Mars24 User's Guide

(Updated Jan. 14, 2004)

Mars24 is a cross-platform Java application which displays a Mars "sunclock", a graphical representation of Mars showing the current day- and nightsides of Mars, along with numerical readouts of the time in 24-hour format. Other displays include a plot showing the relative orbital positions of Mars and Earth and a diagram showing the solar angle for a given location on Mars.

Mars24 cannot accurately determine the time on Mars unless your computer's time and time zone, and date are set correctly.

Please read the accompanying Technical Notes on Mars Solar Time for a more detailed discussion about the meaning of the various display values and for definition of Mars time units.


When first launched, Mars24 displays three windows: a settings window for controlling display options, a small display window showing the time on Mars and on Earth, and a large display window which can show either a sunclock map of Mars, an orbital positions plot, or a solar path plot for a location on Mars. If you close a window, you can make it visible again using the appropriate command in the Window menu. The Window menu also allows you to display the two help pages.

On most computers, closing all windows will cause Mars24 to quit. If you are using a Macintosh, however, you must use the Quit command in the application menu to exit the program.

Popup Menus

When viewing either the time display window or the graphic display window, you may right-click (Windows, Linux) or control-click (Mac OS X) on the window content to bring up a popup menu (aka, contextual menu) which offers some choices on changing the display. Among these options is the ability to resize the window to one of several pre-defined sizes.

Saving Settings

When you quit Mars24, it saves the settings of the various controls to a preferences file. The next time you use the program, it will open that preferences file and initialize the controls using the settings it finds.

If you would like to save several different configurations of the controls, you can use the Open and Save Settings commands in Mars24's File menu to do so. Saved settings files must be given the extension .m24. A few sample settings files are included with Mars24. (Macintosh users: if you doubleclick on a .m24 settings file, Mars24 will automatically open it.)

Settings Window

This window is divided into three tab panels: one tab allows you to specify how to display the time on Mars, the second to set the time on Earth, and the third to specify properties of the large graphical display. We'll start by taking a look at the second tab.

The Earth time settings tab gives you three choices for specifying what time on Earth should be used in making all the various calculations that Mars24 must perform, both in determining the Mars time and generating the various displays.

The first (default) Earth time choice is to use the current time, i.e., now, whatever "now" happens to be.

The second choice is to add some offset to the current time on Earth. For example, if you need information about the time on Mars exactly 100 Earth hours from now, you would select this option and enter "100" in the hours field. Note that when you select this option, the clocks in the time display keep on ticking, just offset from the current time by whatever amount have entered.

To find out the time on Mars that corresponds to a specific Earth time, you would select the third choice and enter a time of day in the first input field and a date in the second field. The time and date must be in UTC (Coordinated Universal Time), which is in everyday usage effectively the same as Greenwich Mean Time. Please note that the Java class which we use to parse the date entry is somewhat finicky about the format of the entered date. It understands common date notation such as "Dec. 25, 2003" but not ISO date format such as "2003-12-25".

Also on the Earth time settings tab is a checkbox which allows you to specify how you would like Earth date shown in the display windows. Normally it is shown in ISO format, i.e., "YYYY-MM-DD", but if you opt to show the date as day-of-year then the format changes to "YYYY-DDD".

The Mars time settings tab effectively gives you two choices, to either display the local time for some particular place on Mars or to display the mission time for one of the current lander projects. If you choose the first option, you also need to select a time format. When Mars24 is first launched, the default is to display the local mean solar time on the Mars prime meridian.

For example, if you would like to display the time at a Olympus Mons, you would click the "local time" radio button and enter into the location fields longitude 133.10°W and latitude 18.60°N.1 (Actually, the N-S latitude doesn't matter when calculating the time, so you could just leave the latitude field at 0°N.) The adjacent format menu gives you the option of displaying the time in either local mean solar time (LMST), local true solar time (LTST), or local mean zonal time (LMZT). (See the "Notes about Mars Time" help page for further information about these formats.).

The alternative Mars time setting is to display the mission time for one of the three 2003-2004 lander missions: MER-A (Spirit), MER-B (Opportunity) and Beagle 2. The mission time will be shown as a time of day and a Sol value, the latter being the count of the number of Martian days since landing. "Sol 1" is defined as the Martian solar day on which the lander touched (or will touch) down, and the day before landing is "Sol -1". (Obviously, then, there is no "Sol 0".) The time of day displayed starts ticking at local midnight immediately prior to landing, not at the actual time of landing. If you are displaying the time for an Earth date prior to landing, the time of day will instead read either "T.B.L." (to be launched) or "In Flight".

