ESA ENVISAT Image: Straits of Gibralter

ESA's Envisat radar image shows internal waves in the Strait of Gibraltar, between the southern coast of Spain (top) and the northern coast of Morocco.

The strait is the only connection between the Atlantic Ocean (left) and the Mediterranean Sea (right), making it a place of intense study in order to understand the exchange of water between the two.

As water flows into and out of the Mediterranean, two currents are formed in the strait.

An upper layer of Atlantic water flows eastward into the sea over a lower layer of saltier and heavier Mediterranean water flowing westward into the ocean.

The lower current is called the Mediterranean Outflow water. As it leaves the Mediterranean near the strait’s western end, it flows over a sudden rise in the sea floor, generating a series of internal waves.

Internal waves are not directly visible to the observer because they do not result in large undulations on the sea surface; instead, they induce a horizontal surface current, which changes the surface roughness of the sea.

From space, internal waves can be detected very efficiently using Synthetic Aperture Radar (SAR) instruments that are sensitive to changes in the small-scale surface roughness on the ocean surface.

Internal waves in this image show up as a semi-circular rippled pattern east of the strait’s entrance in the Mediterranean Sea. Additional sets of internal waves generated in the Atlantic Ocean are visible as dark pink lines on the western side of the strait.

With hundreds of vessels passing through daily, the strait is one of the busiest shipping routes in the world. The main shipping lanes can be seen through the concentration of ships (colourful points) in distinct channels.

Several cities are visible in Spain (all as patches of light green), including the British oversees territory of Gibraltar (semi-circle at the entrance to the Strait of Gibraltar), Malaga (on the coast about 100 km east of Gibraltar), Granada (northeast of Malaga) and Seville (left of centre top). The Spanish autonomous city of Ceuta is on the coast in North Africa, directly across from Gibraltar.

NASA Radar Finds Ice Deposits at Moon's North Pole

NASA Radar Finds Ice Deposits at Moon's North Pole

Using data from a NASA radar that flew aboard India's Chandrayaan-1 spacecraft, scientists have detected ice deposits near the moon's north pole.

NASA's Mini-SAR instrument, a lightweight, synthetic aperture radar, found more than 40 small craters with water ice.

The craters range in size from 1 to 9 miles (2 to15 km) in diameter. Although the total amount of ice depends on its thickness in each crater, it's estimated there could be at least 1.3 trillion pounds (600 million metric tons) of water ice.

The Mini-SAR has imaged many of the permanently shadowed regions that exist at both poles of the Moons. These dark areas are extremely cold and it has been hypothesized that volatile material, including water ice, could be present in quantity here. The main science object of the Mini-SAR experiment is to map and characterize any deposits that exist.

Mini-SAR is a lightweight (less than 10 kg) imaging radar. It uses the polarisation properties of reflected radio waves to characterise surface properties. Mini-SAR sends pulses of radar that are left-circular polarised.

Typical planetary surfaces reverse the polarisation during the reflection of radio waves, so that normal echoes from Mini-SAR are right circular polarised. The ratio of received power in the same sense transmitted (left circular) to the opposite sense (right circular) is called the circular polarisation ratio (CPR).

Most of the Moon has low CPR, meaning that the reversal of polarisation is the norm, but some targets have high CPR. These include very rough, fresh surfaces (such as a young, fresh crater) and ice, which is transparent to radio energy and multiply scatters the pulses, leading to an enhancement in same sense reflections and hence, high CPR.

CPR is not uniquely diagnostic of either roughness or ice; the science team must take into account the environment of the occurrences of high CPR signal to interpret its cause.