U.S. astronomers say they've used radar to peer through the choking, toxic cloud atmosphere of Venus to reveal the secrets of its surface features far below.

Visually, Venus just appears as a featureless sphere, covered in a thick atmosphere of clouds of carbon dioxide that obscures the view of the planet's surface.

In a new radar image from the National Radio Astronomy Observatory, details of the face of Venus including dramatic features like volcanoes and craters can be seen.

The new image was generated using two Earth-bound instruments, the National Science Foundation's Green Bank Telescope, a radio telescope in West Virginia, and the powerful radar transmitter of the foundation's Arecibo Observatory in Puerto Rico.

"The radar signals from Arecibo passed through both our planet's atmosphere and the atmosphere of Venus, where they hit the surface and bounced back to be received by the GBT in a process known as bistatic radar," the NRAO explained in a release.

Although Venus's surface has been mapped by radar before, by spacecraft such as NASA's Magellan probe, using radar telescopes on Earth allows astronomer to study a number of images taken over time.

A series of images has been gathered from 1988, 1999, 2001 and most recently from 2012, allowing a comparison that could help find and track changes in the planet's surface, perhaps due to volcanic activity, they say.

It's not an easy process, they acknowledge.

"It is painstaking to compare radar images to search for evidence of change, but the work is ongoing," says Bruce Campbell, senior scientist with the Center for Earth and Planetary Studies at the Smithsonian's National Air and Space Museum in Washington, D.C.

"In the meantime, combining images from this and an earlier observing period is yielding a wealth of insight about other processes that alter the surface of Venus."

Scientists don't yet know what geological processes might be active on Venus today, although a number of volcanoes show evidence of having been active in the last couple of million years, fairly recent in geological terms.

The 100-meter Green Bank Telescope, which received the reflected Arecibo signals, is the world's largest fully steerable radio telescope.

It is located in what has been dubbed the National Radio Quiet Zone and the West Virginia Radio Astronomy Zone to protect the incredibly sensitive instrument from undesirable radio interference, allowing it to make unique observations, the NRAO says.