Saturday, December 22, 2007

Breaking News: Asteroid Threatens to Hit Mars

Science@NASA has a short article on an asteroid that may hit Mars next January 30th. The asteroid, if it does hit Mars, is expected to create a crater about one kilometer wide. CNN International, which aired this story last night, suggested (with tongue firmly in cheek) that we call on Bruce Willis.

Also, be sure to click on the animation link below.

December 21, 2007: Astronomers funded by NASA are monitoring the trajectory of an asteroid named 2007 WD5 that is expected to cross the orbital path of Mars early next year. Calculations by NASA's Near-Earth Object Office at the Jet Propulsion Laboratory indicate that the 164-ft wide asteroid may pass within 30,000 miles of Mars at about 6 a.m. EST on Jan. 30, 2008.

"Right now asteroid 2007 WD5 is about half-way between the Earth and Mars and closing the distance [to Mars] at a speed of about 27,900 miles per hour," said Don Yeomans, manager of the Near Earth Object Office at JPL.

Above: This artist rendering uses an arrow to show the predicted path of the asteroid on Jan. 30, 2008. The orange swath indicates the area it is expected to pass through. Mars may or may not be in the asteroid's path. Image credit: NASA/JPL. [Animation]

There is a 1-in-75 chance of 2007 WD5 hitting Mars; researchers can't be more confident than that because of uncertainties in the asteroid's orbit. If this unlikely event were to occur, however, the strike would happen somewhere within a broad swath across the planet north of where the Opportunity rover is.

"We estimate such impacts occur on Mars every thousand years or so," said Steve Chesley, a scientist at JPL. "If 2007 WD5 were to thump Mars on Jan. 30, we calculate it would hit at about 30,000 miles per hour and might create a crater more than half-a-mile wide." The Mars Rover Opportunity is currently exploring a crater approximately this size.

Such a collision could release about three megatons of energy. Scientists believe an event of comparable magnitude occurred here on Earth in 1908 in Tunguska, Siberia, but no crater was created. The object was disintegrated by Earth's atmosphere before it hit the ground, although the air blast devastated a large area of unpopulated forest. The Martian atmosphere is much thinner than Earth's so a similar sized impactor would be more likely to reach the ground.

Asteroid 2007 WD5 was first discovered on Nov. 20, 2007, by the NASA-funded Catalina Sky Survey and put on a "watch list" because its orbit passes near the Earth. Further observations from both the NASA-funded Spacewatch at Kitt Peak, Ariz., and the Magdalena Ridge Observatory in New Mexico gave scientists enough data to determine that the asteroid was not a danger to Earth, but could potentially impact Mars.

Because the asteroid has been tracked for little more than a month, there is still some uncertainly about the path it will take. "Over the next five weeks, we hope to gather more information from observatories so we can further refine the asteroid's trajectory," says Yeomans. More data could eliminate or confirm the possibility of an impact.

Update: This news actually came out back on January 9th, but I only noticed the information now (January 28th). The potential collision of Asteroid 2007 WD5 with Mars has been "effectively ruled out" by NASA's Near Earth Object Program:

Since our last update, we have received numerous tracking measurements of asteroid 2007 WD5 from four different observatories. These new data have led to a significant reduction in the position uncertainties during the asteroid's close approach to Mars on Jan. 30, 2008. As a result, the impact probability has dropped dramatically, to approximately 0.01% or 1 in 10,000 odds, effectively ruling out the possible collision with Mars.

Our best estimate now is that 2007 WD5 will pass about 26,000 km from the planet's center (about 7 Mars radii from the surface) at around 12:00 UTC (4:00 am PST) on Jan. 30th. With 99.7% confidence, the pass should be no closer than 4000 km from the surface.

Friday, December 14, 2007

MARSIS and Subsurface Geology

One of the purposes of the MARSIS instrument is to probe Mars' subsurface geology to a depth of five kilometers. To do this, MARSIS sends low-frequency radio waves down to the surface and records the echoes that have bounced back to Mars Express. In November 2005, the European Space Agency (ESA) reported that the MARSIS team had discovered buried impact craters and hints of the presence of deep underground water ice.

Credit: ASI/NASA/ESA/Univ. of Rome/JPL

First results revealed an almost circular structure, about 250 kilometers in diameter, shallowly buried under the surface of the northern lowlands of Chryse Planitia (see the map below). Scientists have interpreted it as a buried basin of impact origin. Echo structures, as shown in the radargram images above, form a distinctive collection that include parabolic arcs and an additional planar reflecting feature parallel to the ground, 160 km long. The images were taken in two different orbits, spaced about 50 km apart.

