Hello, friends!  Welcome back to the A to Z Challenge, a month long blogging event.  For this year’s challenge, my theme is the planet Mercury, and in today’s post M is for:

MAGNETOSPHERE

Back in the 1960’s and 70’s, Earth’s magnetosphere seemed like something special.  Both the American and Soviet space programs had sent missions to the Moon, Venus, and Mars.  None of those places had protective magnetic fields, like Earth does.  It was assumed that Mercury would be the same.  But in 1974, as NASA’s Mariner 10 space probe approached Mercury, a charged particle experiment got some unexpected readings, and Mariner 10’s magnetometer picked up a weak magnetic field.

Mercury’s magnetic field is said to be about 1% as strong as Earth’s.  That’s very weak, but 1% is not 0%.  And Mercury even has his own Van Allen belt, a region encircling the planet where the magnetic field gathers and concentrates radiation.  Again, Mercury’s Van Allen belt is not as powerful as Earth’s, but it is there.

For scientists in the 1970’s, the discovery of Mercury’s magnetosphere was difficult to explain.  To generate a magnetic field, a planet needs two things:

• A molten metal core
• A rapid rotation rate

Mars is a rather small planet, and small planets lose their internal heat very quickly.  Since Mars doesn’t have enough internal heat left to maintain a molten metal core, Mars can’t generate any meaningful magnetic field.  Venus, meanwhile, is a very large planet—almost as large as Earth.  She probably does have a molten metal core, BUT her rotation rate is extremely slow.  It takes Venus over 200 Earth days to rotate once.  That’s way too slow to generate a magnetic field.

As we’ve discussed previously, Mercury has a very slow rotation rate.  Mercury is also very small.  That should be a double whammy for Mercury’s magnetosphere, and yet the magnetosphere persists anyway.  Somehow, Mercury retained enough internal heat to have a molten metal core.  And somehow, Mercury overcame his own slow rotation rate to keep a weak magnetosphere alive.  Science accepts that Mercury has done these things.  But how?  How did Mercury do these things?

I don’t think anyone can answer that yet.  Mercury’s magnetic field is still something of a mystery.  Hopefully the upcoming BepiColombo Mission will help find some answers.

WANT TO LEARN MORE?

Here’s a brief article from Scientific American about the magnetic fields of all the planets in the Solar System (well, all the planets that have magnetic fields, at least—sorry, Venus and Mars).

And here’s an article from Space.com about how Mercury’s magnetic field may have changed over time—at some point in the past, it may even have been as strong as Earth’s.