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主题：火星上是否存在液态水

1. Sliding Rocks and Dust

The surface of Mars is often disturbed by objects falling from space. As these space objects collide with Mars, they disturb the Martian surface and cause loose pieces of rock and dust to slide down slopes. The sliding particles leave behind visible streaks that are similar to the recurring slope streaks. Some scientists have suggested that the recurring slope streaks might perhaps be caused by sliding rocks and dust, and not by water.

1. First, the recurring slope streaks differ from streaks left behind by sliding rocks and dust in an important way. Sliding particles are typically set in motion by a space object colliding with mars. But space objects don't keep landing on the exact same part of the Martian surface year after year, right? That means that sliding rock and dust streaks do not appear in the same places year after year. But that's exactly how the recurring slope streaks behave. They do appear and reappear in the exact same location over several years. This strongly suggests that they are actually caused by water that finds the same paths to flow through year after year. This makes a water origin very likely.

2. Cold Temperature

Another issue is that the surface of Mars may be too cold for liquid water to exist there. Water freezes at zero degrees Celsius, while the average temperature on Mars is minus 70 degrees Celsius. For that reason, the only water whose presence on Mars has been clearly established so far is frozen.

2. Second yes, the surface of mars is very cold, but salty water can remain liquid even at very low temperatures. Much lower than zero degrees Celsius. Many scientists think that it is very possible that the recurring slope streaks are actually formed by salty water that remains liquid even in the cold temperatures we find on mars.

3. No Evidence from Specialized Instruments

One of our main sources of information about Mars is a research spacecraft that is currently orbiting Mars. The spacecraft carries specialized instruments that can detect various chemical elements and compounds on Mars's surface. However, the instruments on board the spacecraft have so far failed to detect any liquid water in the recurring slope streaks.

3. Finally, it's true that the instruments on board the spacecraft have not detected liquid water in the streets so far. But there's a good explanation for that. Because the atmosphere on mars is very dry. Water that's directly on the surface of the streets evaporates, and so the instruments cannot detect it. Most of the liquid water that remains in the streaks is mixed with the Martian soil and is located slightly below the Martian surface. The problem is that the instruments on board the spacecraft cannot detect water that's located slightly below the surface.