No Drought of Mars Landing Sites
Mars Exploration Rover 2003 Landing Sites
From the start, the Mars research community had to survey in detail more than 150 landing sites to touch down their 2003 Mars Exploration Rover (MER). Scheduled for launch in mid-2003 (May and June), the MER project’s twin landers and rovers will follow a similarly thrilling descent as the 1997 Mars Pathfinder. Parachutes and a rocket-braking, followed by airbag bounces, will determine the spacecraft’s final resting location. In preparation, the delicate balance must be struck between science and safety as the team narrows the choices for final explorer sites.
Using high-resolution orbital imagery from the Mars Orbiter Camera (or MOC) and Viking, the 150 sites were initially graded for safety and scientific wealth. The MOC takes images of the surface with a resolution as much as 50 times greater (as fine as meters per pixel) than the previously available pictures taken by the Viking spacecraft more than two decades ago. The many factors that influenced their decision-making included high winds, day-to-night temperature extremes, slopes, rockiness, and the hopes of finding ancient evidence of a fascinating geological and potentially water-filled past.
As scientists get more detailed views of the surface, Mars topology is increasingly viewed as a very active landscape–shaped by tornadoes, dust devils, regional and even global dust storms. Particularly bad during the summer and spring, dust can envelop the entire planet on a seasonal cycle of surface and atmospheric heating. As expected from such sweeping winds, fields of dunes and sandstorms have been observed as part of the contouring forces that fill in behind meteor impacts, cratering, and the Red Planet’s volcanic history.
Weather on Mars
High winds pose a dual challenge to mission planners. Because the crafts are scheduled to descend during the Martian afternoon (to maintain clearer Earth-communications), Martian winds can whip and rock the landers just when solar-heating and thus gusts reach their maximum. Like any landing airplane, the winds make true height detection a difficult task.
But any side-to-side swings also pose an additional risk, since the rocket-braking can potentially add rather than subtract from the descent-speed if the landers pitch.
Once safely grounded, weather prediction takes on much importance for mission planners. If an unexpected storm visited the landing site, many of which can last a few days to a couple of weeks regionally without pause, then the mission has to negotiate power, communications and general hardware safety over the length of its expected 90 day life. Interestingly, a 2001 global dust storm on Mars is often compared to the effects of the 1991 Mt. Pinatubo eruption on Earth, because of changes in atmospheric aerosols, temperature changes and planet-wide effects from a single event.
Mars undergoes not only an extreme seasonal range of winds and temperatures, but also a rapid heating and cooling cycle during a single day. It is not uncommon to find a 100 degree F (60 C) difference between high and low temperature on the same Martian day (or sol). Away from the equator, maximum daytime temperatures reach only -22 F (-30°C), while, on the equator, this can rise to over 72 F (22°C). But daytime is nothing compared to the frigid nights on Mars. Because the thin atmosphere is a poor heat retainer, in even the warmest of places night-time temperatures can fall as low as -103 F to -148 F (-75 to -100°C). State-of-the-art insulation is needed to keep the sensitive electronics warm while still not bulking up the mass or hindering the mobility of the rover and lander twins.
The mission planning for a Mars lander has to guess at local conditions years in advance, and compensate for swings that might daunt even the most intrepid Earth-bound weather station.
The Envelope Please
From consideration of the 150 candidate sites, the MER teams appear to have narrowed their choices to four (called Hematite, Gusev, Isidis, and Elysium). All four are within15 degrees of the equator. Two of their science targets highlight the possibilities to explore some of the newly-found sediments and ancient water-eroded areas.
Terra Meridiani: Testing the Wet Hypothesis
One candidate site (Hematite) is located in a larger region called Terra Meridiani (located 3 S-0 N latitude, 352 E to 1 E longitude). Extending for hundreds of miles in extent, Terra Meridiani has rocky outcrops that might roughly be compared to the terrain of parts of Utah or northern Arizona. The patchwork of bright and dark regions seems to be scarred by all the potentially interesting epochs on Mars as layered views into its past: volcanism, sedimentation, wind erosion, and early crustal history. But remarkably free from cratering, many Mars scientists have wondered what forces might have shaped the unique orbital views of this equatorial region. The relative absence of fresh impact craters points to a geologically young area. So un-Mars-like is the selected area, that those MOC scientists who have seen Mars daily at the scale of a small school bus have remarked: "If I’d seen this landscape used in a movie about Mars five years ago, I’d have said the director had no clue what Mars is supposed to look like."
