DM WASHINGTON -  At first glance, it looks like the tracks left by a flock of birds.

However, in fact this incredible image reveals a series of strange shaped craters on Mars.

It shows a surface covered in ‘secondary craters’ - but astronomers are mystified by how they developed their raised appearance.

Nasa’s Mars Reconnaissance Orbiter (MRO) captured the unique region of Mars, sprayed with secondary craters from 10-kilometer Zunil Crater to the northwest. ‘Secondary craters form from rocks ejected at high speed from the primary crater, which then impact the ground at sufficiently high speed to make huge numbers of much smaller craters over a large region,’ Nasa said.

‘In this scene, however, the secondary crater ejecta has an unusual raised-relief appearance like bas-relief sculpture.’

Nasa admits it does not have a conclusive answer to the puzzle.

‘One idea is that the region was covered with a layer of fine-grained materials like dust or pyroclastics about 1 to 2 meters thick when the Zunil impact occurred (about a million years ago), and the ejecta served to harden or otherwise protect the fine-grained layer from later erosion by the wind,’ it said. It comes just a week after scientists said they were investigating a bizarre scaly feature on the surface of Mars that could be the site of a past impact. The circular structure was discovered on the South Polar layered deposits, and a stunning new image from the Mars Reconnaissance Orbiter’s HiRISE instrument reveals a close look at its strangely reptilian appearance.

While it’s thought to be an impact crater, the icy terrain experiences processes that drastically alter the form of surface features such as these, causing uncertainty about its true origin.

The size and frequency of impact craters on Mars can help scientist paint a clearer picture of a landscape’s age, according to Nasa.

But, as they flatten and morph over time, it’s sometimes difficult to ascertain an impact origin.

This is the case with the strange scaly indent captured by the HiRISE instrument.

An image of the feature projected at a scale of 50 centimeters (19.7 inches) per pixel shows how it dwarfs all the other pock-marks that dot the landscape.

It comes just days after Nasa revealed the Mars Reconnaissance Orbiter has completed 50,000 orbits around the red planet since 2006. 

The space agency released a stunning animation that shows how the observations have accumulated over the last 11 years. The mosaic shows the increasing coverage at just one frame per month, revealing how the orbiter’s Context Camera pieced together more than 99 percent of the Martian surface. 

The orbiter’s Context Camera (CTX) ‘exploits a sweet spot in the balance between resolution and image file size,’ according to Nasa.

It achieves a resolution of roughly 20 feet (6 meters) per pixel, and in the years since it switched on, it’s imaged 99.1 percent of Mars – about the equivalent of the land area of Earth.

According to the space agency, no other camera deployed to the red planet has captured such detailed observations as this.

Over about a decade, the camera has taken roughly 90,000 images, with each one covering a swath of ground roughly 18.6 miles wide.

Despite the distance, it can capture features as smaller than a tennis court.

The camera has made a few return trips, observing 60.4 percent of the planet more than once to help scientists better understand the surface, and learn more about future landing sites.

‘Reaching 99.1 percent coverage has been tricky because a number of factors, including weather conditions, coordination with other instruments, downlink limitations, and orbital constraints, tend to limit where we can image and when,’ said Michael Malin, the Context Camera Team Leader, from Malin Space Science Systems, San Diego.

‘Single coverage provides a baseline we can use for comparison with future observations, as we look for changes.

‘Re-imaging areas serves two functions: looking for changes and acquiring stereoscopic views from which we can make topographic maps.’

The Context Camera has observed dramatic changes on the surface, including more than 200 observations of fresh impact craters, which have allowed scientists to calculate the rate of collisions from space rocks.

And, some of these craters have revealed material thought to be water ice.

The Mars Reconnaissance Orbiter was launched August 12,2005, and began capturing images after reaching orbit in 2006.

It eventually revised its orbit after several months, using the friction with Mars’ upper atmosphere.

Its latest adjustment took place just last week, using a 45.1-second burn of six intermediate-sized rocket engines to shift the orbit so the craft can be in the right place to receive transmissions from Nasa’s InSight Mars lander, as it descends to the surface on Nov 26, 2018.

‘After 11 and a half years in flight, the spacecraft is healthy and remains fully functional,’ said MRO Project Manager Dan Johnston at Nasa’s Jet Propulsion Laboratory.

‘It’s a marvellous vehicle that we expect will service the Mars Exploration Program and Mars science for many more years to come.’