(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

E.T., the stranded alien botanist from the blockbuster 1982 movie, may have been the most popular extraterrestrial in recent history; but he had no identified home planet.

In contrast, Mars and Martians have captured the imagination of science fiction writers and the public for centuries. The dramatic 1938 radio adaptation of H.G. Wells’ 1898 novel, War of the Worlds, convinced some listeners that an actual alien invasion by Martians was in progress.

Scientists, too, have long speculated about the possibility and current status of life on Mars, owing to the planet’s proximity (one step beyond Earth in orbit around the sun) and similarity to Earth.

For instance, the two planets share similar surface features, such as volcanoes, polar ice caps, mountains, and valley networks. A Martian day is only 40 minutes longer than an Earth day.

Due to a similar tilt on its axis, Mars experiences seasons just as Earth does, although nearly double the length, as the Martian year is nearly twice that of Earth.

Both Earth and Mars have large temperature fluctuations, with Earth’s ranging from -60 to 120 degree Fahrenheit (-50 to 50 degree Celsius) and Mars’ temperature ranging from -220 to 68 degree Fahrenheit (-140 to 20 degree Celsius).

Mars has an atmosphere very different than Earth’s, consisting mainly of carbon dioxide (95 percent). The atmosphere of Earth is mainly nitrogen (78 percent), oxygen (21 percent), and argon (1 percent), with a small amount of carbon dioxide (0.039 percent).

Mars’ thin atmosphere has a pressure of about one-thousandth that of the Earth’s surface, which means that liquid water can not exist in most places on the planet’s surface.

Were Mars’ surface conditions previously —or are they currently — supporting life? A series of exploration missions over the decades has continued to tantalize space enthusiasts with interesting findings.

In the 1960s, Mariner 4 transmitted close-range images of the red planet; in the 1970s, Viking searched for (and some say found) signs of Martian life; and in 2008, the Phoenix lander confirmed the presence of ice near the planet’s surface.

Researchers have been using Hawaii’s landforms as Mars analogs for many years, having recognized that they are the closest earthly counterparts to Martian volcanoes. Recent studies look to Hawaiian pit craters to reveal new information that may help confirm the existence of Martian life.

Typically, a pit crater is a circular depression formed by the collapse of the ground surface lying above a void. On Mars, they are generally bowl-shaped with floors that curve gradually to the upper rims.

A small number of Martian pit craters, known as Anomalous Pit Craters (APCs), look and behave differently than Typical Pit Craters (TPCs), as imaged from space. For instance, they are deeper than expected for their diameters, appear to have cylindrical (rather than bowl-shaped) interiors with vertical or overhanging inner walls, and exhibit thermal behaviors that are strikingly different from those observed in Typical Pit Craters. The difference in thermal behavior between Anomalous and Typical Pit Craters may be significant.

Space-based thermal-infrared observations show that Typical Pit Crater temperatures fluctuate widely with the dramatic warming and cooling cycles that occur during a Martian day.

However, the Anomalous Pit Craters experience smaller variations, suggesting that the interior of the crater controls, or at least heavily damps, the Anomalous Pit Crater temperatures.

On Earth, cave systems show the same lack of daily and seasonal temperature variations that are observed in the Martian Anomalous Pit Craters. At Kilauea, there are pit craters that have extensive systems of underground caverns and lava-tube caves.

Ongoing work strives to identify the thermal signature for these pit crater-cave combination features to see whether they are similar to the thermal behaviors of the Martian Anomalous Pit Craters. If so, the Martian Anomalous Pit Craters may also contain subsurface cave systems.

Caves in the Martian landscape are important because they may serve as hospitable habitats for potential future human visitors and because caves may be among the places on Mars that contain preserved evidence of past or present microbial life.

Currently, Mars is dimly visible in the evening twilight. While a space party in the Anomalous Pit Craters of our neighbor planet may not be imminent, the future may bring discoveries that seem lifted straight from the pages of a science fiction novel.