Life on Mars?

A group of scientists led by David McKay of NASA's Johnson Space Center has published an article in the August 16 issue of Science magazine announcing the discovery of evidence for primitive bacterial life on Mars. An examination of a meteorite found in Antarctica and believed to be from Mars shows: 1) hydrocarbons which are the same as breakdown products of dead micro-organisms on Earth, 2) mineral phases consistent with by-products of bacterial activity, and 3) tiny carbonate globules which may be microfossils of the primitive bacteria, all within a few hundred-thousandths of an inch of each other. Based on age dating of the meteorite, the following scenario has been proposed:

The following <ol> tag begins the construction of an Ordered List. In an Ordered List, each list item is preceded by a sequential number, which the browser generates. Within the list, each list item is preceded by an <li> "list item" tag. The list is closed (ended) with the </ol> "End-Ordered-List" tag. Note that, unlike most tags, the <li> "list item" tag does NOT have an </li> "End-List-Item" counterpart.

The original igneous rock solidified within Mars about 4.5 billion years ago, about 100 million years after the formation of the planet. (Based on isotope ages of the igneous component of the meteorite)
Between 3.6 and 4 billion years ago the rock was fractured, presumably by meteorite impacts. Water then permeated the cracks, depositing carbonate minerals and allowing primitive bacteria to live in the fractures.
About 3.6 billion years ago, the bacteria and their by-products became fossilized in the fractures. (Based on isotope ages of the minerals in the fractures)
16 million years ago, a large meteorite struck Mars, dislodging a large chunk of this rock and ejecting it into space. (Based on the cosmic ray exposure age of the meteorite)
13,000 years ago, the meteorite landed in Antarctica.
The meteorite, ALH84001, was discovered in 1984 in the Allan Hills region of Antarctica.

The full article about life on Mars is available from NASA.

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An Unordered List

Current and Previous Missions to Mars

The following <ul> tag begins the construction of an Unordered List. In an Unordered List, each list item is preceded by a bullet of some sort, which the browser generates and whose style can be selected/changed in the tags. As with the Ordered List, each list item is preceded by an <li> "list item" tag. The list is closed (ended) with an </ul> "End-Ordered-List" tag.

Mars Pathfinder -- NASA Mission to Mars (1996)
Mars Global Surveyor -- NASA Mars Orbiter (1996)
Mars Observer -- NASA Mission to Mars (1992)
Phobos -- Soviet Mission to Mars (1988)
Viking -- NASA Orbiters/Landers to Mars (1975)
Mariner 9 -- NASA Mars orbiter (1971)
Mariner 7 -- NASA Mars flyby (1969)
Mariner 6 -- NASA Mars flyby (1969)
Mariner 4 -- NASA Mars flyby (1964)

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Using a TABLE to present information

Mars Global Surveyor Spacecraft Characteristics

(Note: The full MGS reference page is also available.)

The table construction begins with the <Table> tag. Here, the "border=5" specifies the width in pixels of the box-like frame of the table cells. "Border=0" would generate a table without any frame at all.

Each row within the table begins with a <TR> "Table Row" tag and ends with a </TR> tag.

Within a table row are the table elements, which can be either "Table Headers" or "Table Data". The <TH> "Table Headers" are used for row and column header elements, and the text of such an element appears in a bold font. Each of the "Table Header" and "Table Data" tags has corresponding </TH> "End-Table-Header" and </TD> "End-Table-Data" tags.
Within a table row, each column element is specified, from left to right. Thus, a 3 column table would have three <TD>bla bla</TD> types of entries for each table row. The first "Table Header" element in this case has nothing in it, so the <TH> and </TH> are right next to one another.
To make the html source code more readable, it is helpful to use an indenting scheme to display table data and rows.

This is the end of the first row of the table. This table has 2 elements per row. In the first row, the first element was blank, and the second element was a Header whose contents were "Mass".

