/// <summary>
 /// The main entry point for the application.
 /// </summary>
 static void Main(string[] args)
 {
     using (Game1 game = new Game1())
     {
         game.Run();
     }
 }
        MarsTerrain terrain; //added

        #endregion Fields

        #region Constructors

        public Game1()
        {
            instance = this;
            graphics = new GraphicsDeviceManager(this);

            graphics.PreferredBackBufferWidth = 1024;
            graphics.PreferredBackBufferHeight = 768;
            graphics.PreferMultiSampling = true;
            graphics.SynchronizeWithVerticalRetrace = true;
            graphics.ApplyChanges();

            Content.RootDirectory = "Content";
        }