly all bid for it. According to BI, Penguin “flew someone out to meet with Musk at SpaceX’s California headquarters,” and they sealed the deal.

The book deal is worth a reported $3.5 million, but BI‘s sources also say that number is “low,” and that Musk intends to write the book himself and not use a ghost writer.