Gravitational Anomalies of Mercury . ... Credit Copyright: NASA , GSFC 's SVS , JHU's APL , Carnegie Inst. Washington Explanation: What's that under the surface of Mercury? The robotic MESSENGER spacecraft that had been orbiting planet Mercury for the past four years had been transmitting its data back to Earth with radio waves of very precise energy. ... Red hues indicate areas of slightly higher gravity, which in turn indicates areas that must have unusually dense matter under the surface. ...