Launching Lasers to Mars

Lynn M. Savage

Scheduled for launch five years from now, the ExoMars spacecraft is designed to study the Martian environment, from the planet’s tectonics to its geochemistry. One of the primary goals, in fact, is to search for signs of past or present life. To accomplish its objectives, the spacecraft’s lander will carry a wide assortment of detection and analysis instruments, many of which are photonic.

These instruments — including a Raman laser-induced breakdown spectrometer, an x-ray diffractometer and a panoramic camera suite called PanCam — mainly depend on destructive analysis techniques, nonreplenishable test chemicals or both. To search for hints of Martian life without these constraints, investigators with the Kinohi Institute in Altadena, Calif., with Mullard Space Sciences Laboratory in Holmbury St. Mary, UK, and with Oregon State University in Corvallis are suggesting that another instrument be added to the mission: a laser diode.

The scientists believe that a small, lightweight laser — operating in the 365- to 375-nm-wavelength range — would be a valuable addition to the mission because it could be used to induce fluorescence in polycyclic aromatic hydrocarbons (PAHs) that may lie beneath the surface of the Martian soil.

On Earth, PAHs are mainly known as the resulting pollution formed by combusting oil, coal and similar materials. The hydrocarbons, however, also are thought to be possible precursors to early life, and finding them on other planets may point to new paths of discovery.

Excited by UV radiation, most common forms of PAHs produce visible fluorescence. According to team member Jan-Peter Muller of Mullard Space Sciences Laboratory, the proposed UV-emitting lasers would be mounted next to the filter wheel accompanying each of the PanCam’s two wide-angle cameras. From their mounted position, the lasers would have a direct line of sight to soil samples drawn to the surface by a drill on the lander. Whether one or two lasers would be included in the mission depends on the lander’s available power and load capacity.

The ExoMars mission scheduled to launch in 2013 will carry a lightweight drilling system, a sampling and handling device, and a set of scientific instruments to search for signs of past or present life. Among the latter may be a UV diode laser package that will look for polycyclic aromatic hydrocarbons by making them fluoresce. Artist’s rendition courtesy of the European Space Agency AOES Medialab.

The filter wheel contains filters with center wavelengths of 448, 530 and 655 nm, but the researchers found that the 530-nm one was the only one to show clearly fluorescence from PAHs with either three, four or five aromatic rings — respectively, anthracene, pyrene and perylene. They tested the system using crushed peridotite — a rock rich with the mineral olivine — which closely mimics soil from one of the areas on Mars where the lander may be sent.

The group also considered using an LED as a source of UV radiation but, according to Muller, the laser diode is preferable because it has higher intensity, which enables accurate measurements even during daylight hours. Additionally, said the institute’s Dr. Michael C. Storrie-Lombardi, the laser’s longer reach makes it possible to study soil samples in otherwise inaccessible places, such as outcrops, cliffs, cracks and crevices.

If the laser goes with the ExoMars mission in 2013, it should begin looking for the signs of early life by sometime in 2014.

Geophysical Research Letters, June 20, 2008, L12201.