More horror? Okay. What's horror?

I've gotten a good rejection, with a suggestion I'd like to implement, but I'll need some help from horror writers.

My novel is SF, but I've got a scene early on (I consider it the novel's hook) where something bad happens. The publisher recommended I "ratchet up the initial horror" of this scene.

So, here's a basic question: What makes a scene horrifying?

Here's my specific scene: An asteroid miner searches for his son and finds him inside an apparently ancient, derelict alien spacecraft. The son, dressed fully in a spacesuit, is unresponsive. Then, the father gradually realizes that: 1) It's not his son inside the spacesuit; 2) His son must have been killed by what is in the suit; and 3) The father is trapped in a small space with this thing.

I thought I filled the scene with good mentions of gore and bloodshed, disgusting sounds from inside the suit, unhuman motions of the arms and legs, and a good sense of the MC being trapped.

But they wanted more. Suggestions?

Wouldn't that be telling instead of showing? I've got my MC looking around like a caged animal, going all cold, and being paralized by emotional shock. (Probably not in that order.)

Would lengthening the scene do it? Build up the tension, maybe?

Here's a sample. It's 1,100 words, so should read pretty quick.

http://absolutewrite.com/forums/showthread.php?t=37465

Thanks for all the replies.

Thanks!