Mars was explored like never before in 2004. NASA landed two rovers -- Opportunity and Spirit -- on the red planet, while the European Space Agency placed Mars Express in orbit around our near neighbour. Only the Beagle 2 lander failed to overcome the curse that has plagued so many Mars missions in the past. All three missions found evidence for ancient water on the planet.

Images from Opportunity revealed numerous layered outcrops, similar to sedimentary rocks on Earth, suggesting that liquid water had once flowed through the rocks. Meanwhile, on the opposite side of the planet, Spirit found signs that water may have altered volcanic rocks at Gusev crater. Moreover, the OMEGA instrument on board Mars Express obtained the first direct evidence for frozen water on the surface of Mars by analysing the near infrared spectrum of sunlight reflected from the planet.

Highlights in low-temperature physics included the creation of a new "supersolid" phase of matter by Eun-Seong Kim and Moses Chan of Pennsylvania State University. Supersolid helium-4 behaves like a superfluid -- a liquid that flows without resistance -- but has all the characteristics of a crystalline solid.

Elsewhere in the low-temperature world physicists at the JILA laboratory in Colorado created the first "fermionic condensate" when they persuaded fermionic atoms -- which obey the Pauli exclusion principle -- to form bosonic pairs, which don't. This allowed the atoms to "condense" into the same quantum ground state. The ultimate goal of such research is to gain a better understanding of superfluidity and superconductivity, and physicists at the University of Innsbruck in Austria took a major step forward in this direction when they observed the equivalent of a superconducting energy gap in a low-temperature gas for the first time.

Meanwhile astronomers at the Parkes telescope in Australia and the Lovell telescope in the UK discovered the first ever double pulsar system. Pulsars are extremely dense, rapidly rotating neutron stars that are a million times more massive than the Earth, yet measure just tens of kilometres across. The discovery will allow physicists to perform the most stringent ever tests of Einstein's general theory of relativity.

PhysicsWeb will publish its highlights of 2004 next week.