why is there no water on the moon
The bigger the delta-V the more energy you need. You can use diagrams
such as these to get an idea of the best ways to move around the
Earth-Moon system. Note: these values don't take into account the drag caused by
the Earth's atmosphere. You can see immediately that it is easier to leave the Moon than
leave the Earth. The Moon's escape velocity is only 2. 38 kilometers
per second. To escape from the Earth requires a velocity almost
five times as fast (11. 2 kilometers per second). That's a lot
of energy and we haven't even taken into account the drag. I've also drawn in the delta-Vs for some orbits. Let's take a
closer look at them. First you will see that it takes 2. 2 kilometers
per second to reach lunar orbit from the surface of the Moon.
Just a little more delta-V (0. 18 km/s) and you could go from lunar
orbit to complete lunar escape (2. 38 km/s)! That means once you reach lunar orbit it's a simple matter to
go to other parts of the solar system. Compare that with the delta-Vs from the surface of the Earth to
a low Earth orbit. The Space Shuttle achieves these low Earth
orbits and the Space Station will be parked in a low Earth orbit
too. You could get a payload up to the Station in the Shuttle
but it takes a delta-V of 8. 6 kilometers per second. Once the
payload is there it needs an additional 2. 6 kilometers per second
to escape completely from the Earth on an interplanetary mission. Clearly it takes less energy to launch an interplanetary expedition
from the Moon than from the Earth.
For a real surprise, let's look at the delta-Vs to get to a low Earth orbit. It takes 2. 2 kilometers per second to achieve lunar orbit and
another 4. 1 kilometers to move on to the low Earth orbit. That's
a total delta-V of 6. 3 kilometers per second. (Just add them up. )
Compare that to the delta-V to achieve the same orbit from the
Earth's surface - 8. 6 kilometers per second. That means it takes
LESS energy to go from the surface of the Moon to the Station
than to get there from the Earth!
Why is there no liquid water on Mars at present? Here on Earth, we have water as liquid in the oceans, in solid form (ice) in the ice caps and glaciers, and as vapor in the atmosphere. On Mars, the story is different.
However, the diagram also illustrates that the typical pressure and temperature on the surface of Mars are relatively close to the point where water is stable as liquid, solid and vapor (called the ). This means that if the temperature and pressure increase for a short period of time, for example on a warm summer day, it is possible for ice to melt instead of going directly from ice to vapor. An example of this can be seen on the picture below. The two pictures show the same crater side with 6 years in between. In those years, a new pattern formed on the side of the crater, probably originating from subsurface ice melting and running down the crater slope. You can read more about the newly discovered patterns on the.
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