[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Meteorites on Mars

E.P. Grondine wrote:

>      Since Mars has only a very thin atmosphere, it is
> quite likely that most impactors make it to its surface
> intact, as you point out.  But since they have been slowed down very
> little by an atmosphere, they hit the surface of Mars at a speed of
> around 26 miles per second, plus or minus the speed of Mars in its
> orbit around the Sun.


Interesting.

Let's do a simple model.  Treating the drag force F as quadratic in
velocity V, the instantaneous deceleration magnitude goes like

    F = 1/2 (density of atmosphere) (Cd) (Area) V^2

We should actually assume an entry angle and do an integration as the
atmosphere and velocity each varies (and to a lesser extent other
factors like the drag coefficient Cd and area as the object could be
ablated or altered), but let's do a crude guess and let the object hit
rather obliquely to minimize at least atmospheric density variation over
a long stretch and give the atmosphere a chance to act at slowing  the
meteor.  Since we are not being exact, it's reasonable to take half the
drag coefficient as being of order unity.  Take as an example an area of
a hand sized object:  .1 m x .1m = 10^-2 square meters.  You chose 26
miles per second, so I'll use about 40 km/s = 4 x 10^4 meters per
second.  Let's go through a stretch of Martian atmosphere that has a
density roughly around 1/1000 the Earth's near sea level.  Let's say
this is perhaps one  gram per cubic meter.

    F = (1) (10^-3) (10^-2) (10^4)^2 = 10^3 Newtons   

Give our object about a five kg mass, and the deceleration a is
drag/mass or 

    a =  10^3 N / (5 kg) = 2x10^2 m/s/s

This is a hefty deceleration even for a thin atmosphere, however...  If
the deceleration is crudely treated as constant at these angles, then 
after even a few tens of seconds, the product of acceleration and time
is only a few km/s so the object is not greatly slowed relative to its
initial velocity. Our crude model was good enough to illustrate that
appreciable speeds remain for this particular meteorite. Even if the
lower altitude density on Mars were an order of magnitude greater so
that the deceleration was 2 km/s, the object would have to spend an
unreasonable length of time in this dense part to be stopped- it will
strike at many km/s. 

On the other hand, if the object were only tens of grams the
acceleration would go up by two orders of magnitude offset somewhat by
the fact that a slower drag results from the decreased area.
Maybe an order of magnitude higher deceleration would be the net result
and now a long oblique streak would be appreciably slowed, and more so
near the surface.

My point for this exercise is to say it seems there is much opportunity
to get high speed strikes at the Martian surface, if not for meteorites
weighing grams, then at least for pieces that are a little bigger but
still numerous when considering we are talking about billions of years.
I conclude that iron introduced to the surface by meteorites is a
distinct expectation.  How *they* subsequently rust depends on the
Martian environment.   
    




>       Most likely, when they hit, the impactors explode into dust and
> very small fragments. 

If the above calculation is at all close, your statement does not sound
bad.  Meteorite hypothesis as partial explanation for Red Mars seems to
hold up.

TO UN-SUBSCRIBE: 
----------------
Send an email To: 
meteorite-list-request@meteoritecentral.com 
with UNSUBSCRIBE in the subject field of your 
email. That's all there is to it!
----------------
Thank You
References: