Gocv！ RGB到灰度

I'm having troubles to convert images to grayscale, can you help me at this? I'm new at Go and in go cv, but I have done a semester at image processing, I'm doing a project in the best path to take some supplies in 7 different spots.

package main

import (
    "gocv.io/x/gocv"
    "path/filepath"
)

func main() {
    pizzaPath := filepath.Join("Mars.bmp")
    window := gocv.NewWindow("Hello Mars")
    windowB := gocv.NewWindow("Hello Gray Mars")

    MarsImg := gocv.NewMat()
    Marsgray := gocv.NewMat()

    MarsImg = gocv.IMRead(pizzaPath, gocv.IMReadColor)
    Marsgray = gocv.ConvertTo(*Marsgray,0) 

    window.IMShow(MarsImg)
    window.WaitKey(0)

    windowB.IMShow(Marsgray)
    windowB.WaitKey(0)
}