Admittedly, your image of May 8, 2010, recently posted on APOD, is a dramatic one. It is hard to imagine our tire tracks across a desert on the surface of another world. It is a truly amazing achievement for which you are to be congratulated. That said, we do not find the image in its present condition suitable for publication. If you want to correct the following technical errors and resubmit it we will be happy to reconsider our decision.
Tilted Horizon Line
The most obvious error and most objectionable is the tilted horizon line. You should turn on the grid display to avoid this in the future. That said, your image could still be corrected using the Crop & Straighten tool in Lightroom.
The second problem with your image is the noticeable vignetting. In certain circumstances, vignetting can be a useful artistic technique. Not so here. This can be corrected in Photoshop, but consider using a different lens next time.
This image has a wide dynamic range, from nearly pure white to total black. In this type of situation, you should always consider making an HDR image. There is plenty of information on the Internet about this technique. Otherwise, use the Adjustment Brush in Lightroom to dodge and burn the horizon line.
Black and White
Frankly, these days black and white is only suitable for “art” photography, which this is certainly not. Most of the images of this sort, which appear in National Geographic, are in color. Besides that, your choice deprives the viewer of the subtle reds of the Martian landscape. Please consider shooting in color all of the time.