THE BEST ASTRONOMY PHOTOS OF THE YEAR

Star trails are photographed by pointing a camera skywards and waiting. As the Earth rotates, stars produce circular trails.

This image was shot over Yosemite falls in Yosemite National Park, California. The long exposure gives the illusion of daylight, but it's actually the light of a full moon that is creating a rainbow in the mist. The image was constructed from 500 individual photographs shot over several hours.

(Image: Jeffrey Sullivan)

 

The Orion nebula, at the right of the image, is the closest region of massive star formation to Earth and can be seen with the naked eye.

To the left of the image, the bright stars of Orion's belt can also be seen. In addition, the tiny silhouetted Horsehead nebula is just visible against the pink hydrogen clouds just left of centre.

A nebula is an interstellar cloud of dust and gases. Many nebulae form as the interstellar medium collapses under its own weight. As the material shrinks, massive stars form. Ultraviolet radiation emitted by these stars ionises the surrounding gas, making it visible to telescopes back on Earth.

(Image: Fabian Neyer)

This bright streak across the night sky was caused by Japan's Hayabusa spacecraft returning to Earth in June 2010. It was returning from a seven-year mission to visit the asteroid Itokawa. Hayabusa touched down on the surface of the asteroid in 2005 and brought dust samples back to Earth, landing safely in the Australian outback.

Other spacecraft had visited asteroids before, but this was the first time that material from one had been returned to Earth.

(Image: Kouji Ohnishi)

The northern lights, or aurora borealis, are a natural light display caused by highly charged electrons in solar wind interacting with the Earth's atmosphere.

Solar winds stream away from the sun at speeds of around 450 kilometres per second. Upon reaching the Earth, some 40 hours later, they follow the lines of magnetic force generated by the Earth's core and flow through the magnetosphere, a teardrop-shaped area of highly charged electrical and magnetic fields. The electrons in the solar wind then interact with oxygen and nitrogen in the Earth's upper atmosphere, producing spectacular light displays.

In the southern hemisphere, the analogous phenomenon is known as aurora australis.

(Image: Christian Salomonsen)

This image shows the Milky Way rising over the village of Oneroa on the coast of Mangaia in the Cook Islands. It is a spiral galaxy, and our solar system sits in a spiral arm known as the Orion-Cygnus arm.

The Milky Way contains something between 200 and 400 billion stars, and is estimated to have at least 50 billion planets. It takes our sun 225 to 250 million years to complete a full orbit around the centre of the galaxy.

Its name comes from Greek mythology. According to legend, the galaxy is milk spilt by Zeus's wife Hera while suckling the infant Heracles.

(Image: Tung Tezel)

 

 

Perseid meteors have been observed for over 2000 years; they are shed by the comet Swift-Tuttle. Because its orbit takes it close to Earth, astronomer Gerrit Verschuur described Swift-Tuttle as "the single most dangerous object known to humanity".

The showers occur from mid-July onwards each year and are best viewed in the northern hemisphere. Last year, at its peak, the shower produced over 100 meteors an hour.

In the foreground is St Michael's tower, atop Glastonbury Tor, UK, a site famously associated with Arthurian legend.

(Image: Mike-DT6)

 

 

Dark streaks on Mars bolster case for liquid water

Mars's dust bowl image may need a makeover. Dark streaks seen forming in summer and fading in winter might be signs of water flowing just beneath the surface.

The sudden appearance of streaks on sloping ground have been attributed to present-day liquid water before, although their origin is still debated. Light streaks have been seen appearing on steep slopes in images taken years apart by NASA's Mars Global Surveyor spacecraft. And seasonal dark streaks have emerged in the north polar region.

Liquid water flowing downhill might explain both types of events, but dust or sand avalanches could also be to blame.

Now, new images have revealed a previously unknown population of seasonal dark streaks in Mars's southern hemisphere, with characteristics that seem to tie them to liquid water.

Salty antifreeze

Researchers led by Alfred McEwen of the University of Arizona, Tucson, found slopes where dark streaks appear every summer and disappear each winter in images from NASA's Mars Reconnaissance Orbiter (MRO).

They have been seen mainly on slopes in the southern hemisphere at mid-latitudes, though some have been seen near the equator, as well.

Since they appear in the summer when the ground is warmest, one possibility is that they result from ice that melts and drains down slopes. The researchers believe any flowing water lies below the surface because if it were above it, MRO probably would have spotted its spectral signature.

Some of the streaks form when temperatures are only -23 °C, well below the freezing point of pure water. But salty water can remain liquid at such temperatures, possibly explaining the streaks.

Shifting grains

The researchers think salty water flowing just beneath the surface might shift dust grains up above, causing the dark streaks. However, the explanation is not watertight – it is not clear why the streaks would then disappear in colder weather.

"The best explanation we have for these observations so far is flow of briny water, although this study does not prove that," says McEwen.

The seasonal streaks, which the team calls recurring slope lineae, show no preference for dusty areas, where dust avalanches would be more likely.

However, the streaks do occur in the same zone of the southern hemisphere – between 30 and 60 degrees latitude – where there is previous evidence for buried ice, including what appear to be glaciers buried under rock.

Toehold for life?

The ice deposits may have formed when past climate swings deposited snow on the planet's mid-latitudes.

"They've added to the case for subsurface ice and salts in the mid-latitudes, which, not too surprisingly, can melt and flow short distances downhill," says Phil Christensen of Arizona State University in Tempe, who was not involved in the study.

"The correlations with known deposits of snow and ice look very strong," agrees James Head of Brown University in Providence, Rhode Island, who was also not involved in the study.

The discovery of what might be liquid water on present-day Mars raises the possibility that life may have a toehold there. Keeping water liquid at very low temperatures requires very salty water, in which life as we know it would have difficulty surviving, however.

But researchers are not ruling out the possibility. "This is a very exciting discovery because it is our first chance to see an environment on Mars that might allow for the expression of an active biological process if there is present-day life on Mars," says Lisa Pratt of Indiana University in Bloomington, who was not involved in the study.