QUESTION: 
Are the lava tubes on Mars similar in number, size etc
to those found on the Earth, Moon and Venus?

ANSWER from Jeff Plescia on February 11, 1997:
Lava tubes on Mars would be produced in a manner similar to
those produced on the earth -  channels lavas flows roof over from
solidification of lava resulting in a tube which holds lava until the
eruption ends, afterwhich the lava drains out and the tube remains.

We see them on the large shield volcanoes, but the resolution of the
images is not sufficient to determine much detail. Presumably they
occur just as frequently, but we really don't know.

Jeff Plescia
Jet Propulsion Laboratory