Saturn has a rocky surface, but it's deep beneath the clouds. That makes it hard to tell exactly how long the day is. NASA hide caption

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Friday
NASA astronaut Scott Kelly is seen inside a Soyuz simulator at the Gagarin Cosmonaut Training Center on March 4 in Star City, Russia. Kelly, along with Russian cosmonaut Mikhail Kornienko of the Russian Federal Space Agency, are scheduled for launch Friday aboard a Soyuz TMA-16M spacecraft from the Baikonur Cosmodrome in Kazakhstan. NASA/Bill Ingalls hide caption
Wednesday
Cuban artist Tania Bruguera poses for a photograph near the statue of José Martí in Havana's Revolution Plaza. She was arrested in December for planning a political performance there. Eyder Peralta/NPR hide caption
In Havana, A Journey Into The Forbidden With A Provocative Artist
Water molecules between two layers of graphene arranged themselves in a lattice of squares — unlike any other known form of ice. NPG Press via YouTube hide caption
Sunday
This booking photo provided by the Orleans Parish Sheriff's Office shows Robert Durst, after his arrest Saturday in New Orleans on an extradition warrant to Los Angeles. AP hide caption
Friday
Ferguson Mayor James Knowles announces the resignation of police Chief Thomas Jackson during a news conference on Wednesday in Ferguson, Mo. Jeff Roberson/AP hide caption
Ferguson Mayor Knowles Slams 'Hostile Language' From Eric Holder
Thursday
The moon Ganymede (right) orbits the giant planet Jupiter in this artist's rendering. Scientists say a saline ocean lurks under the moon's icy crust. NASA/ESA hide caption
Terry Pratchett at the 2012 South Bank Sky Arts Awards in London. Ian Gavan/Getty Images hide caption
Police in riot gear respond to demonstrators blocking traffic during a protest Wednesday outside the Ferguson Police Department in Missouri. As the protests were ending, someone fired at the police and wounded two officers. Michael B. Thomas/Getty Images hide caption
Friday
Astronomers have known about Ceres for centuries, but they don't really know what to make of it. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA hide caption
Thursday
Fans are celebrating World Book Day on Thursday. Here, a man browses through books at the Albertine, a French bookstore and library at the French Embassy in New York. Jewel Samad/AFP/Getty Images hide caption
Wednesday
Misao Okawa, the world's oldest living person, poses for a photo with her son Hiroshi Okawa, 92, (left) and other family members and friends on her 117th birthday celebration at Kurenai Nursing Home in Osaka, Japan. Buddhika Weerasinghe/Getty Images hide caption
Protesters gather in front of the Los Angeles Police Department Headquarters Tuesday, to express anger over the fatal shooting of an unarmed homeless man Sunday. David McNew/Getty Images hide caption
In this courtroom sketch, Boston Marathon bombing suspect Dzhokhar Tsarnaev (third from right) is depicted with his lawyers and U.S. District Judge George O'Toole Jr., as O'Toole addresses a pool of potential jurors. The trial begins Wednesday. Michael Dwyer/AP hide caption
Tuesday
Former CIA Director and retired four-star Gen. David Petraeus speaks at the University of Southern California on March 26, 2013, his first public speech after resigning as CIA director. Kevork Djansezian/Getty Images hide caption