The discovery of water on Mars has long fascinated scientists, but recent evidence suggests that liquid water may exist in subsurface aquifers beneath the planet's polar ice caps. This claim is supported by three main arguments. First, radar soundings from the Mars Express spacecraft detected bright reflections beneath the south polar layered deposits, which are consistent with the presence of liquid water. The strong radar signal, similar to that seen over subglacial lakes on Earth, indicates a high dielectric constant typical of liquid water. Second, theoretical models of Mars' internal heat flow suggest that the planet's geothermal heat, combined with the insulating effect of thick ice, could keep water liquid at depths of 1.5 kilometers, despite surface temperatures averaging -60°C. Third, the presence of hydrated salts, such as perchlorates, detected in the polar regions would lower the freezing point of water, allowing it to remain liquid at temperatures far below 0°C. These salts, known to exist on Mars from spectral analysis, could create brines that persist in the harsh Martian environment. Together, these lines of evidence make a compelling case for the existence of liquid water beneath Mars' polar ice.
🎧 听力讲座
🎯 模拟考试环境：先点「播放讲座」听完内容，再作答。听力原文默认隐藏。
The reading passage presents an exciting hypothesis about liquid water on Mars, but the evidence it cites is far from conclusive. First, the bright radar reflections are not unique to liquid water. They could equally be explained by layers of carbon dioxide ice or clay minerals, both of which produce similar radar signatures. In fact, the radar data used in the study were ambiguous, and subsequent analyses by other teams have shown that the reflections can be reproduced without invoking liquid water. Second, the theoretical models of geothermal heat are based on assumptions about Mars' interior that are highly uncertain. Recent measurements of Mars' crust from the InSight mission indicate that the planet's heat flow is much lower than previously estimated, making it unlikely that temperatures at depth would reach the melting point of ice. Third, while perchlorates do lower the freezing point of water, the concentrations required to form liquid brines are far higher than those observed in the polar regions. Moreover, the presence of perchlorates in the soil does not guarantee they are in contact with the ice in a way that would allow melting. In summary, the evidence for subsurface liquid water remains speculative, and alternative explanations are more consistent with the data.
📝 写作提示 Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
