Christmas Day 2018 marks the 15th anniversary of ESA's Mars Express orbiter arriving at the Red Planet. To mark the occasion the space agency has released a suitably festive image of a crater that seems filled with fresh snow. In fact, the Korolev Crater is filled with perpetually frozen ice due to a remarkable meteorological phenomenon.

Launched on June 2, 2003 atop a Soyuz rocket from Baikonur Cosmodrome in Kazakhstan, Mars Express arrived in orbit around Mars on December 25 when it began its science mission. The first planetary mission conducted by ESA, its job was to deliver the ill-fated British Beagle 2 lander and to make a high-resolution imaging and mineralogical survey of the surface, conduct subsurface radar probes, and study the Martian atmosphere.

One feature that stuck out during its photography work was Korolev Crater. Captured by the High Resolution Stereo Camera (HRSC), it was photographed in five strips as the orbiter made successive passes and included perspective, context, and topographic views.

Korolev Crater is located in the northern lowlands of Mars to the immediate south of the Olympia Undae dune fields. It's 82 km (51 mi) wide and plunges 2 km (1.2 mi) at its deepest. But what makes it interesting is that this perfectly preserved meteor crater is perpetually filled with ice to a depth of up to 1.8 km (1.1 mi) and a volume of 2,200 cu km (530 cu mi).

This is because Korolev is a cold trap. That is, the winds blowing across the ice is cooled and then sinks into the crater, forming a freezing layer of air that both chills and insulates the ice. The result is a frozen filling that never thaws.

First discovered during the NASA Mariner missions and named in 1973, Korolev Crater is named after Sergei Korolev, the "Chief Designer" and father of soviet rocketry. Though unknown even to the Russian people due to official secrecy, he was responsible for a remarkable string of space firsts, including the first artificial satellite, the first living creature in orbit, the first man in space, the first woman in space, the first space walk, and many others.

Source: ESA

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