Image copyright NASA/UCLA/MPS/DLR/IDA/PSI/LPI Image caption The bright spots in Occator Crater contain magnesium sulphate

The US space agency's Dawn satellite continues to return remarkable images from the dwarf planet Ceres.

Now just 385km above the surface (lower than the space station is above Earth), the probe has revealed new features inside the mini-world's Occator Crater.

This is the 92km-wide depression that has multiple bright spots of what are thought to be exposed salts.

The new imagery reveals a dome in a smooth-walled pit in the centre-most bright area of the crater.

With a resolution now of 35m per pixel, Dawn can make out numerous fractures that cut across the top and down the flanks of this dome.

Very prominent cracks also surround the dome and run over the crater floor, extending to the other bright spots in Occator.

Image copyright NASA/UCLA/MPS/DLR/IDA/PSI/LPI Image caption The dome is clearer in this enhanced colour view of Occator Crater

"Before Dawn began its intensive observations of Ceres last year, Occator Crater looked to be one large bright area. Now, with the latest close views, we can see complex features that provide new mysteries to investigate," said Ralf Jaumann, planetary scientist and Dawn co-investigator at the German space agency (Germany provided Dawn's cameras).

"The intricate geometry of the crater interior suggests geologic activity in the recent past, but we will need to complete detailed geologic mapping of the crater in order to test hypotheses for its formation."

Scientists think the bright spots are deposits of epsom salts (magnesium sulphate), the trace remains of briny water-ice that at one time became exposed on the surface.

Image copyright NASA/JPL-Caltech/UCLA/ASI/INAF Image caption Low neutron detection (blue) indicates more hydrogen at higher latitudes; the equator has a higher neutron count (red), which suggests less ice is buried in this region

With no atmosphere on the dwarf planet, the water content would have rapidly vaporised, leaving only the magnesium sulphate spots.

Ceres likely has quite a lot of buried water-ice.

This idea is being investigated by the satellite's GRaND instrument, which senses neutrons and gamma rays produced by cosmic ray interactions with surface materials. It is a means to understand the chemistry of the top metre or so of Ceres' rocky "soil".

Image copyright NASA/UCLA/MPS/DLR/IDA/PSI/LPI Image caption Dawn is now closer to Ceres than the space station is to Earth

Already, GRaND is detecting larger concentrations of hydrogen in this layer, at higher latitudes. Being a key component of water, the hydrogen signature could very well indicate the presence of shallow-buried ices.

Scientists reported the latest results from the Dawn mission at the Lunar and Planetary Science Conference in The Woodlands, Texas.

It is a year since the Nasa spacecraft went into orbit around the 950km-wide world - the largest object in the asteroid belt between Mars and Jupiter.