New Messenger images of Mercury

[StevenB]

NASA's Messenger probe had another fly by of the planet Mercury a few days ago, and the pictures have been posted at http://messenger.jhuapl.edu/gallery/...p?gallery_id=2. Some of the pictures show parts of the surface nobody has seen before. I like the horizon pictures. Looking at those pictures, it almost seems like you are flying past Mercury yourself.

The cratered surface looks very much like the Moon, but unlike the Moon Mercury has a very tenuous atmosphere with magnetic tornadoes.

[Hans]

Beautiful pictures!

Magnetic tornadoes? Ugh.

[P'ter]

To what extent is Mercury a twin of our Moon?

[StevenB]

Originally Posted by P'ter

To what extent is Mercury a twin of our Moon?

Not at all. It looks like the Moon because it is also a rocky body that has been bombarded and cratered by meteorites for millions of years. But looks can be deceptive: Mercury has a large iron core, which makes it a lot denser than the Moon and gives it a magnetic field.

The Moon is rather like a solid chocolate M&M and Mercury like a peanut M&M. They look the same on the outside, but bite into them and you'll find the inside completely different. (Disclaimer: Don't do this at home. Eating planets can give you indigestion and be bad for your health.)

[Hans]

Steven, if I understand you right Mercury, Moon and maybe other planets are cratered because being hit by meteorites for n=millions of years? If that is so, why don't we (on our little planet) have no more contact with meteorites?

Not that I'm not thankful we don't

[StevenB]

We do, but the Earth has a thick atmosphere which conveniently burns up the small ones (see the other thread about the Perseid meteor shower). The larger ones still create craters on the surface of the Earth (see Meteor Crater in Arizona), but they are gradually eroded away over millions of years by wind and water.

Each planet has a different bombardment history, depending on its size and location in the Solar System. Jupiter is probably the most bombarded planet because of its gravity, but its atmosphere is so thick any impacts only leave traces for a few days or weeks (e.g. the Comet Shoemaker Levy 9 impact).

Because the Moon is close to the Earth it has had roughly the same bombardment history. It's sobering to think that all the impacts which created the craters on the Moon, including the gigantic collisions which carved out the lunar maria, also happened here on the Earth. Thankfully it was all a long time ago. Our friend Jupiter has cleared up much of the crud floating about the Solar System, so these huge impacts don't happen anywhere near as frequently as they used to.

[Hans]

Ah, of course, the atmosphere. Thank you And let's indeed be grateful to Jupiter

[P'ter]

According to ancient legend, Jupiter gave birth to Venus (torn from his forehead). A certain Immanuel Velikovsky took this literally to mean that the planet Venus was derived from the planet Jupiter.

[Cheryl]

The vast majority of the bombardment was early in the history of the planets right, while they swept up all the debris from the birth of the solar system, right?

Since then, plate tectonics and sediment deposition (and subsequent sedimentary rock formation) has wiped the crust clean of that early bombardment, and only the periodic later impacts are nicely visible.

[StevenB]

That's right. The worst bombardment happened during the birth of the solar system, and it reduced over time as the bits and pieces collided and merged to form the planets or were ejected from the Solar System by an encounter with one of the planets. Some of the bits and pieces still exist today in the form of comets, asteroids and meteoroids.

I had overlooked plate tectonics as a means of mopping up the old craters. Other bodies have different ways of erasing the bombardments of the past: Europa periodically melts and refreezes its watery surface; Io periodically covers its surface with sulphurous lava. It's when you find a body with a really old surface, such as the Moon or Callisto, that you realise the full horror of the bombardment that happened in the past.