American Idol judge Randy Jackson tells Us Weekly that producers have been trying to get Mariah Carey on the hit Fox show for the past couple of seasons.

Their time has come.

Carey, 38 — whose new disc E=MC2 drops today — pops by the show tonight to mentor the seven remaining contestants.

"She's going to bring a wealth of knowledge about singing to these kids," says Jackson of the newly svelte singer, who will also perform. "It's amazing for us to have her on.

"She's kind of what the show's about — she's that girl who grew up in New York, who struggled to become that singer."

Now, Jackson says, "she's won the Holy Grail. She sold 175 million albums! I don't know if anyone will ever get to that point ever again because the music business is in such disarray."

On tonight's show, contestants will perform songs from Carey's extensive catalogue.

So will David Cook belt out "Vision of Love?"

"You never know!" Jackson tells Us. "Just because you're a rocker or a country singer doesn't mean you can't sing these songs - you adapt them to what you do best."

Adds Jackson, "I'm sure these kids will have a good time."