Visible to the naked eye and known to early astronomers

Helped Kepler derive his laws of planetary motion in 1609

By the late 1800s telescopes were powerful enough to observe surface features

Before we had such data, Mars remained largely a mystery until the robotic exploration of the 1960s and 1970s.


Mars Exploration

The ice caps are composed of water ice overlain at times by "dry ice" (carbon dioxide ice)

Each cap has a permanent and seasonal component

There is a great deal of ice off the cap, especially in the Northern Lowlands

This ice is frozen throughout the martian year leading to a condition in the soil known as permafrost

Mars is both Earthlike and Moonlike

Like the Moon, Mars is divided into old, heavily cratered highlands and younger, lightly cratered lowlands

Mars' surface is divided into two provinces:
 
       ancient, heavily-cratered highlands (similar to the Moon's) located mainly in the southern hemisphere

       younger plains located mainly in the northern hemisphere.


Close to the equator of Mars we find the Valles Marineris, the largest canyon in the solar system.

As elsewhere in the solar system, the number of craters on a surface can be used to date that surface

Largest is Olympus Mons – the largest volcano in the solar system

Two other main complexes of Volcanoes

Olympus Mons and Valles Marineris (the largest and deepest canyon system) suggest significant activity in the past

We believe that Mars has a thick lithosphere, somewhat like the smaller planets Mercury and Moon.

The evidence for the thick lithosphere is the enormous shield volcanoes.

They build up to great heights because the thick lithosphere can't move much over "hot spots".

Here is our best guess about Mars' interior structure:

Here is a view of the summit of Olympus Mons, the largest shield volcano on Mars (and, in fact, the largest volcano in the solar system).

 

Notice the "wrinkle ridges" inside the caldera, and the lava flow tubes on the slopes (analogous to lunar rilles).

We will discuss the difference between an impact crater and a caldera.

 It is important to note that lava flow channels always get narrower downstream, because the lava cools and solidifies, becoming part of the channel walls.

Lava tubes get narrower downstream, like this:

In contrast, water flow channels always get wider downstream, and have a dendritic (branch-like) structure:

Mars continued

More on how to tell a water flow feature from a lava flow feature:

Apollo 15 metric camera picture of sinuous rilles (at bottom of image) east of the Moon's Aristarchus Plateau. The rilles have the appearance of lava channels or collapsed lava tubes, and most originate on craters. The largest rille in this southward-looking oblique view is Rima Prinz, which starts at the center of the image at the crater Prinz (25.5 N, 44.1 W; 46 km in diameter) and trends generally northward (down). Oceanus Procellarum is at upper left and the bright 40 km diameter Aristarchus crater is at upper right, at 24 N, 47 W. (Apollo 15, AS15-2606)

 

And, here's a close-up look at a lunar rille (lava tube):
Apollo 15 astronaut James Irwin and the rover at the edge of Hadley Rille on the Moon. David Scott took this Hasselblad picture looking northwestward from the flank of St. George crater. The rille has been widened and made shallower over time by wasting from the walls. Talus blocks are visible on the walls and on the floor of the rille. (Apollo 15, AS15-85-11451)

Water on Mars in the past

In addition to Phoenix, there have been a number of successful Mars surface experiments.  We have the two Mars Exploration rovers, "Spirit" and "Opportunity".  Spirit is now dead, but Opportunity is still working.  Both landers worked well into their extended missions, exploring opposite sides of Mars in regions that are now known to have been once inundated in liquid water.

The newest Mars Rover is the much larger Mars Science Laboratory Curiosity, which is examining evidence for ancient water flows in Gale Crater:

Landing site link

A "fanciful" map of Wet Early Mars (William R. Johnston):


However, there is relatively little evidence of shoreline features like those we see on Earth. Thus basins may have been only periodically flooded

MER Spirit landing site:

Opportunity landing site:

Hematite is a mineral that may be associated with liquid water.
View of layered terrain near Opportunity landing site:

Opportunity has finished exploring Victoria crater (download movie) and is moving on to another site.

The key question:  how long ago did Mars have liquid water?  Did it have liquid water more than once?


In the northern hemisphere are found dendritic flow channels (especially in the Chryse region).


And here is an image of dendritic flow channels:


The image shows numerous channels carved by liquid.  It also shows windblown dust patterns.  Notice how wind has scoured dust from crater bottoms and blown it out toward the southwest.

It is important to note that liquid water cannot exist on the present-day Martian surface. The temperatures are too low and the atmospheric pressure is too low.  We will discuss this in more detail later.

Nevertheless, water is the only substance likely to have produced the liquid erosion. Now here is more evidence for water on Mars:


This is a high-resolution view of a "river bed" on Mars from Mars Global Surveyor.  The image is only a few km across.  It looks like a feature produced by a long-term stream, rather than by terrestrial flooding.

There is also evidence for subsurface frozen water, like tundra.  Here is an image of the crater Yuty, a so-called pedestal crater with lobate ejecta.  Also notice the flow patterns to its right:


Here is very dramatic evidence for water from Mars Global Surveyor.  This shows a crater which was buried under deposits, and then unburied by floods:


The area in the box (only about 10 km across), enlarged:

Evidence for subsurface water from Mars Global Surveyor?

gullies in a crater wall

Some planetary scientists think these may be sand flows rather than water flows.