8 June, 2012
These pictures, made with only one the ALMA antennas, show Venus shortly before it moved in front of the Sun (the famous Venus transit) a couple of days ago on June 5th. At that stage Venus was not visible through optical telescopes because its dark side was turned towards us. ALMA detects millimeter and submillimeter waves which are given off by the hot atmosphere of Venus (and of course even more strongly by the hot surface of the Sun). The two images which compose this animation were taken about 15 minutes apart and the relative motion of Venus towards the edge of the Sun is easily seen.
ALMA astronomers Tsuyoshi Sawada and Shin’ichiro Asayama made these pictures by instructing the antenna to make multiple scans across the face of the Sun and the region where Venus was expected to be. Actually, the scanning produces the slightly ragged edge on the image of the Sun. They were delighted by the result but just sorry that they were not able to continue making pictures after the transit had begun. Unfortunately by that time the Sun had set in Chile and even ALMA cannot make observations of objects through the Earth!
The brighter patches visible on the solar images are “active regions” – effectively storms on the surface of the Sun which give off stronger millimeter-wave signals than the “quiet” regions. In future, scientists will use ALMA’s full array of antennas to study these regions in far greater detail.