<p>Liquid hydrogen (LH₂) is a highly preferred fuel for propelling large-ton Martian landers. Currently, composite insulation structures (CISs), consisting of multilayer insulation (MLI) and spray-on foam insulation (SOFI) are used for the thermal protection of cryogenic propellant tanks. However, the Martian environment poses significant challenges for the thermal protection of LH₂ tanks, primarily because of the presence of rarefied carbon dioxide (CO₂). CO₂ can condense within the MLI microchannels, leading to significant deterioration in the insulation performance of CIS. This study investigates this unique phenomenon occurring on Mars. Two thermal-insulation schemes are designed for potential applications in Martian landers. Numerical models are developed to predict the insulation performance of CIS on Mars by coupling CO₂ condensation and conduction calculations within microchannels. Several key findings emerge: (1) The heat leakage decreases by 82.22% as SOFI thickness increases from 5 to 50 mm. (2) The heat leakage initially rises sharply, and then increases gradually with ambient Martian temperature. (3) The insulation performance deteriorates rapidly and then stabilizes as the number of MLI layers increases, but the heat leakage of CIS exceeds 10 W/m². (4) A new type of insulation scheme (NIS) aimed at solving the issue in finding (3) is designed for the Mars lander by prioritizing the minimum weight which results in an NIS heat leakage of 0.298 W/m²—two to four orders of magnitude lower than that of CISs. These findings provide valuable technical insights into enhancing the thermal protection of LH₂ propellant tanks.</p>