The particles are being accelerated away by interactions with the solar wind, a constant stream of electrically charged particles released by the Sun. The solar wind in earth does not directly interact with the atmosphere. It is diverted by Earth's natural cloak of magnetism. Neither Mars nor Venus have appreciable magnetic fields generated inside the planet, so each planet's atmosphere suffers the full impact of the solar wind. Interestingly, this full-on interaction does create a weak magnetic field that drapes itself around each planet and stretches out behind the night-side in a long tail, Science Daily reported. The atmosphere of Venus is thick and dense, whereas that of Mars is light and tenuous. Despite the differences, the magnetometer instruments have discovered that the structure of the magnetic fields of both planets are alike.
The proximity of Venus to the Sun, however does create an important difference, however. The solar wind thins out as it moves through space so the closer to the Sun it is encountered, the more concentrated is its force.
This creates a stronger magnetic field, making the escaping atmospheric particles move collectively like a fluid.
There are also illuminating difference between Mars and Venus.
The former displays strong small-scale magnetic fields locked into the crust of the planet. In some regions, these pockets protect the atmosphere, in others they actually help funnel the atmosphere into space.