PARIS — The Mona Lisa, Leonardo da Vinci's portrait of the unknown woman with the enigmatic smile, is sparking a new kind of mystery: What is causing the Renaissance masterpiece to deteriorate so quickly?

The thin, poplar panel on which the Mona Lisa is painted in oil has changed shape since conservation experts last evaluated it, the Louvre Museum said. Leonardo's masterwork — now nearly 500 years old — is inspected every one to two years.

The Louvre said the "state of conservation" of the most famous artwork in France's most famous museum "has aroused some worry."

The state-run Center for Research and Restoration of Museums of France will conduct a study to better determine what materials the painting is made of and evaluate its vulnerability to temperature changes.

Some seasoned visitors say they have noticed changes.

"We lived in Paris in 1962 and 1963 — she seemed brighter back then," said Enid Kushner, 74, a retired lawyer from Cleveland.

First-time visitor Kristy Vander Ploeg, 23, of Toronto, said: "I didn't expect it to look like that — it's a lot more faded than I thought it would be."

The painting has yellowed from layers of varnish applied over the centuries, but the Louvre has resisted pressure to touch it up. The last real work on the Mona Lisa dates to the mid-1950s, when experts removed several age spots.

The Louvre says the most recent analysis can be done without taking the painting out of the public eye. The Mona Lisa now has its own wall; next year it will get its own room.

For the museum's crown jewel, little is left to chance: The painting is housed in an air-conditioned glass case, and visitors are held back by a waist-high barrier.

The Mona Lisa is seen each year by nearly 6 million people. Just last year, it was on the cover of Dan Brown's best seller, "The Da Vinci Code," and figured in the plot.

On Monday, as usual, rubbernecking tourists peered at the painting, and camera flashes went off with the speed and frequency of a Paris fashion show.

Louvre spokeswoman Aggy Lerolle said the flash photography — which is "theoretically forbidden" at the museum — is not believed to be the cause of the painting's deterioration.

But some visitors, after hearing about the Mona Lisa's woes, felt a little guilty for using their flashes.

"It's our fault, I know," said Mikhail Kouzmenko, a Moscow executive, after a friend snapped him, smiling, in front of the painting. "It's bad for the picture, I know."

Most days, security guards and ushers keep the flow of tourists to a regular pace. Often lines to see the Mona Lisa stretch for dozens of yards.