ASTR 1210 (O'Connell) Study Guide 16


Crescent Mars during Viking 2 approach 1976.
Clouds trail downwind of volcano Ascraeus Mons.

Mars is the most intriguing planet. From Earth, it exhibits the largest brightness variations of any planet and has the most distinctive color (red/pink). Telescopes have revealed it to be the most Earth-like planet, with a transparent atmosphere, varied terrain, polar caps, and seasonal changes.

Although Mars is smaller than Earth, it has no oceans, so its land area is comparable to Earth's. Space missions have revealed fantastic topography, including the largest canyon and the largest mountain in the solar system.

Finally, for over 100 years, Mars has been the favorite candidate for another biosphere. Claims by some astronomers that there were "canals" on Mars had tremendous impact on popular culture. The canals were optical illusions, but recent evidence for possible fossil lifeforms and for abundant water on its surface in the past finally have given real credence to speculations about life on Mars. Consequently, Mars is now under intense scrutiny by spacecraft for evidence of a favorable habitat (now or in the past).

A. Mars: Introduction

Small: 50% Diam(Earth); 10% Mass(Earth)

Orbit: Semi-major axis 1.5 AU. Orbital period 1.88 yr.

Atmosphere: thin. Mass ~1% Earth's. Mainly CO2; some H2O vapor.

Surface: easily visible since CO2 is transparent. Has been explored in ever increasing detail by Earth-bound telescopes and by spacecraft, including orbiters, landers and rovers.

B. Major Spacecraft Missions to Mars

Mars has been the target of over 40 robotic spacecraft missions, many of which (especially the USSR's) failed (wow! incompetence or conspiracy??). All types: flybys, orbiters, landers, rovers. Successful missions have mapped nearly its entire surface and have sampled its atmosphere and soil and even subsurface layers using special techniques.

Important Earlier Missions:

Current Missions:

C. Percival Lowell and Canals on Mars

"What goes on upon all those distant globes? Are they worlds, or are they mere masses of matter? Are physical forces alone at work there, or has evolution begotten something more complex, something not unakin to what we know on Earth as life? It is in this that lies the peculiar interest of Mars." --- Percival Lowell (1895)

Astronomer Percival Lowell devoted his career & his observatory (ca. 1890-1915) to study of features on Mars' surface which he believed to be artificial canals engineered by an advanced civilization for survival on a desert planet. He made numerous sketches using a medium-sized telescope (at left; an example of a Lowell sketch is at right). Another, much more detailed, Lowell map can be seen here.

Lowell was not the only astronomer claiming to have seen the canals. The visibility of any such features is strongly affected by the blurring effects ("seeing"---see Study Guide 14) of Earth's atmosphere. The canal enthusiasts claimed to have seen the sharp, straight-line features crossing the planet's face emerge during brief moments of atmospheric stability. However, many careful observers were never able to see them.

Lowell's efforts to popularize the idea of civilizations on Mars had a great impact on the public imagination. They were the stimulus for H. G. Wells' War of the Worlds, the prototype story of alien invasion, and a tidal wave of subsequent science fiction and fantasy stories (see Study Guide 18).

But the canals are optical illusions!, created by the tendency of the human brain to link threshold markings together. "Pattern recognition" by the eye-brain system is an important survival adaptation for human beings. However, in many situations (e.g. at the telescope or in sudden emergencies) it can produce misleading memories of what was seen. The canals were never photographed even with the largest Earth-bound telescopes (see comparison below).

A photograph (left) and drawing of Mars made on the same night.
Click for an enlarged comparison of an HST image to classical drawings.
The "canals" are an object lesson in dealing with marginal evidence, a very common situation in science:

It turns out that few of the real topological features on Mars---such as the enormous mountains, canyons, and craters revealed by spacecraft imaging---are even visible in Earth-based telescopes despite their scale. Instead, the patterns conspicuous from Earth are large-scale differences in albedo (reflectivity) caused by surface dust and mineral deposits. The pink/orange dust is very fine. Winds carry it easily across the surface, causing continuous small changes in appearance.

There is good evidence for water and possibly primitive lifeforms on Mars in the distant past (discussed below and in Guide 17)---but not for civilizations.

Topographic charts of Mars, color-coded for altitude, with
main features identified (from the MGS MOLA Altimiter)

D. Martian Topography

Mars features an amazing landscape. Martian topography has been surveyed by many spacecraft (see above), now reaching an accuracy of about one meter.

ASTR 1210 Mars Images Page. For illustrations, click here or on highlighted items below.

E. Evidence for Water on Mars

Click for illustrations.

Discussion of Mars is continued on Study Guide 17.

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Last modified April 2017 by rwo

"Red Mars" image © 1997 by Calvin J. Hamilton. Mars orbit graphic by A. Huffman. Text copyright © 1998-2017 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.