Credit: ASI/NASA/ESA/Univ. of Rome/JPL/MOLA

The topographic map, based on Mars Orbiter Laser Altimeter (MOLA) data, shows the MARS Express groundtracks and the arc structures detected by MARSIS that are interpreted to be part of the buried impact basin. The topographic relief represented in the image is 1 km, from low (purple) to high (red). The projected arcs are shown in red for orbit 1892 and white for orbit 1903. There is no obvious feature in the surface topography that corresponds to the buried feature identified with MARSIS data.

The parabolic arcs correspond to ring structures that could be interpreted as the rims of one or more buried impact basins. Other echoes show what may be rim-wall 'slump blocks' or 'peak-ring' features. The planar reflection is consistent with a flat interface that separates the floor of the basin, situated at a depth of about 1.5 to 2.5 km, from a layer of overlying different material. It is possible that this planar feature is a low-density, water-ice-rich material at least partially filling the basin.

Credit: ASI/NASA/ESA/Univ. of Rome/JPL/MOLA Science Team

MARSIS also probed the layered deposits that surround the north pole of Mars, in an area between 10º and 40º East longitude. The interior layers and the base of these deposits are poorly exposed. Prior interpretations could only be based on imaging, topographic measurements and other surface techniques. However, MARSIS results (above) show two strong and distinct echoes coming from the area corresponding to a surface reflection and subsurface interface between two different materials.

The MARSIS radargram image (top) shows data from the subsurface of Mars in the layered deposits that surround the north pole. The lower image shows the position of the ground track on a topographic map of the area based on MOLA data. The total elevation difference shown in the topographic map is about 2 km, between the lowest surface (magenta) and the highest (orange) over an area 458 km wide.

The MARSIS echo trace splits into two traces to the right of center, at the point where the spacecraft's groundtrack crosses from the smooth plains onto the elevated layered deposits on the right. The upper trace is the echo from the surface of the deposits, while the lower trace is interpreted to be the boundary between the lower surface of the deposits and the underlying material, believed to be basaltic regolith. The strength of the lower echo suggests that the intervening material is nearly pure water ice. The time delay between the two echoes reaches a maximum of 21 microseconds at the right of the image, corresponding to a thickness of 1.8 km of ice. This conclusion appears to rule out the hypothesis of a melt zone at the base of the northern layered deposits.

Wednesday, December 12, 2007


Credit: ESA

Continuing with our discussion of instruments aboard Mars Express:

MARSIS, the Mars Advanced Radar for Subsurface and Ionosphere Sounding, was developed by the University of Rome, Italy, in partnership with NASA’s Jet Propulsion Laboratory (JPL). It is the first instrument to actually look below the surface of Mars, using low-frequency microwaves reflected by the different layers of matter. Its primary objective is to map the distribution of water, both liquid and solid (ice), in the upper portion of the crust of Mars; the instrument is also designed to probe Mars' subsurface geology and to measure the planet's ionosphere. MARSIS consists of three antennas: two "dipole" booms 20 meters long, and one seven-meter "monopole" boom oriented perpendicular to the first two. The instrument works by sending a coded stream of radio waves towards Mars at night, and analyzing their distinctive echoes. From this, scientists can then make deductions about the surface and subsurface structure. Operations are conducted on both Mars' day-side and night-side. The night-side is for deep subsurface sounding: during the night the ionosphere of Mars does not interfere with the lower-frequency signals needed by the instrument to penetrate the planet's surface, down to a depth of five kilometers. Day-side operations use higher frequency radio waves, which allows MARSIS to conduct shallow probing of the subsurface and atmospheric sounding. The MARSIS operation altitudes are up to 800 kilometers for subsurface sounding and up to 1200 kilometers for studying the ionosphere.

The extension of the three MARSIS booms was originally planned to deploy in April 2004. However, computer simulations pointed to a risk that the booms could lash back and harm the spacecraft and its instruments during deployment. The ESA then delayed deployment until the boom supplier (JPL) and the spacecraft prime contractor (Astrium, France), together with ESA’s experts, had conducted further analyses and simulations of the boom behavior during deployment and the possible impact on the spacecraft. Once the magnitude of the risk involved had been assessed and the relevant mitigation scenarios defined, ESA decided to proceed with releasing the MARSIS antennas in May 2005. Deployment of the first boom was started on May 5, 2005. At first, there was no indication of any problems, but later it was discovered that one segment of the boom did not lock. Using the Sun's heat to expand the segments of the MARSIS antenna, the last segment locked in successfully on May 10th. The second 20-meter boom was successfully deployed on June 14th, and the third boom on June 17th. On June 22nd, the ESA announced that MARSIS was fully operational, and the instrument began science operations on July 4th.

