The topic of today's note is Martian meteorites and the age of the current surface of Mars.

Now when we speak of Martian meteorites, what generally comes to mind are those pieces of Mars which were ejected from that planet by massive impacts and then found their way to our own planet, Earth. But there is another type of Martian meteorite, and that is meteorites which fall onto the surface of Mars instead of onto the surface of our planet, the Earth.

Now every year tons of meteoritic material falls onto the Earth. Most of it is reduced to dust as it ablates away while plunging through our dense atmosphere, but some of it is large enough and tough enough to survive that entry. Even so, that small part of it which survives this plunge is then usually eroded away by water processes on Earth and is seldom recovered intact by man.

On Mars, the situation is different. While Mars has only 1/3 Earth's gravity, this makes little difference, as the meteoritic material is already traveling at speeds of 10's of kilometers per second by the time it reaches Mars. Mars also has a very thin atmosphere compared to that of the Earth, but again this matters little in what reaches Mars' surface. The key here is that Mars has almost no water, so that the meteorites which reach it's surface ontinue there unchanged for many millenia.

Now the large iron asteroids which have hit Mars have been turned into a dust which covers everything on Mars, and give Mars an appearance which looks deceptively like that of the desert Southwest of the United States. Along these lines, it is interesting to note how many Mars "Enthusiasts" have been all too willingly taken in by this dusty deception. But scrape away that iron dust, and what you find is gray meteoritic residue which looks exactly like that found on the Moon.

Further, just as on the Earth, larger meteorites survive their encouter with Mars atmosphere and find their way to Mars' surface. You can easily identify them in the images of Mars returned by Viking 1 and 2 and Pathfinder, as they compromise most of the rocks seen in those images.

Some may find this surprising, but it must be remembered that for the most part Mars' has no water based "geological" processes to speak of, as it has alomost no water. And thus meteorites falling onto the surface Mars stay there for a very long long time, until disturbed by other larger falls.

While it was earlier thought that the reason for the pock marked appearance of many boulders on the surface of Mars was that they had been pounded by dust ejected at very high speeds from nearby large, it is now understood that in many cases these "pock marks" are the result of collisions between these mini-asteroids which took place during the billions of years they were still in space before they hit Mars.

EP