text: It has spent four years orbiting the closest planet to our sun, but despite desperate attempts to extend the life of Messenger spacecraft, it is slowly spiraling towards Mercury's scorched surface. Yet even as its mission draws to an end, the final weeks of Nasa's Messenger spacecraft are revealing the planet in unrivaled detail. Messenger is currently 18 miles (29.1km) above Mercury after Nasa engineers used the last of its fuel in a last ditch effort to push the spacecraft higher into orbit after it dropped to within nine miles (15km) of the surface. Among the images sent back by Messenger as its mission draws to an end is this picture of 2km high cliffs cutting through the Carnegie crater. They are thought to have formed as Mercury has cooled and contracted . The spacecraft, however, is expected to smash into the planet's surface on 30 April. But as messenger drops ever closer to Mercury's surface, it is sending back some of the most detailed images ever obtained of the smallest planet in our solar system. Researchers have uncovered the mystery of Mercury's 'stealth' colour scheme. The say a steady dusting of carbon from passing comets has slowly painted Mercury black over billions of years. It solves the mystery of Mercury's unusual colouring - which has baffled scientists. On average, Mercury is much darker than its closest airless neighbour, our Moon. Airless bodies are known to be darkened by micrometeorite impacts and bombardment of solar wind, processes that create a thin coating of dark iron nanoparticles on the surface. But spectral data from Mercury suggests its surface contains very little nanophase iron, certainly not enough to account for its dim appearance. As comets approach Mercury's neighborhood near the sun, they often start to break apart. Cometary dust is composed of as much as 25 percent carbon by weight, so Mercury would be exposed to a steady bombardment of carbon from these crumbling comets. Scientists have calculated that after billions of years of bombardment, Mercury's surface should be anywhere from three to six percent carbon. Among the features it has revealed to scientists in the past month are patterns of distinctive hollows - each around a couple of hundred metres wide - in the bottom of a huge impact basin. It has also sent back detailed images of huge 1.2 mile (2km) high cliffs that cut across a crater named Duccio on the surface. Another image shows a 621 miles (1,000km) long cliff that rises 1.8 miles (3km) above the surface, called Enterprise Rupes. Data sent back by Mercury suggests these cliffs, or lobate scarps as they are called, have formed as Mercury has cooled and contracted over time, creating dramatic tectonic faults on the surface. Another image shows a meteorite crater that has almost completely filled with lava on Mercury's northern volcanic plains, leaving just the hint of a rim behind. It also shows how the contracting surface of the planet has also created strange formations where ridges and cliffs have hit other lava filled craters. Andy Calloway, Messenger Mission operatoins manager at Johns Hopkins Applied Physics Laboratory, said: 'The MESSENGER spacecraft operates in one of the most challenging and demanding space environments in our Solar System, and we have met that challenge directly through innovation and hard work, as exemplified by the stunning discoveries and data return achievements. 'Our only regret is that we have insufficient propellant to operate another 10 years, but we look forward to the incredible science returns planned for the final eight months of the mission.' This image by Messenger shows the distinctive hollows on the floor of the Zeami impact basin on Mercury . This ghostly crater filled with lava oozed from volcanic vents on Mercury leaving just the outline of the rim . This crumpled crater has been deformed after it filled with lava and then ridges formed as the surface cooled . Launched in August 2004, Messenger has traveled more than 4.9 billion miles (7.9 billion kilometers) during its mission to Mercury. The above graphic was released by Nasa to mark ten years since the spacecraft was launched in 2004 . Its journey included 15 trips around the sun before it entered orbit around Mercury in 2011 - the first spacecraft to ever do so. During its mission is has acquired more than 250,000 images and collected more than 10 terabytes of data with its suite of seven instruments. It has helped to reveal volcanic vents that measure up to 15 miles across that were once sources for the large volumes of lava that have covered the surface and carved out valleys. It has also revealed Mercury's complex internal structure and that the planet has an unusually large core that is still partially liquid. On 6 April engineers used the last of the Messenger spacecraft's hydrazine fuel to try to push it into a higher orbit. However, the tanks ran dry before they could raise it to more than 11 miles (18km) above the surface. In an attempt to increase its height further the team then ejected helium gas from the spacecraft through its thrusters to raise it to its currently height. This image shows some of the mysterious irregularly shaped hollows on Mercury's surface in greater detail . They hope to raise it further with another burst of helium on April 14. Karl Whittenburg, deputy mission operations manager at Johns Hopkins Applied Physics Laboratory, said: 'To our knowledge, this is the first-ever use of a pressurant for a planned propulsion of a spacecraft, so we could only theorize how it might perform. 'We are now fully confident that future use of gaseous helium will continue to provide Messenger with a unique vantage point for studying Mercury.' This image shows fine-grained volcanic material that has blown across impact craters almost like snow . This colour image of Mercury was one of the first sent back by Messenger as it approached the planet in 2008 . The above artist's impression shows Messenger as it orbits above the surface of Mercury. It is due to impact on the surface at the end of April as the planet's gravity slowly pulls it down out of its current orbit . Messenger has also revealed the temperature of Mercury's surface. This image of the planet's north pole shows the extreme heat it is subjected to with red indicating temperatures greater than 126°c and blue -220°c .
target: Messenger is expected to impact on the surface of Mercury around April 30 .
  Spacecraft has been orbiting Mercury since 2011, taking 250,000 pictures .
  As it gets lower it is sending back images of the surface in greater detail .
  Engineers have attempted to extend the mission by using up its last fuel .