You might not have heard of Nasa’s latest Mars mission. Unlike the hoopla that surrounded the arrival and landing of their 2012 Curiosity Rover, Maven has slipped into orbit relatively quietly. It will not land on the surface but, arguably, is the more important of the two missions.

If it achieves its scientific goals it could allow scientists to make full sense of Curiosity’s individual discoveries.

Maven stands for Mars Atmosphere and Volatile EvolutioN (you have to pinch the ‘n’ off the end of “evolution” to make the acronym). Its goal is to investigate the upper reaches of Mars’s atmosphere in an attempt to understand how much water has been lost into space.

Technicians work on the Maven spacecraft. Photograph: John Raoux/AP

It has been 40 years since Nasa collected its first incontrovertible evidence that water once flowed on the red planet.

Mariner 9 entered orbit on 14 November 1971, becoming the first spacecraft to orbit another planet. During the next 349 days it photographed 85% of the planet’s surface, revealing meandering channels that were clearly dried-up rivers. The pictures also showed that the planet today is a cold, dry desert.

Thus began the great mystery of what happened to the water on Mars. There are two possibilities. Either the water seeped down into the interior, or it was driven into space.

To find subsurface water, Esa’s Mars Express and Nasa’s Mars Reconnaissance Orbiter were designed with ground-penetrating radars, Marsis and Sharad. Marsis looked down to around five kilometres, while Sharad scoured the top kilometre. Unfortunately, neither instrument found any evidence of a planet-wide subsurface water system.

The Echus Chasma pictured by Esa’s Mars Express in 2005. Gigantic waterfalls may once have plunged over these cliffs on to the valley floor. Photograph: ESA/Getty Images

Either the water is much deeper than expected, or it escaped into space. Maven will investigate this second possibility by studying the way the upper atmosphere of Mars interacts with particles thrown from the sun. Known as the solar wind, this constant stream erodes Mars’s atmosphere.

Mars Express’s Aspera 3 instrument has already shown that the solar wind penetrates very deeply into the Martian atmosphere, and could be responsible for stripping away most of the water during the past 3.8bn years. Maven will confirm and extend these studies.

A full moon rising behind the Atlas V rocket that launched Maven from Cape Canaveral, Florida, on 18 November 2013. Photograph: Nasa/Bill Ingalls/EPA

Mars is becoming ever busier with spacecraft traffic. There are already three spacecraft working there, two American (Mars Reconnaissance Orbiter and Mars Odyssey) and one European (Mars Express), and later this week India’s Mars Orbiter Mission (known informally as Mangalyaan) mission will aim to join them.

Launched in November 2013, Mangalyaan is India’s maiden voyage to Mars. On 13 September, it was said to be “in the pink of health”. The engine burn to place it into orbit will take place on 24 September.

The mission is a technology demonstrator but does carry some scientific instruments, including a camera. One instrument is designed to estimate the amount of water Mars has lost to space.



The Curiosity rover’s alpha particle X-ray spectrometer, which measures the abundance of chemical elements in rock and soil. Photograph: Handout/Reuters

While on the ground Curiosity continues to scout for local evidence of past and present water, Maven’s investigation, along with those of Mangalyaan, will allow these to be placed in their correct global context. Together, these missions could reveal the entire planet’s geological history.

Stuart Clark is the author of Is There Life on Mars? (Quercus Books)