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European Space Agency

Press Release

Giants joust in the cold
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18 October 2002

A new giant was born recently in the coastal waters of Antarctica. A
series of images captured from May through the beginning of this month 
by ESA's Envisat satellite shows the subsequent duel between the new 
iceberg and another as it breaks free of the Ross Ice Shelf and tries 
to move north. 

ESA1IUTHN6D
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Christened C-19 by the US National Ice Centre in Maryland, the new
iceberg measured 200 x 32 km, and about 200 m thick.

As seen in the accompanying animation of images acquired by Envisat's
Advanced Synthetic Aperture Radar (ASAR) from 3 May through 7 October, 
C-19 jostles for position with an older iceberg, B-15a, resting 
aground in shallow water off Ross Island. C-19 scrapes along the side 
of B-15a and, failing to move its firmly anchored adversary, finally 
heads off into open sea and an eventual demise in warmer waters.

Iceberg "calving"
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Several different processes are important in causing an iceberg to
form, or "calve" as it is conventionally known, according to Dr David
Vaughan, principal investigator of the physical sciences division of
the British Antarctic Survey, based in Cambridge, UK.

"They can form as a result of the action of wind and waves, or simply
because the ice shelf has grown too large to support part of itself,"
Vaughan explained. "Once in a while, an older iceberg collides into an 
ice shelf and breaks off a new iceberg."

The ASAR imagery also clearly outlines that the C-19 berg is not
moving through open water. The white swirls captured in the image
represent sea ice, offering more resistance to the movement of the
iceberg, but winds and ocean currents finally prove too strong to
overcome. Also evident in the radar image is the difference between
Antarctic ice that is resting on land or water. The Ross Ice Shelf,
for instance, is seen as a smooth surface. To the left-hand side of
the images, however, the radar image shows the rougher terrain of
Antarctic ice resting on land.
 

Part of the normal Antarctic ice cycle
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Such iceberg calving like this one occurs in Antarctica each year and
is part of the natural lifecycle of the ice sheet. Scientists are
eager to understand if the mass of ice lost in such events is balanced 
by new snowfall accumulating on the continent. Any imbalance would 
imply a change in world sea level.

Since C-19 was already floating before it calved, however, it will not 
cause any rise in world sea level, according to the British ice
expert. In addition, since this ice shelf has shown no progressive
retreat in recent years, scientists are expected to view this event as 
part of the natural lifecycle of the Ross Ice Shelf. 

"However, if similar events continue to occur then we may begin to
believe that this is a result of climate change," Vaughan cautioned.
"For the moment, the jury is still out."

Envisat's ASAR sensor has several key advantages over optical sensors: 
it can penetrate cloud cover, which is especially useful in studying 
polar regions, and can capture imagery at night. ASAR is the first 
permanent spaceborne radar to incorporate dual-polarisation
capabilities - the instrument can transmit and receive signals in
either horizontal or vertical polarisation. This Alternating
Polarisation (AP) mode can improve the capability of a SAR instrument
to classify different types of terrain. The sensor also can record
imagery on-board the spacecraft and the images can be made available a 
few hours after acquisition. 

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