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Midnight on the Red Planet

Recording the deep chill of the witching hour on Mars is an evocative enough idea-surely there's a short story there somewhere-but it is also important for an understanding of the planet. It is not so much a matter of simply knowing the Martian temperature in the dead of night as of finding out how the different parts of that diverse world cool down from the lesser chill that passes for the warmth of day. By taking temperature readings at selected times around the clock, it is possible to find out how rapidly the surface materials absorb and give off heat-a property called thermal inertia. This can help determine whether given parts of the surface are dominated by sand, rocks or boulders, while relating also to such factors as the role of the winds in distributing heat around the planet. The Viking orbiters each carry a heat sensor-an Infrared Thermal Mapper (IRTM)-for this purpose, and have been scanning the planet since the day they arrived, weeks before the landers descended to the surface. In August of 1976, for example, it was the IRTM temperature measurements that led to the conclusion that the Martian residual north-polar cap consists almost entirely of water, since the readings were too high for frozen carbon dioxide but low enough for water ice. The polar caps, however, respond more to seasonal than to diurnal effects. It is the rest of the surface that requires careful day-and-night study. Readings just before dawn show how cold the ground has gotten during the night; sunrise measurements reveal the beginnings of the day's warming, while readings during the afternoon show just how much heat the surface material has actually taken in. Sundown and the coming of night show the curve reversing, as the stored heat is given off. To complete the cycle, a midnight reading should show the surface approaching its coldest point, which will be monitored in the pre-dawn hours.