Muito fiel, mas com algumas diferenças notáveis.
Depois de ler o livro e ver o filme, posso dizer que é uma adaptação bastante fiel. As principais diferenças que notei são:
The novel starts after Watney has been abandoned, and does not explain exactly how and why he was left on Mars until over 100 pages into the novel. The film, on the other hand, starts off almost immediately with the dust storm that forces the evacuation.
Venkat Kapoor is named "Vincent Kapoor" in the film.
The book has more concerning the deal with the Chinese space program. (It is less well-received in the novel than in the film.)
The relationship between Beck and Johannsen is known to the Hermes crew in the novel (Lewis knows and reveals it to everyone), but it is kept secret in the film.
The journey from the HAB to the MAV is not shown in the film, but described in great detail in the novel (and there is a setback along the way involving a crater).
The space rescue is much more dramatic in the film (e.g. Watney pokes a hole into his suit to use the escaping oxygen as thrust).
The (latest eBook / audio book version of the) novel ends immediately after the rescue, but the film shows Watney teaching cadets and also reveals some of what the other members of the crew are doing with their lives, post-rescue.
Generally speaking, the novel devotes more time and energy to explaining the science behind Watney's actions and innovations.
Razões para diferenças:
O diretor Ridley Scott não comentou diretamente se as mudanças foram por extensão ou razões dramáticas / videográficas. Pode-se imaginar que as motivações foram um pouco de ambos:
Cutting down on the scientific explanations helps with length — i.e. trying to squeeze a nearly 400-page book into 140 minutes — and with making the film appeal to a broad audience (not everyone wants to hear scientific details, unfortunately). Cutting down on the negotiations with the Chinese space agency and on Watney's journey from the HAB to the MAV also help with the length.
Watney's "Iron Man" stunt during the rescue (i.e. using his suit's oxygen supply as thrust) makes for good drama.
Você pode achar interessante este artigo sobre as diferenças gerais de tom entre o romance e o filme:
- Vox - Como Ridley Scott ajudou O marciano para quebrar a maldição do filme de Marte
Finalmente, Scott foi recentemente desafiado. sobre o porquê ele decidiu mudar as raças de dois personagens (veja a segunda e terceira tag de spoiler acima), mas ele ainda não comentou oficialmente sobre isso.