Li há tempos no resumo de uma palestra de divulgação, que a região de Tharsis, em Marte (uma das mais importantes do planeta em termos de vulcanismo), podia ser o resultado do impacto de Hellas, um cratera gigante que existe nos antípodas de Tharsis e cuja ondas de choque podiam ter erguido a referida região. Fiquei perplexo com a ideia e, por isso, comecei a meditar sobre o assunto e a colher opiniões. Para mim, trata-se de uma região resultante da acção de uma superpluma, ou seja, de um mecanismo semelhante ao que tem actuado na criação das ilhas do Havai. Em Marte teria sido mais intenso e nada teria a ver com o impacto de Hellas. Aqui fica a primeira opinião. Outras surgirão nos próximos dias.

I have not heard this theory and, although I am not an expert on Mars, it doesn't sound very likely. I have read in a textbook that the Hellas impact may have substantially altered Mars' crust, possibly accounting for the fact that the southern hemisphere generally has higher elevations than the northern hemisphere, but I do not understand the mechanism. Perhaps it created a region of thin crust at the Tharsis region through which magma could easily rise. Given that Mars has no plate tectonics, a region of thin crust could persist for a long time (that helps explain why Tharsis has been a region of volcanic activity for so long). There may be a problem with ages, though, as the Hellas Basin is thought to be about 4 billion years old, whereas volcanic activity in the Tharsis region is thought to have started only about 3 billion years ago. I'm not sure if the 1 billion year delay can be explained in some way. Also, the motions that produced the Tharsis Bulge are also thought to have produced the Valles Marineris, so again there may be a timing problem in that those regions may not have formed at anywhere near the same time as the Hellas Basin. And the physical mechanism by which the impact caused the crust to thin at Tharsis and crack at Valles Marineris would need to be explained. So I'm afraid I'm dubious right now.

Richard Rand
Associate Professor of Astronomy
University of New Mexico