2. LOOKING AT MARS

The two Viking spacecraft that were sent into orbit around Mars in 1976 provided spectacular images of the entire planet. These images typically show surface features as small as 100 meters (about 100 yards). You can browse the surface of Mars as seen by Viking on the interactive program "PDS Mars Explorer for the Armchair Astronaut". This photo archive is more complete and uniform than anything comparable we have for the Earth! To help you interpret what you're looking at on Mars, here are a few pointers.

What's Up, What's Down?

How can you tell what's at high elevation and what's low when you look at pictures of Mars, the Moon, or any solar system body with a solid surface? Sometimes it's hard to tell a raised ridge from a depressed valley, or a volcanic mountain from a sunken crater floor. But once you know what to look for, it's usually easy to tell.

Because mountains stick out from the surrounding surface, they look bright on the slope that faces the incoming sunlight. They look darker, or even in black shadow, on the slope tilted away from the Sun. The lighting is just the reverse for valleys. Because they fall below the surrounding surface, valleys look bright on the slope opposite the Sun, and dark or in shadow on the side of the valley floor nearest the Sun. So, if you know where the Sun is, it's easy to tell mountains from valleys. You simply pay attention to where the surface is highlighted and where it's in shadow.

But how can you tell where the Sun is? Most pictures of Mars that you see aren't labelled to tell you. The first thing to do is look for impact craters, which provide the most consistent clues. Most impact craters are circular and have a raised rim and a sunken floor. The raised rim has its own shadows and highlights, and their positions depend on where the Sun is. In this image of a crater, the raised rim on the left side casts a shadow on the crater floor. That means that the sunlight is coming in from the left side. The interior of the crater rim on the right side faces the incoming sunlight and is therefore highlighted, while the outside of the same rim casts a shadow on the surrounding plain. So now you know where the Sun is.

To see how to use this principle, consider the Viking Orbiter image which shows one of the smaller Martian volcanoes, called Uranius Tholus, in the Tharsis volcanic province. Notice the circular features that are each illuminated on their right side and shadowed on their left side. These are impact craters with raised rims. The highlights and shadows around these craters tell us that the sunlight is coming in from the left side. That information in turn tells us that the large circular feature in the center of the image cannot be another crater but must be a mountain with a flat top. Its left side is brighter and therefore tilted toward the incoming sunlight. Its right side is darker, and so must be sloping away from the sunlight. The circular volcanic cone of Uranius Tholus is about 60 km across and 3 km high.

What's Young, What's Old?

You can also determine the relative ages of many surface features on Mars. In general, if one feature appears to intersect or overlap another, it must be younger. For example, if the circular rim of one crater is interrupted by that of another, the interrupted crater is the older one. Similarly, if a large surface crack cuts across a crater on the surface of Mars, the crack formed after the crater. If the crater overprints and erases a surface a crack, the crater is younger than the crack. You can generally use this principle of "superposition" to tell the relative ages of surface features, but not the time interval between the formation of the two features, or their absolute ages.

In the above image of Uranius Tholus, the circular impact craters around the slopes of the volcano must all be younger than the volcano itself. The irregular depression on the flat top of the mountain is a volcanic crater (caldera), made during the eruptions that built the mountain. However, the smaller circular crater inside the caldera was made by a later impact from space. The crater at the foot of the volcano at top center of the image has been partly flooded by material from the surrounding plain. Therefore, this crater must be younger than the volcanic mountain but older than the flooding from the plain.

Geologically, Mars is almost two different planets. The southern two-thirds is densely cratered like the Moon; the northern third has volcanoes and giant rift valleys like the Earth. In general, the southern hemisphere is much older than the northern hemisphere.

How do we know that? Because most of the impact craters on Mars were made by collisions with asteroids and comets billions of years ago. Such collisions with the planets gradually swept up most of the original population of small bodies from the inner solar system. This heavy bombardment was actually the final stage in the formation of the planets. The asteroids that remain today are only stragglers from a population once much greater.

The densely cratered Martian southern hemisphere preserves a record of the end of the ancient heavy bombardment. It has not been resurfaced much since then.

In contrast, the northern Martian hemisphere has been resurfaced in relatively recent geological times, and this has erased its original densely cratered surface. The widely scattered craters we see in the northern hemisphere today are not remnants of the early bombardment, but were made more recently by the stragglers in the asteroid belt and by comets.

In general, the more densely cratered a surface region on Mars (or any solar system body with a solid surface), the older it is.