QUESTION:
Why does it take 10 hours to study one rock?  Also does the info come back 
immediately?
					
ANSWER from Wendy Calvin on April 30, 1997:
Well, I'm not sure I'm the right person to answer this, but here's my
guess.  I'm presuming the 10 hours to measure one rock has to do
with first, looking through a camera to figure which rock might
be interesting enough to measure, then getting over there. Getting
there could actually take a long time as Sojourner has to drive 
around any rock that is bigger than a certain size, say about 
your two fists put together. Then, once you get there you have to
let the x-ray counter sit on the rock awhile (this should be
confirmed with someone that knows more about the APX). Let's say
only one x-ray comes out every second, and you need to count 
10,000 before you have enough x-rays that they begin to show a 
distinct pattern (this is called getting a statistically significant
reading).  At that rate it will take you 10,000 seconds to 
get enough "signal" which is close to 3 hours.

As far as information coming back, it goes to the lander, then
the orbiter and then to earth.  The Earth-Mars link time is 
anywhere from a few minutes to half an hour, depending on where Earth
and Mars are in their orbits.  That is just the time it takes the
radio wave to travel.  Usually since Earth is not in a good spot
to "listen" to the spacecraft, the data goes onto a tape recorder
and then gets played back later.

ANSWER from David Mittman on June 27, 1997:
The way Sojourner studies a rock is like drawing a picture with a pen that's
just about out of ink: the picture comes in slowly and you have to go over
the lines again and again. It takes about ten hours before enough "lines"
have been drawn and we get a clear "picture" of the rock.

Sojourner uses an instrument called an Alpha Proton X-Ray Spectrometer
(APXS) to study the rock, but it doesn't produce a picture of the rock like
a photograph. What is produced is graph which shows how much of many
different chemicals are present in the rock. The presence of certain
chemicals in rocks tell the Mars Pathfinder scientists about how the rock
was formed.

About those "pictures"...since Sojourner also carries three different
cameras (two black-and-white and one color), we'll also get nice pictures of
the rocks that we've spent 10 hours studying!