From the April 2004 issue

Changing Mars

Exploring Mars by spacecraft destroyed an entire world that existed only in humankind's imagination — and it gave us a real world loaded with complexities.
By | Published: April 26, 2004 | Last updated on May 18, 2023
Mariner 4: first glimpse of Mars
In 1965, Mariner 4 shocked everyone by showing that Mars was a cratered, Moon-like world.
Robert Burnham

The exploration of Mars began earlier than many people think. Just three years after the launch of Sputnik in October 1957, the beginning of the space age, the first Mars probe was launched (by the Soviet Union, now Russia). It failed on launch, however — as did a second Soviet Mars launch attempt a few days later. Over succeeding years, the Soviets tried three more times, each flight failing one way or another. Meanwhile, the United States looked on, while making plans.

First glimpses
The first successful Mars probe was Mariner 4, launched by the United States in November 1964. Three weeks earlier, its twin, Mariner 3, had failed when its aerodynamic shroud refused to come off after the spacecraft left Earth’s atmosphere. Mariner 4, however, successfully flew past Mars in mid-July 1965 and took 22 images.

Harsh reality
These images — grainy, loaded with electronic noise, and low in resolution — nonetheless shocked scientists and the public alike because they showed Mars was heavily cratered. Abruptly, any lingering fantasies of a warm Mars with parallels to Earth, possible life, and perhaps even canals (such as the ones envisioned by Percival Lowell) vanished. In its place was a cold, bleak Mars that looked a lot more like the Moon than any place on Earth.

Mariners 6 and 7, launched by the United States in February and March 1969, were the next successful flights, both craft flying past Mars in July-August 1969. Using cameras higher in resolution than Mariner 4’s, each probe took more than one hundred images. The two probes’ results confirmed the cratered appearance of Mars and added tantalizing hints of other features.

Mariner 9 changes our view of Mars
In the early 1970s, Mariner 9 again surprised scientists by showing that Mars had a dynamic geological history — and that maybe it wasn’t quite so lunar after all.
Yet it wasn’t until Mariner 9 in 1971 that scientists realized how complex Mars would be to understand. Designed as an orbiter, Mariner 9 was launched in May 1971 and arrived in November — only to find the planet shrouded in a global dust storm. As the atmosphere cleared, Mariner 9’s cameras recorded the basic geography of Mars emerging from the dust. It’s a geography that’s now familiar to us — gigantic volcanoes; deep, labyrinthine canyons; channels that unquestionably saw running water; and complexly layered polar caps.


Mars Pathfinder explores Ares Vallis
Getting down on the planet’s surface showed abundant traces of wind, dust, erosion — and perhaps water. The Sojourner rover proved extremely useful and was a hit with the public.
Subsequent probes — the U.S. Viking orbiters and landers in the mid-1970s, the Soviets’ partly successful Phobos orbiters in the late 1980s — added details to this picture without altering it greatly. The Viking landers and 1997’s Mars Pathfinder, with its rover named Sojourner, took us down to the planet’s surface, where we saw a rock-strewn desert sprawling toward the horizon under a tawny sky.

This Mars clearly posed geological challenges for any idea of life. The soil was highly oxidizing, and basaltic boulders told of a heavily volcanic past — and yet, the outflow channels pointed to abundant water at one time in martian history.

Opportunity examines El Capitan
Opportunity digs into an outcrop of sulfate-rich rock dubbed El Capitan. As robotic geologists, the current Mars rovers each carry a sophisticated toolkit and have enough mobility to drive a couple miles from their landing sites. Both have found evidence for lots of water in the martian past.
These puzzles linger now as we get new looks at the Red Planet with two Mars Exploration Rovers dubbed Spirit and Opportunity. They approximate human geologists as they roll across the floor of Gusev Crater and the flat expanse of Meridiani Planum. Once again, the picture is changing. In both locations, the rovers have found rocks that were strongly affected by lots of water. What this means for life’s chances — ancient or modern — remains an exciting, open question.