Final answer:
Mars's hydrosphere is markedly different from Earth's, as most of Mars's water is now frozen and its surface lacks the continuous presence of liquid water, making Earth's robust and life-sustaining hydrosphere unique among the terrestrial planets.
Step-by-step explanation:
The hydrosphere of Mars is vastly different compared to Earth's hydrosphere, primarily because Mars does not have liquid water on its surface today. Mars's past is intriguing because it likely had a thick atmosphere and seas of liquid water, which are crucial for the development of life. However, due to its distance from the Sun and its thin atmosphere, most of Mars's volatiles are now frozen, contrasting sharply with Earth, which enjoys ample liquid water and a dynamic atmosphere capable of supporting life.
Our search for habitable environments and life beyond Earth often involves the pursuit of liquid water, which has been historically present on Mars's surface and is still believed to exist in subsurface reservoirs. Comparatively, Mars's current surface is mostly dry and cold, with water only occasionally flowing under specific conditions. This stark difference in their hydrospheres makes Earth unique in its ability to maintain liquid water continuously on its surface, which is essential for sustaining life as we know it.