Complex sentence: The only two other areas on the equator and at low elevation, Isidis Planitia and Valles Marineris, are too rocky and exposed, in addition Valles Marineris has too steep a gradient to allow safe landing.
Split into simpler sentences: The only two other areas on the equator and at low elevation, Isidis Planitia and Valles Marineris, are too rocky. In addition, Valles Marineris has too steep a gradient to allow safe landing.