For centuries, people claim to have heard pops, crackles, whooshes and sizzles coinciding with meteor falls. One mystery is why there is no time delay, as it should take several minutes for the sound to travel many kilometres to the ground.

In an attempt to nail down the phenomenon, a team from the Croatian Physical Society and the University of Kentucky set out in November 1998 to observe that year’s rich Leonid meteor shower. In the quiet of a frozen, lifeless plain in central Mongolia, the team set up a sensitive digital video camera to watch the sky, along with two microphones to capture any sounds, and radio receivers to monitor electromagnetic signals.

In an article accepted for publication by the Journal of Geophysical Research – Space Physics, they report that at least two deep, popping sounds coincided with bright fireballs tearing across the sky. But the radio equipment, which was tuned to very low frequencies (VLFs), failed to pick up any signals.

It’s the very first time anyone has picked up these sounds under controlled conditions, says Dejan Vinkovi´c of the University of Kentucky. “We proved it is possible.”

Turbulent wakes

But an explanation of the phenomenon seems as far away as ever. According to Australian meteor researcher Colin Keay, very low frequency radio waves produced when the turbulent ionised wakes of meteors tangle with the Earth’s magnetic field could generate sounds on the ground by making plants or other objects vibrate. The lack of such radio signals puts this theory in doubt.

The work has got people taking notice. “It’s certainly a step in the right direction,” says meteor specialist Martin Beech of the University of Regina in Canada. But he cautions that with so many fireballs in the Leonid shower, the timing of the sounds and the meteors could have matched by chance.

Nor is he yet convinced that Keay’s theory is wrong. Unlike the slow, deeply penetrating, large meteors Keay was describing, the Leonids are swift sand grains that do not make it far into the atmosphere. That’s why you get no VLF transmissions, says Beech. “You wouldn’t expect Keay’s theory to apply.”

Todd Stevens and Jim McKinley of the US Government’s Pacific Northwest Laboratory in Richland, Washington, found the bacteria – which they call a subsurface lithoautotrophic microbial ecosystem, or SLIME – in deep aquifers within basalt formations near the Columbia River in Washington. The organisms apparently survive without energy input from the Sun, geothermal warmth from the Earth’s interior, or chemical energy stored in buried organic matter. Instead, they get their energy from chemical reactions between rock and water. If life exists on Mars, the researchers suggest, it may look something like this.

Using nothing but water from the deep aquifers and sterile crushed basalt, Stevens and McKinley cultured thriving bacterial communities in the laboratory. Most of the bacteria in mature cultures required no organic food to survive. Iron-rich silicates in the basalts break down spontaneously in the presence of water to release hydrogen gas. The bacteria may use this to convert dissolved carbon dioxide, or another carbon source in the water, into methane gas. The reaction yields energy which the bacteria use to grow and to synthesise organic compounds, says Stevens.

Even mature cultures contained some bacteria that need organic compounds synthesised by other groups, Stevens and McKinley found. This suggests, but does not prove, that an entire ecosystem may have evolved deep within the basaltic aquifers.

The existence of such an ecosystem may explain why the Columbia River region contained large deposits of natural gas, which was commercially exploited earlier this century, says Stevens. Usually, such gas is found in sedimentary rocks with a high organic content, not in volcanic rocks with virtually no organic compounds. If the researchers’ guess is right, the Columbia River basalts – covering some 422 000 square kilometres and extending as far as 5 kilometres below ground – may contain a lot of bacteria and hence a lot of methane.

The most intriguing aspects of the new ecosystem concern its implications for our understanding of the origin of life, and the possible existence of extra-terrestrial SLIMEs. “All the ingredients should be present in subsurface Mars,” Stevens says. NASA scientists have told him they have long suspected that SLIME-like ecosystems could be lurking under the Martian surface.

But just how bacteria could get deep into the Columbia River basalt – let alone subsurface Mars – is still a profound mystery. If SLIMEs are ever found on Mars, would it imply that life on both planets started underground? At present, says Stevens, no one has any plans to lug a drilling rig to Mars and find out.