# Toward Sustainable Mars Exploration: A Perspective on Collaborative Intelligent Systems
## 3 Energy Management
### Alternative Energy Sources  
At present, Mars exploration missions are relatively limited in terms of the number of rovers deployed and the extent of collaborative swarming activities. Nevertheless, as we progress toward a scenario illustrated in Figure 2, the energy demand is bound to increase. Although solar and RTG power generators are capable of powering autonomous vehicles, their applications are limited due to the drawbacks discussed before. Consequently, a dense and deeply collaborative ecosystem of autonomous vehicles would need to include alternative sources of energy to ensure a long-term operation. To accomplish this, it is necessary to build a multi-source energy harvesting system and employ an efficient power management scheme capable of handling solar, RTG, and other durable energy sources resilient to the challenging conditions on Mars. **Thermal energy generators** possess the unique capability of generating energy from temperature differences, making them reliable for energy generation in applications where such differences are available. There are many thermal energy reservoirs identified on Earth that can be potential sources for thermal energy harvesting. Among these, the temperature difference between Earth's soil and air is considered a viable source of energy that can power many applications [26; 27]. Earth's soil has a high thermal inertia, causing its temperature to lag behind air temperature. This leads to a consistent temperature difference, which can be converted into electrical energy. When exploring the ground and atmosphere temperature from Mars, a similar phenomenon can be observed [28]. For instance, the data collected by the Curiosity rover show the atmosphere temperature changing from \(-2\,^{\circ}\)C to \(-75\,^{\circ}\)C, while the surface temperature can go as high as \(-3\,^{\circ}\)C and as low as \(-91\,^{\circ}\)C [29]. Moreover, soil temperature on Mars lags behind the atmospheric temperature, creating a noticeable temperature difference that can be converted into electrical energy with the help of a TEG. While the energy produced from the soil-atmosphere temperature difference may not be sufficient to perform energy-demanding tasks, many real-time monitoring and surveillance applications may benefit from it. Although TEGs can convert temperature gradients into energy, they can only work with spatial gradients. Thus, extracting a temperature gradient between the atmosphere and the ground surface requires installing a heat engine with one of its heat reservoirs connected to the atmosphere and the other connected to the soil. This limits the mobility of the energy generators. If this temporal difference could be converted into energy, it could serve as an energy source for autonomous vehicles. A conversion of temporal temperature difference into electrical energy can be accomplished with thermal resonators [30]. Thermal resonators typically employ two thermal masses, for instance, phase change materials (PCMs), to create a thermal circuit tuned to a particular temperature fluctuation frequency, converting temporal gradients into spatial gradients [30]. This spatial gradient can be then converted into energy with the help of any heat engine, or it could even be transformed into mechanical energy to actuate autonomous vehicles [31]. Converting thermal energy to electric energy using TEG is a relatively inefficient process [19]. Consequently, the power generated by thermal generators from ambient heat sources on Mars may not be sufficient to compensate for the solar energy blackout during dust storms. While dust storms reduce the efficiency of solar generators, they themselves can be a potential source of energy. Mars's environment is abundant in mechanical vibrations produced by frequent winds and dust storms, making this **vibrational energy** a valuable resource. However, traditional electromagnetic generators may not be the right choice for Mars due to their heavy weight and large form factor, which results in an increased cost of transportation.