April 2004

As NASA's Mars rover Opportunity explores the Red Planet and beams valuable data back to Earth, scientists are reporting the exciting news that Mars was once a much warmer, wetter planet than it is today. And while the search for evidence of life on the Red Planet continues, strong evidence of the presence of water has been found. Photos from Mars, such as the one below, show sediments that look very similar to sediments formed on Earth in the presence of water. And chemical probes have identified salts containing chlorine that probably formed from ancient, evaporating, salty, martian water.

Salt Stories

Photo from Mars: Scientists believe this is a sedimentary rock
that formed in water when Mars was a wetter planet.

Salts formed by the slow evaporation of seawater over long periods of time are common on Earth. Sodium chloride --the mineral halite, NaCl, also known as common table salt-- is one example. Another natural salt found with halite is potassium chloride--KCl--also known as the mineral sylvite.

Potassium-Chemical Symbol K?
You may wonder why potassium is designated "K" in chemical shorthand. The "K" is for kalium, a word derived from "alkali," or the Arabic "al-qali," meaning "ashes of the saltwort plant." Historically, potassium carbonate (K2CO3), called potash, an important plant fertilizer, was obtained by stirring the ashes left after a fire in a pot of water (pot of water + ashes = potash) and then evaporating the water.

It is not surprising that the same chemical elements found on Earth are present on Mars because scientists believe all of the planets of the solar system were "born" from the same starting matter. But, we might wonder how a chloride mineral formed on Mars might differ from one formed on Earth. Science is a never-ending hunt for answers to questions that continually arise as soon as new information is gathered.

Elements: Earth Recycles Them

We know that potassium and chlorine are two of the many chemical elements transported from Earth's continents to its seas as the forces of nature wear down rocks. Potassium and chlorine are two chemical elements found in minerals comprising rocks. Natural waters slowly dissolve these and other elements from minerals and carry them, by the force of gravity, to the oceans. Earth processes may be thought of as nature's large-scale recycling projects: Minerals in continental rocks are broken down by wind, water and living organisms into simpler components, only to be re-formed into new minerals, such as those found in salt deposits.

Salt For Life

Here on Earth, the mineral sylvite is a useful source of potassium. Many living organisms depend on potassium for their existence because potassium is critical to the functioning of living cells--the microscopic building blocks of life. Human beings, for example, are made of skin cells, muscle cells, bone cells, blood cells, and the cells of various organs. To function properly, cells must admit and release a steady stream of chemicals--nutrients, for example, must be admitted into cells and waste products released. Potassium has the important role of regulating this "chemical traffic" in and out of living cells.

KCl is a prescription drug when it is used to raise blood levels of potassium in people who are deficient in this element. It also may be used as a sodium chloride salt-substitute by people who should not take in sodium chloride for medical reasons. The taste of KCl is similar to that of NaCl, but slightly bitter. (Geologists have been known to distinguish sylvite from halite by this "taste test.") Human blood and ocean water contain the same concentrations, or levels, of salts, so it is not surprising to learn that when there is a medical need for potassium, that need is satisfied by a natural product of the seas.

KCl: Case Study in Ionic Bonding

Chemical bonding is the process that links elements together to form compounds. KCl is an example of ionic bonding.

Below is a simple diagram of potassium and chlorine atoms. Electrons are shown to be arranged in rings around a central nucleus. Electrons fill these rings, or shells, as they are know, in a particular order. For many elements, the most stable state is one in which there are eight electrons in the outermost electron shell. Notice that potassium has one electron in its outermost shell and chlorine has seven. When the two atoms are in contact, potassium readily transfers its outer electron to chlorine which readily accepts it, resulting in both atoms achieving a state of eight outermost electrons. With this electron transfer, the ionic bond in KCl is formed.

Atomic Number 19                 Atomic Number 17
   (19 electrons)                         (17 electrons)


NASA photo of the Opportunity

Follow-up Activities:

  1. How might natural processes on Mars, such as weathering of rocks, differ from those on Earth? What kind of information is needed to evaluate the differences? For example, how does gravity on Mars compare to gravity on Earth?

  2. Draw an accurate diagram to show how ionic bonding occurs in NaCl. Use the Periodic Table to obtain the atomic number of Na and Cl. The atomic number will tell you how many electrons to draw for each element.

  3. Why is Mars called the Red Planet?

Check It Out:

A Mars Internet site for kids:

All about the Hubble Space Telescope:

For a list of previous "Chlorine Compound of the Month" features, click here.


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