The mission time for the two Mars Exploration Rovers ticks away using the mean values for the Martian time units. However, the MER mission times are not the same as local mean solar time for the landing sites. Instead they are offset from local mean solar time by several minutes (and not by the same amount each) so that at the middle of the planned mission duration (three months each), the mission time and local true solar time would just about match up.

For Beagle 2 mission time, the time of day displayed is simply the local true solar time for the 269.5°W meridian.

We'll come back to discussing the graphic display settings tab below when we discuss the graphic display window.

Time Display Window

This window is divided into four quarters, grouped two above and two below. Let's take a look at the bottom two first.

The most prominent items in the bottom two quarters of the time display are the Earth time readouts, at left in UTC and at right using the user's local time zone, according to the choice selected in the Earth time settings tab. Below each of these is the corresponding date. Remember, both UTC and local time (and consequently the time on Mars) will not be correct unless your computer's time and time zone settings have been set correctly.

Below the Earth UTC time and date is the corresponding Modified Julian Day (MJD). The Julian Day (JD) is the number of days that have passed since noon on Jan. 1, 4713 BCE. This count is a very useful value in astronomy and is often used to indicate the dates of astronomical events and observations, especially those which predate use of the modern Gregorian calendar. However, in the 20th and 21st centuries, the Julian Day is a large and unwieldy number, so we subtract 2,400,000.5 to obtain the easier-to-use Modified Julian Day, the number of days since midnight (UTC) starting Nov. 17, 1858.

Below the user's local time and date is the light distance between Earth and Mars, i.e., the amount of time a photon would take to traverse the distance between the two planets. Depending on the current relative orbital positions of the two planets, this value ranges between 3 and 22 minutes.

At the top of the time display window are the Mars time read-outs. If you have selected the default choice in the settings, then just as for Earth the top left will show the "standard Mars time" and the top right a local time. The latter is determined by the location you specified in the Mars time settings tab.

The "standard Mars time" is given using the designation "MTC", or Coordinated Mars Time. In analogy to Earth's UTC, this is the local mean solar time at Mars' prime meridian, which is defined by the location of the crater Airy-0.

As we noted above, the local Mars Time readout is given for a specific longitude and latitude and uses one of three different time formats which you may specify in the settings. The lon-lat chosen is shown immediately below the local time.

The upper half of the time display window also contains a few other numeric readouts. At left, below the MTC, are three items giving some information about the Mars "date". At present, there is no recognized Mars calendar so these data are shown in an alternative fashion. First, in analogy to the Julian Day on Earth, there is a readout for MSD, or Mars Sol Date, i.e., a count of the Mars days -- sols -- that have elapsed since humans began to make useful observations of the surface features of Mars in the late 19th century. MSD 0.0 is approximately the same moment in time as MJD 5521.5 (noon on Dec. 29, 1873) on Earth.

Below the MSD is a readout of the Areocentric Solar Longitude, denoted (LS), the angle that Mars has reached in its orbit relative to vernal (spring) equinox in the northern hemisphere. Below the LS value is an indication of the corresponding Martian. The abbreviation "NH" or "SH" which appears here stands for "northern hemisphere" or "southern hemisphere", as appropriate.

On the right side of the Mars time info, below the local time readout, are entries for the solar elevation and solar azimuth. For the location on Mars at which the local time is being calculated, these two values indicate the current position of the Sun, the elevation indicating its position above the horizon and the azimuth its position relative to due north.

The layout of the upper half of the time display, i.e., the sections displaying the time on Mars, will re-arrange itself if you have used the settings controls to specify a mission time display. The only change in what is actually displayed is that the local time and solar position are replaced by the appropriate lander mission time and that the solar position is omitted.

Graphic Display Window and Settings

The largest window displayed by Mars24 is the graphic display, which initially shows a sunclock of Mars. The other two types of plots which may shown in this window are an orbital position plot and a solar path plot. Use the menu at the top of the graphic display tab in the settings window to choose which plot to display.

Sunclock Display

The sunclock has three sets of options which may be variously specified using the controls in the graphic display tab. The first option is which global map projection to apply. At start, Mars24 defaults to using an orthographic projection, which renders a fairly realistic view of how Mars looks from space. The projection menu provides nine other choices, the two most popular of which are probably the equirectangular and Mollweide projections.

The second option in the sunclock settings is the location on which the map projection should be centered. The controls allow you to select from a menu of landmarks, including a number of landers and geographic sites, or to specify a particular longitude and latitude. Alternatively, you can shift-click on the map itself, and the map will re-center at location where you clicked. (Note: only the orthographic projection can be centered at a latitude off the equator.)

