ADVANCED APPLICATIONS - ASTRONOMY: Lasers probe the universe

For the astronomical community, 1999 may well have been the year of the laser. During that year, some of the world's largest telescopes began routine use of the laser-guide-star technique, which compensates for atmospheric turbulence to improve resolution. Also, ongoing experiments using intense laser pulses were able to simulate conditions in the heart of exploding stars, allowing astronomers to check theoretical predictions and computer simulations.

Perhaps the most dramatic discovery of planetary science, though, was the still-controversial evidence supporting the existence of a vast ancient ocean on Mars provided by the Mars Orbiting Laser Altimeter (MOLA). Since 1997, this laser rangefinder onboard NASA's Mars Global Surveyor spacecraft has mapped Mars from a nearly polar orbit to produce a topographic map of the entire planet accurate to about 13 m. By May 1999, the laser-altimeter data had produced a three-dimensional map of Mars with vertical accuracy hundreds of times better than any previous terrain map.¹ This map, which has more detail than any comparable topographic map of the entire Earth, has allowed planetary researchers to reach some spectacular conclusions about the history and geology of Mars.

Examining the Martian surface

Today Mars is bone dry, with an atmosphere only a tiny fraction as dense as Earth's and too thin to allow liquid water to exist on the surface. Astronomers have theorized for years, though, that water must have existed on the planet's surface several billion years ago. Supporting this theory are photographs taken by the Viking and other spacecraft that show vast channels that could only have been cut by running water. The MOLA maps have helped to reveal just how much water Mars once had.

To develop a map, the MOLA sent 8-ns-long, 40-mJ pulses of 1.064-µm radiation down to the Mars surface and then timed the return pulse. Combined with the known location of the spacecraft, these data allowed computers to calculate the altitude of each 300-m-wide swath of surface.

The MOLA data showed that a huge northern basin on Mars lies nearly 4 km below the southern hemisphere plateau and is as smooth as the abyssal plains at the bottom of the Earth's oceans. Researchers do not believe the basin is an impact crater because its outline is far from circular. Rather, it is surrounded by two sharp scarfs, each thousands of kilometers long and up to 2 km high, where the ground slopes down as steeply as 20°.

A careful analysis of MOLA maps by James W. Head of Brown University (Providence, RI), David E. Smith of NASA Goddard Space Flight Center (Greenbelt, MD), Maria T. Zuber of the Massachusetts Institute of Technology (Cambridge, MA), and their colleagues indicates that the northern basin on the planet's surface was left behind by an ocean. They showed that the elevation of the inner scarf, which was the more recent feature, had sufficiently few variations in altitude to very likely be an ancient shoreline. The outer scarf may well represent the shoreline of a still earlier and slightly large ocean where subsequent uplifts have destroyed any consistent altitude. "Not everyone agrees that this ocean definitely existed," Smith says, "but MOLA's evidence is consistent with such an ocean, and none of it contradicts the idea, which is being taken very seriously for perhaps the first time."

By measuring the profiles and slope of the kilometer-deep channels carved around the edge of the northern basin, researchers were able to calculate that just one channel could have disgorged water (presumably from some underground reservoirs) at a rate of 5 km³/s—40,000 times the flow of the Amazon river with enough volume to fill the northern ocean in a month. In all probability, though, much shorter catastrophic inflows from many channels did the job during a warming period in Mars' early history. Other measurements of the planet's northern and southern ice caps indicate that about one-third of the ocean's water remains locked in the polar ice. The rest may have escaped into space or been trapped in a permafrost layer.

Guided by a laser star

On Earth, lasers increasingly are helping astronomers fine-tune telescope feedback. Until recently, atmospheric turbulence had restricted the angular resolution of large telescopes to not much better than a backyard telescope. Helping to resolve this problem are adaptive optics that can automatically compensate for atmospheric turbulence by distorting a mirror in the light path (see Laser Focus World, April 1999, p. 15). The basic premise is to look at the distortion of the wavefront from a bright point source and then compensate for the distortions to bring the wavefront to a point focus. This corrects for turbulence over the whole field of view.

Because stars bright enough to serve as guide stars are few and far between, researchers create an artificial guide star with a laser. The best approach still uses the 589-nm line of sodium to excite sodium atoms in the mesosphere. At 90 km above the Earth's surface, this area is above nearly all atmospheric turbulence. At least in the near-infrared band, correcting with laser guide stars can improve angular resolution by a factor of ten—to the limit set by diffraction and close to the 0.1 arc sec achieved by the Hubble Space Telescope.

At the Keck telescopes in Hawaii, a laser guide system uses a dye laser pumped by five frequency-doubled Nd:YAG lasers to generate an average power of 20 W in 26-kHz pulses. The Max-Planck-Institutes for Astronomy and Extraterrestrial Physics (Heidelberg, Germany) run another laser-guide system at the smaller 3.5-m Calar Alto telescope in southern Spain. Called ALFA (adaptive optics with a laser for astronomy), it relies on an argon-ion laser pump.

To improve reliability and ruggedness of laser-guide-star systems, several research groups are trying to find all-solid-state alternatives to dye lasers for generating 589-nm radiation. One concept considered by the European Southern Observatory (Garching, Germany) for its Very Large Telescope pumps a double-clad ytterbium-doped fiber with a 100-W laser-diode bar. This fiber then feeds into a germanosilicate fiber. Stimulated Raman scattering in the fiber produces the 1.178-µm radiation that is then frequency-doubled for the output.

Supernova in the laboratory

Laser-based technology is also helping astronomers move toward active experiments in addition to the passive observations they have traditionally been limited to because of the scale of the phenomena observed. Laboratory astrophysics involves modeling astrophysical events at vastly reduced scales. Recent experiments have shown lasers to be central to such modeling because of their capability to concentrate large amounts of energy in small space and time.

One application involves the study of supernovae, a process that results when a massive star runs out of nuclear fuel and its core suddenly collapses and then rebounds. The shock wave moves outward through in-falling material, producing complex instabilities. While computer simulation has helped researchers understand this critical phase of the process, experimental modeling is crucial to checking the simulation results.

With hydrodynamic processes such as the shock wave, the time scale of a process scales linearly with the length scale, as well as the square root of the ratio of the density divided by the pressure. With adjustments to pulse length, the size of the focal area, and pulse energy, high-intensity laser pulses can reproduce the substantially larger, much-slower processes in a supernova.

Recent laser experiments with a 20-ns pulse focused on a 50-µm target reproduced the hydrodynamic conditions in a 1 million-km-wide supernova about 40 minutes into the detonation.³ Another experiment with even-larger scaling factors modeled the interaction of the remnants of a supernova with interstellar matter on a time scale of a year.

Scientists also are exploring the use of petawatt lasers with power densities of 1020 W/cm² to produce plasma fireballs with similar energy to that generated by gamma-ray bursters. Successful simulation of these phenomena with high-power lasers would be quite an achievement when one considers that these bursters may be the most energetic processes in the universe.