This is the end of the 2nd row of the table. In this second row, the first element has contents "Spacecraft Bus" and the second element has contents "598.4 kg".

Mass

Spacecraft Bus 598.4 kg

Fuel 387.7 kg

Science Payload 75.8 kg

Total (at Launch) 1,061.9 kg

The table is closed or completed with the </TABLE> "End-Table" tag. The browser decides how big the table needs to be in order to properly display the elements in the table.

	Mass
Spacecraft Bus	598.4 kg
Fuel	387.7 kg
Science Payload	75.8 kg
Total (at Launch)	1,061.9 kg

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Using a TABLE for layout

The following table is used just to accomplish a certain layout on the page. The table has just one row, and has three elements (columns or cells) in that row. The first and last elements are used to hold images, while the middle element contains text. This achieves a page layout with images on the left and right and text in the middle, all aligned vertically. In the <Table> tag, the "cellpadding=10" statement sets the amount (in pixels) of white space or "padding" there is between the edges of the cell and the placement of the cell contents. No cellpadding (or "cellpadding=0") results in text or images placed right next to the cell border or frame. The greater the cellpadding, the more white space there is between the cell frame and the cell contents.

As a December 2, 1996 CNN article says, "Sojourner is not much bigger than a toy radio-controlled car, but instead of spinning around a suburban driveway, it will -- if all goes well -- cruise the rocky surface of Mars."
All did go well, and on July 5, 1997, Sojourner rolled down a ramp and into history, "becoming humanity's first autonomous vehicle to travel on the surface of another planet." (CNN July 6, 1997.)
It didn't take long for the Rover to spring into action!

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Using the UL tag for indents

Destination: Mars!

An advanced formatting feature is alignment of the text on the page accomplished by using <ul> "unordered list" list tags. The browser automatically indents and left justifies the text placed between <ul> and </ul> tags. If no <li> list items are specified, there are no "items" in the list, but the text between the <ul> and </ul> tags is still formatted by being indented and left justified. If a list is placed within a list, the "inner" list is indented further, and the contents of that list are again all left justified. Thus, an outline type of indentation scheme is easy to accomplish by using a series of nested lists which don't contain any list items.

Here is the first level of indentation, used to set up the topic. The first question in this topic is "What are these missions all about?"

A decade of intensive Mars exploration has begun in 1996 as the United States successfully launched the Mars Global Surveyor in November and the Mars Pathfinder in December.

Note the use of different font sizes and styles in this section. Key points or phrases are emphasized by using a larger font, and the leading questions are set off in italics.

What are these two missions all about?

The next level of indentation is used to highlight each of the two missions. Each mission is described in a paragraph whose justification is 2 levels of indentation.

Mars Global Surveyor: The Mars Global Surveyor is the first of 10 NASA probes bound for Mars in the next decade and was designed to replace the NASA Mars Observer probe which disappeared mysteriously in 1993 just days before it was to reach the orbit of Mars. Surveyor will take 10 months to reach Mars and another six months to settle into a planetary mapping orbit. Later, it will descend into the Red Planet's thin atmosphere, using its wing-like solar panels as brakes. Surveyor will study the Martian surface and atmosphere, but will not land. (More information...)

Mars Pathfinder: The Mars Pathfinder spacecraft has successfully cruised directly to Mars, entered the atmosphere with a Viking-derived heat shield, and landed with the aid of parachutes, rockets and airbags! Its spectacular landing occurred on July 4, 1997, on a rocky plain in an area known today as Ares Vallis. Mars Pathfinder is the first spacecraft to return to the surface of Mars in almost 21 years and has begun an exciting era of exploration of the Martian surface! (More information...)

This is the end of this second level of indentation. The first question, "What are these two missions all about?" has been answered. Now the next question, "What other missions are planned?" can be asked at the first level of indentation, and answered at the second level of indentation.

What other missions are planned?

The answer to the second question can be addressed here at the second level of indentation.

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