The third set of options in the sunclock settings is a table of checkboxes indicating which, if any, of a set of locations should be marked on the map. The first two checkboxes are to mark the "Subsolar point" and the "Sub-Earth point", by a yellow circle and blue circle, respectively. The subsolar point is the location on Mars for which the Sun is directly overhead. Likewise, the sub-Earth point the location at which Earth is directly overhead, or for an observer on Earth looking at Mars, it is the spot directly in the middle of the hemisphere in view.

If you are currently viewing the sunclock using the Equirectangular projection, the next checkbox provides the option to turn on/off marking of a longitude-latitude grid (at 30° graticule) on the map.

The next ten points of interest in the table are lander sites, in order the three 2003-2004 mission sites, the three past successful landers (Viking 1 and 2 and Pathfinder), and the four failed landers2. Following the landers is a selection of notable surface features.

When examining the sunclock, please keep in mind that the base map used to render this plot comes from a composite of Mars Global Surveyor imagery acquired during northern hemisphere winter, when the north polar ice cap was relatively large and the solar polar ice cap relatively small compared to their average extents. During southern hemisphere winter, the south polar cap becomes significantly larger, with a layer of frost extending almost all the way to the edge of Hellas Planitia.

Orbital Positions Display

The second graphic display option is a plot of the orbital positions of Mars and Earth. It has no options of its own in the settings panel. The outer ellipse in this plot indicates Mars' orbit, and the position of Mars itself is marked with an "M". The inner ellipse is Earth's orbit, with Earth's position marked by an "E".

The other markings on Mars and Earth's orbits in the positions plot indicate the locations of perihelion, marked "peri", and the northern hemisphere vernal (spring) equinox, marked "nhve". Perihelion is a planet's closest approach to the Sun; its aphelion, or farthest distance from the Sun, is indicated by a tickmark on the far side of the orbital elipse from perihihelion. Similarly, the start of the other seasons are indicated by tickmarks at 90° intervals from the vernal equinox mark.

You might find it interesting to use the Earth time settings to specify an Earth time and date in the morning of Aug. 27, 2003, and then examine the orbital positions plot. You'll see that Earth and Mars lie on the same line from the Sun, with Mars at its perihelion and Earth about 45° from its aphelion. This was the the Great Opposition of Aug 27., 2003, when Earth and Mars were the closest they had been in about 59,000 years. The light distance between the two planets on that date was just 3 minutes and 6 seconds.

Solar Path Display

The final graphic display is a plot of the path that the Sun traces in the sky during the current sol, as it would be seen from the location of Mars you have specified in the Mars time tab. The current position of the Sun is marked by a yellow circle; the small circles marked along the path indicate where the Sun will be at intervals of one Mars-hour.

When you first display the solar path plot, it is centered looking south (note that the vertical line at center is marked by an "S"). If the location on Mars for which you are examining the solar path is in the southern hemisphere, you may wish to use the display control to re-center this plot to look north.

The solar path display also includes a collection of numerical readouts indicating at what time true midnight, sunrise, true noon, and sunset occur during the sol. Please note that the determination of sunrise and sunset does not adjust for any refraction of sunlight by Mars' atmosphere, nor is there any accounting for local topography at the location selected.

However, the determination used in Mars24 of sunrise and sunset does account for the fact that the Sun is not a pinpoint light source but has, on average, an apparent radius of 0.175° as seen from Mars. (It ranges from 0.193° at perihelion to 0.160° at aphelion.) Denoting sunrise as the time when the limb of the Sun reaches the horizon, then on average sunrise begins when the center of the Sun is 0.175° below the horizon.

Drawing the solar path and calculating the times of sunrise and sunset relies on specification of correct planetographic latitude and longitude.


Footnotes

1. You can look up the coordinates of various features on Mars using the US Geological Survey Gazetteer of Planetary Nomenclature's Mars page. Be sure to reference the index which uses planetographic latitude with west longitude.

2. Following is the roster of lander missions and few notes about each. Again, all locations are in planetographic coordinates.

2003-2004 Lander:

Past Successful Landers:

Past Unsuccessful Landers:

There is also a NASA Mars Scout lander called Phoenix planned for touchdown in May 2008. It is expected to land in the Vastitas Borealis, in the vicinity of 70°N 120°W..


The current version of Mars24 may be found at www.giss.nasa.gov/tools/mars24/

Mars24 was written by Dr. Robert B. Schmunk. If you wish to be notified when new versions of Mars24 are released or if you would like to report a bug, please contact:

Robert B. Schmunk
NASA Goddard Institute for Space Studies
2880 Broadway
New York, NY 10025 USA
rschmunk@giss.nasa.gov

Technical questions related to the definition of Mars Solar Time and the mathematical algorithms adopted for its calculation by Mars24 may be directed to:

Michael D. Allison
NASA Goddard Institute for Space Studies
2880 Broadway
New York, NY 10025 USA
mallison@giss.nasa.gov