Mars
What happened to Mars' atmosphere?Nasa

Nasa is set to make another big announcement about Mars, with the space agency remaining coy about details of what it has found. It is likely scientists have had a breakthrough in working out what happened to the red planet's atmosphere.

The announcement will be broadcast live online, and you can watch a livestream of the event here. In a statement, Nasa said it will be providing "details of key science findings from the agency's ongoing exploration of Mars". It will begin at 7pm GMT, 2pm ET.

It is inviting members of the public to submit their questions during the press briefing via social media, using the hashtag #AskNASA. Participating scientists include Michael Meyer, from the Mars Exploration Programme, Bruce Jakosky, principal investigator on Mars Atmosphere and Volatile Evolution (MAVEN), along with Jasper Halekas, Yaxue Dong and Dave Brain, all from the MAVEN team.

Mars
Dark narrow streaks called recurring slope lineae on the walls of Garni crater on Mars. The streaks are up to few hundred metres in length, and thought to be formed by flow of briny liquid waterNasa/JPL/University of Arizona

The mystery announcement follows a similarly cryptic message it sent out before revealing the possible presence of liquid water on Mars. The team published their findings in the journal Nature Geoscience and showed how the planet appears to have water flowing intermittently down the slopes of a mountain during warm seasons. It should be noted they did not find liquid water itself.

How Mars lost its atmosphere has been a long-held mystery. Scientists know it once was a world with a vast ocean of surface water and an atmosphere, but over time it became the freezing and bone dry planet we see today. In 2009, Jakosky said: "The goal of MAVEN is to figure out what processes were responsible for those changes in the climate."