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 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 about "canals" on Mars had tremendous impact on popular culture. The canals were 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.

"Oppositions" every 2.1 years

Mars' orbital ellipticity (10%) implies a large variation in distance & brightness at opposition. See diagram at right. (Click for a different illustration.)
It is a very conspicuous, red-pink object when brightest (hence its association with the God of War). Mars can be brighter than Jupiter.

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

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

Mars' red color is caused by iron oxide compounds such as hematite on its surface. This is equivalent to rust(!). Other conspicuous markings include white polar caps and large dark areas, some appearing greenish to the eye in a telescope.
Mars is distant enough even at opposition that telescopes on Earth yield relatively poor resolution. This led to a long and controversial history over whether or not there was evidence for "canals" or other artificial features on its surface. (See Section C below.)
The image below was taken from Earth orbit by the Hubble Space Telescope and shows the main kinds of features visible from Earth.
Click here to see a global Mars video composited from multiple HST data frames.

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 & soil.

Important Earlier Missions:

Mariner Missions: 3 flybys, 1 orbiter, 1964-71; preliminary imaging reconnaissance of Martian surface
Viking Missions: 2 orbiters/2 landers. 1976. Sampled soil and searched for chemical signatures of lifeforms.
Mars Pathfinder: a lander with the first rover. Sampled soil and rocks. 1997.
Mars Global Surveyor: orbiter. Mapped surface at high resolution with a camera and the MOLA laser altimeter. 1998--2006.

Current Missions:

2001 Mars Odyssey: orbiter. Studying mineralogy, elemental abundances (including hydrogen, a water tracer). 2001-- .
Mars Express: first European mission to Mars. Lander failed (2003), orbiter (2004-- ) studying surface features, mineralogy, atmosphere. Carries a stereoscopic camera and the MARSIS radar instrument for probing subsurface materials.
Mars Exploration Rovers Mission: orbiter plus two landers and rovers ("Spirit" and "Opportunity"), on opposite sides of planet in areas thought to be associated with large water flows in past. Geological analysis ongoing. 2004-- .
Mars Reconnaissance Orbiter: Entered Mars orbit in March 2006. Instrumentation includes the HIRISE camera for high resolution (3 ft on the Martian surface) imaging and experiments for mineral mapping and subsurface analysis by radar.
The Mars Science Laboratory, successfully landed its "Curiosity" rover in August 2012 in Gale Crater, an ancient impact crater which contains a mountain of sedimentary deposits.

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 the 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. They were never photographed even with the largest Earth-bound telescopes (see comparison below).

A drawing and photograph of Mars made on the same night.
Click for an enlarged comparison of an HST image to classical drawings.

Beginning in the 1960's, close-up images from spacecraft proved there to be no artificial structures on Mars.
"Green" markings are likewise an optical illusion, caused by the color contrast between reddish and grey-brown areas.

The "canals" are an object lesson in dealing with marginal evidence, a very common situation in science:

overinterpret marginal data

It turns out that few of the enormous real topological features on Mars---such as the mountains, canyons, and craters revealed by spacecraft imaging---are even visible in Earth-based telescopes. Instead, the patterns conspicuous from Earth are large-scale differences in albedo 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.

here

modern topographic maps

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

Topographic charts of Mars 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.

The image above shows the main topographic features of Mars.
The image at right illustrates the red color, craters, mountains, plains, and dust-laden atmosphere of Mars. Click for a larger view.
Impact Craters: widely distributed with more in the southern hemisphere, which is therefore older; weathered, dusty. The Hellas Basin is the largest impact crater.
Volcanos: 5 large "shield" volcanos plus a number of smaller ones. All are dormant now.
- Olympus Mons is the largest mountain in the solar system (85,000 ft altitude).
- Origin of OM & other "Tharsis bulge" region volcanos: upwelling of a mantle plume, 1-2 Byr ago
- The plume's effect was concentrated into a huge uplift region because there was minimal motion of the crust past the plume.
- This qualifies as proto-tectonic activity. But Mars' surface is not broken into tectonic plates, and mantle activity has been dormant for most of the last 1 Byr.
Valles Marineris: largest canyon in solar system. Not produced by water (though some "tributary" canyons may be). Instead, VM is a surface rift, like the African Rift Valley, created during Tharsis upwelling.
Polar caps: the caps in winter are a mixture of frozen CO₂ and H₂O. They melt and refreeze with seasonal change. Water ice melts at a higher temperature than CO₂ ice, so any ice visible between the two melting points must be water ice.

E. Evidence for Water on Mars

Click for illustrations.

No open bodies of water now
Long known to be some water ice at poles. Trace of water vapor in atmosphere [in percent, 30 times smaller than on Earth but much larger than on Venus]
Layered terrain is evidence for sedimentary rocks
Erosion channels are evidence of massive floods at some time in the past.
Evidence for ocean basins
Radar evidence for water ice layers buried thousands of feet deep under the Martian poles.
Lander missions (5 to date, see Study Guide 17) find mineralogical evidence for water. The Phoenix lander (2008), with soil-sampling scoop, detected subsurface ice crystals; its images of changing droplets are possible evidence for liquid (saline) water.
All the evidence points to large amounts of water in the past, perhaps one billion years ago. Water is present in the soil and rocks now. Large permafrost reservoirs of water under surface? Melted by volcanic activity? Flooding possible cyclic.
Independent sources of evidence concerning water: SNC meteorites (see Guide 17).

Discussion of Mars is continued on Study Guide 17.

Reading for this lecture:

Reading for next lecture:

Web Links:

The Planet Mars: A History of Observation and Discovery

Text of "Mars" by Percival Lowell

Mars Scientific & Cultural Chronology

Mars in Literature & Film

Selected Mars Images For ASTR 1210

3D Viewing of Mars on Google Earth

Mars Info at Views of the Solar System

Mars Info at The Nine Planets

Mars Geology Tutorial at FAS

Mars Map-A-Planet (USGS)

Mars Science Laboratory/Curiosity

Previous Guide

Guide Index

Next Guide

Last modified May 2013 by rwo

"Red Mars" image © 1997 by Calvin J. Hamilton. Mars orbit graphic by A. Huffman. Text copyright © 1998-2013 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.