The European Space Agency's Mars Express spacecraft has discovered that Mars' atmosphere glows during the night. The discovery was made using the spacecraft's SPICAM instrument, which is designed to study the chemicals in Mars' atmosphere and ionosphere. This nightglow occurs because atoms combining to form molecules in the Martian atmosphere release energy in the form of photons. A similar nightglow had been discovered on Venus, and give scientists an insight into the kinds of chemical reactions that are going on, and refining atmospheric circulation models.