Well, you have to get to Mars anyway, so USE the yellow crystal again and
click on the Great Face. You will be teleported into a chamber with three doors
and, yes: more strange yellow markings! USE your yellow crayon to recreate the
figure you saw in the map room inside the Sphinx, and all three doors will
open. Go through the centre door, and you find yourself in the maze of coloured
doors. TURN ON your lighter, go through the green door on the far left, and the
yellow door on the far left of the second room; go all the way to the left, and
you'll be in the Great Chamber. Zak can meet up with Melissa and Leslie at long
last.
  No time for chitchat, though. Have Leslie put her helmet back on and WALK to
the big pile of sand in front on the hostel. USE the broom alien on the sand,
to reveal solar panels, then send her over to stand by the tram. Next, PUT ON
Melissa's helmet, and send her over to the the tram. Finally, you have to send
Zak outside, but he needs a spacesuit. PUT ON the wet suit, the oxygen tank and
the fishbowl, then USE the duct tape on the fishbowl. Perfect! Send Zak to the
shuttlebug where he can use the oxygen valve to freshen his tank and bring it
up the full 15 minutes; then, send him to the monolith, and have him USE his
CashCard in the slot. He needs only one token. Zak should then join the others
by the tram. Save the game here!