Discovered an alien race

When the A.R.L.(American Research Lab) finds footprints on the planet Jupiter, are astronauts Rick and Harry send to that planet. When they arrive they take pictures and go back to Earth. The A.R.L. finds a monster in the space ship. It is successfully captured and put in a cage. When one night someone forgets to close the highly secured cage, the monster escapes in the huge open world. Will the human race survive or not? Read it in this story.