ROME — The Venice Film Festival marks its 70th edition with films starring George Clooney and Sandra Bullock as astronauts adrift in space, Scarlett Johansson as a seductive alien roaming the Scottish countryside and Judi Dench as a single Roman Catholic woman searching for a son she was forced to give up decades before.

Festival director Alberto Barbera concedes that many of the films in the lineup announced Thursday are bleak in their outlook.

"Perhaps one of the strongest themes that emerges from all the films is the lack of prospects. There's an acknowledgment of an extremely problematic situation, in some cases tragic," Barbera told a news conference.

"Cinema mirrors reality, so we can't complain if auteur films today gives us an image of our times that is not consoling."

Twenty films will vie for the coveted Golden Lion at the world's oldest film festival, which opens Aug. 28, with the jury headed by Oscar-winning director Bernardo Bertolucci.

Golden Lion contenders include Terry Gilliam's "The Zero Theorem," starring Christoph Waltz as a reclusive computer genius, and Peter Landesman's "Parkland," which examines the chaotic events at Dallas' Parkland hospital the day John F. Kennedy was assassinated. Indie director Kelly Reichardt returns to Venice with "Night Moves," starring Jesse Eisenberg, Dakota Fanning and Peter Sarsgaard as environmental activists plotting to blow up a dam.

Johansson appears in Jonathan Glazer's highly anticipated science fiction thriller "Under the Skin," while Dench stars in Stephen Frears' "Philomena."

Director Alfonso Cuaron's techno-thriller "Gravity," starring Clooney and Bullock, will open the festival and is among 17 films showing out of competition. Thierry Ragobert's documentary "Amazonia" closes the event on Sept. 7.

The lineup is strong on independent, auteur films and light on blockbusters, a delicate point for organizers competing with Cannes, which shows in the spring, and Toronto, which overlaps with Venice.

"Venice on paper takes more risks — for example, taking two documentaries in competition, a first by any festival, and organizing an auteur cinema to support and promote these films," said Barbera.

Barbera, who is in the second year of a new mandate after directing the festival from 1998-2002, is intent on securing the festival's role as a launch pad for new films. Venice has premiered such films as the "The Hurt Locker," which went on to win an Academy Award for best film, and the highly acclaimed "Black Swan."

The festival will showcase 53 new feature films, all but two world premieres, with 33 countries represented in the overall selection, which also includes sections on new trends and short films. Of the 20 films competing for the Golden Lion, five are American, four British and three Italian.

The lineup includes films by three Oscar-winning directors: Hayao Miyazaki with an animated film about a Japanese fighter plane designer, Errol Morris with a documentary about former U.S. Defense Secretary Donald Rumsfeld, and Poland's Andrzej Wajda with a movie depicting the life of Solidarity leader Lech Walesa. The first two are in competition.

The other documentary in competition is Gianfranco Rosi's "Sacro GRA," about life on the highway that rings Rome.

Three previous Golden Lion winners are returning to the Lido. They include Italy's Gianni Amelio with "L'intrepido," or "The Intrepid," and Taiwan-based director Tsai Ming-liang with "Stray Dogs," both in competition. Last year's winner, Kim Ki-duk, premieres "Moebius," his follow-up to "Pieta" that has generated controversy at home in South Korea for its graphic content.