But foremost, it is Terra Meridiani’s rich source of a particular grey crystalline mineral called hematite that caught the attention of scientists. Considered one of the key discoveries of the Mars Global Surveyor mission, hematite detection has revealed the grey crystals mainly concentrated around Terra Meridiani. On Earth, hematite is almost always formed in ways that require aqueous fluids and groundwater leaching to give it the unique crystalline qualities. But in some terrestrial examples (as unearthed in Chile and Mexico), iron-rich fluids can also give the same layered effects as seen on Mars, even without water. Indeed the May 2002 publication by Brian Hynek, et al. (Wash Univ, St. Louis) in the Journal of Geophysical Research (PDF) concluded that the mineral may have originated from oxidation (or rusting) of lava outflows, followed by precipitation and deposit from fluid flow much later.
So unraveling the climate history of Mars will depend on understanding the complex interplay between its ancient volcanism and water balance–a ‘wet’ hypothesis to be tested actively at Terra Meridiani because of its rich hematite deposits.
|(Click to view details) Terra Meridiani, hematite-rich and a probable test for the Mars’ wet hypothesis
Credit: Malin Space Systems/ARC/JPL/NASA
Gusev Crater: Ancient Lakebed?
A further interesting candidate, called Gusev Crater (located at 14.6°S, 184.6°W), appears to be the site of an ancient lakebed. In Greek, Gusev refers to ‘cup’. Since the early Mariner and Viking images, the crater and adjoining valley first revealed a site rich to explore further in search of sedimentary deposits. The crater stretches approximately 150 kilometers (93 miles) across. Ma’adim Vallis, in the martian southern cratered uplands, is one of the largest valley networks on the planet, and first became classified as a runoff channel in 1975 (Sharp, R. P., and M. C. Malin, 1975, Channels on Mars. Geol. Soc. Am. Bull., v. 86, p. 593-609). Ma’adim Valley, is over 900 km long and named after the Arabic name for the Red Planet.
One of the first things noticed about the crater was its marked asymmetry–perhaps a legacy of a half-filled shoreline or pond concentrated on a tilt. Nathalie Cabrol and colleagues at NASA Ames Research Center, the Vernadksy Institute in Moscow, and Arizona State University published in 1998 (Icarus) their study of the valley and impact crater on Mars which together point to a prolonged history of water-related activity. The researchers established a sequence of events for the Ma’adim Vallis/Gusev crater area that included flowing water, ponding, and sedimentation over a period of a couple of billion years. Because of the sheer variety of smooth terraces and layering, water in Gusev Crater might have ponded around 1.8 billion years ago–following a huge impact that formed the crater and rim (about 3.5 billion years ago). This natural collector basin supported the Ma’adim Vallis sediment, which could be hundreds of meters thick.
Such a varied history makes Gusev crater a prominent depositional site and, a key location for future astrobiological explorations on Mars. In 1995, Gusev crater was included in NASA’s report, "An Exobiological Strategy for Mars Exploration" (NASA Publication SP-530) as a priority site for future biological exploration.
One enhanced feature of the MER mission plan is more mobility for the rovers. These bigger Mars Exploration Rovers (MER) can trek up to a football field–330 feet (100 meters)– per Martian day. Making remote manuevers over those distances means getting very good topological maps, while knowing where every interesting rock or hazard might tip and block the rovers’ paths. Seen globally, the darker areas on Mars are generally more rocky while the bright areas are dusty, but a much enhanced topography goes into site selection beforehand, and then much later after landing to roam the surface. Potentially hundreds or thousands of pebbles and boulders can pock mark a landing site on the scale of a 100 yards per day. In total, the football field milestone is almost as far in one Martian day as the 1997 Sojourner rover did over its entire, many-month-long lifetime. Starting in January 2004, MER surface operations will last for at least 90 Martian days, or longer if hardware health is maintainable.
Once an interesting target is identified on the ground, the Mars Rovers’ will employ what is their primary science payload, a collection of 5 instruments (and a rock abrasion tool) called the Athena package. Mission planners look forward to even more close-up views of the two primary sites slated for the early 2004 rendezvous.
Cabrol, N. A., E. A. Grin, R. Landheim, R. O. Kuzmin, R. Greeley, 1998, Duration of the Ma’adim Vallis/Gusev Crater Hydrogeologic System, Mars. Icarus, v. 133, p. 98-108.