To measure mass-density you need to measure both the mass itself, and
the volume. It's then just a matter of division: mass/volume. The
densities of astronomical objects are measureable. As an example, the
volume of planet Jupiter is determinable by measuring its angular size
in a telescope, then measuring its distance via trigonometric methods,
and therefrom its dimensions can be gotten and then the planet's
volume. Knowing how big it is, its mass is then found by measuring
both the radius and period of a moon's orbit. This information yields
the mass.
Here on Earth the masses of large mountains have been estimated. A
mountain's volume can be roughly determined by surveying its geometry,
and some measure of tis mass can be made at *some* distance by noting
the deflection of a plumb-bob from vertical.
But doing the same for something such as a mound of sand may be
less effective. You could measure its volume at a distance by knowing
that distance and noting its angular dimensions, perhaps through a
telescope. But its mass would be rather hard to determine by gravitic
effects. In this particular case the most effective thing would be to
know, by experiment, the mass-density of a typical mound of sand, and
plug that into the volume of the target mound.
Now here's something I just thought of, though it's a little
far-fetched at the moment. If you fire a bullet from a gun into a
mound of soil, the rate at which it decelerates as it borrows into the
grains will be a function of the bullet's initial energy and the drag
imposed upon it by the heap of sand. It's conceivable to design a
bullet with a small radio transmitter inside. The muzzle energy of the
projectile would be known ahead of time by knowing the bullet's
overall design. The transmitter would generate a known base-frequency,
which would be monitored with a receiver/recorder. The bullet is fired
from a distance into the mound, and the Doppler shift in the frequency
recorded. This then could be reduced to the density of the soil. |