The Tip of the Iceberg

Long before spacecraft reached Mars, astronomers were debating about the white ice masses that cap the Martian poles. Were the permanent caps made of water, as on earth, or of frozen carbon dioxide? Last week scientists learned the answer when data radioed back by the Viking 2 orbiter erased any remaining doubt: The summer ice cap of Mars' North Pole is composed of frozen water. The discovery confirms that Mars has far more water than had previously been believed, and suggests that even more lies in a shell of permafrost below the planet's rocky surface. In fact, scientists said, the polar caps might be merely the visible tips of an iceberg-like Mars.

Viking scientists base their new assessment of Mars on readings made by the orbiter as it passed over the ice cap. One set of sensors aboard the craft found surprisingly high amounts of water vapor in the atmosphere near the north pole, good evidence that it had risen from water ice below. Other Viking instruments found that the temperature of the ice cap was --100DEG F.--low enough to freeze water but a full hundred degrees too warm to freeze CO^2.

Viking also discovered that the Martian atmosphere contains measurable quantities of krypton and xenon, two inert gasses also found on earth--an indication that the planet's atmosphere was once dense enough to allow water to flow on its surface. Those readings, coupled with the finding of substantial amounts of water, seem to confirm that the sinuous channels and tear-shaped '"islands" shown by earlier Viking orbiter photos were indeed carved out by rushing waters. The orbiting mother ship of the Viking 2 lander made other news last week. It shot the clearest picture yet of Phobos, the inner of the two Martian moonlets, showing mysterious parallel streaks in its northern regions and chains of small craters near its equator.

Closer Look. From the Martian surface, Viking 2 was sending back life-search data surprisingly similar to Viking 1's findings. Though Biologist Norman Horowitz called results from one experiment "marginally positive," scientists still did not have enough evidence to determine if the activity observed thus far is biological or some exotic chemical reaction.

In an attempt to clear up the ambiguity, J.P.L. engineers have programmed Viking 2's metabolism experiment to run longer than that of its sister ship, allowing any Martian microbes that might be around more time to carry out the activities that would identify them. The scientists are also ordering Viking 2 to look a bit harder than Viking 1 did. The lander, its balky arm repaired, will reach out to a patch of crusty soil and scratch under it in search of organic molecules that above ground would probably have been destroyed by the ultraviolet-rich Martian sunlight. Later, if all goes well, Viking 2 will do what any earthbound explorer would to find life in an apparently barren area: the lander will turn over a rock and look underneath it.

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