The above drawing is an impression of the completely deployed MARSIS experiment on board ESA's Mars Express orbiter with the two 20-meter and one 7-meter booms sprung out and locked into place.

Sunday, December 9, 2007

SPICAM Detects Ozone on the Earth

Credit: ESA/CNRS Verrieres

On July 3, 2003, thirty-one days after launch and from a distance of about 7 million km, the SPICAM instrument on board Mars Express was turned toward Earth. The main scientific objective of SPICAM is to observe both ozone and water vapor in the atmosphere of Mars. This test was to see how well SPICAM could detect ozone on Earth. Here, ozone forms a natural screen that protects life on Earth from harmful ultraviolet (UV) solar radiation. However, on Mars, the quantity of ozone is about 100 times less than that on Earth, making survival on the surface of Mars very difficult for any lifeforms.

In this first graph, above, the red line shows the simulated results that would be expected from SPICAM if the Earth's atmosphere had no ozone; i.e., if the atmosphere was not able to absorb ultraviolet radiation. The blue line is the simulated results with a realistic Earth atmosphere containing ozone.

The second graph shows the actual results obtained by SPICAM. The blue line indicates the spectrum (the intensity of light as a function of wavelength) of the Earth in ultraviolet light. This light comes from solar light scattered by the atmosphere of the Earth back to outer space. Only the peak just above 300 nm is significantly above the level of "noise." The red line, on the other hand, is the spectrum of the Sun in ultraviolet light. This is the amount of ultraviolet light the Earth would receive if the Earth didn't have the thick atmosphere with ozone that it has. (Conversely, the red line indicates the amount of ultraviolet radiation the moon does receive as, of course, the moon has no atmosphere to protect it.) The difference between the two lines, then, is the amount of ultraviolet radiation the Earth's atmosphere absorbs. As you can see, the ozone layer absorbs most of the ultraviolet radiation below 300 nm, preventing harmful effects on the DNA molecules of all living species exposed to the Sun.

As Jean-Loup Bertaux, principal investigator from Service d'Aéronomie/IPSL (the agency responsible for SPICAM), drolly said, "Together with the OMEGA infrared spectrometer detection of water vapor and oxygen, the detection of copious amounts of ozone in the atmosphere indicates that this planet that we call Earth could sustain life."

Saturday, December 8, 2007

SPICAM and the Aurora at Terra Cimmeria

Credit (Map): NASA/MGS
Credit (Photo): NASA/ESA

Another instrument aboard Mars Express is SPICAM (Spectroscopy for the Investigations and the Characteristics of the Atmosphere on Mars), developed by the Service d'Aéronomie du CNRS/IPSL (Verrières-le-Buisson, France) ,the Belgian Institute for Space Aeronomy (BIRA-IASB; Brussels, Belgium), and the Space Research Institute of the Russian Academy of Sciences (IKI; Moscow, Russia). SPICAM was originally on board the ill-fated Mars 96. However, a new instrument was placed aboard Mars Express, and a similar instrument (SPICAV) is on the European Space Agency's (ESA) Venus Express.

SPICAM is a lightweight (4.7 kg) ultraviolet-infrared dual spectrometer dedicated primarily to the study of the atmosphere of Mars. SPICAM makes measurements of the Martian atmosphere mainly through stellar and solar occultations. Through this method, SPICAM can make measurements of the Martian atmosphere's chemistry, the atmosphere's structure and dynamics (including its density and temperature) through vertical profiles, measurements of aerosols and dust particles and their vertical distribution, and measurements of the ionosphere and the rate of escape of atmospheric molecules. The ultraviolet sensor also measures the level of ozone in the Martian atmosphere, and the infrared sensor measures water vapor.

On August 14, 2004, SPICAM detected a new type of aurora on Mars. On Earth and among the gas giants (Jupiter, Saturn, Uranus, and Neptune), aurorae occur along the planetary magnetic field lines near the poles, and are produced by charged particles (electrons, protons and ions) precipitating along those lines. Venus also produces aurorae, although of a different type. Because Venus has no "intrinsic" (planetary) magnetic field, Venusian aurorae appear as bright and diffuse patches of varying shape and intensity, sometimes distributed across the full planetary disc. Venusian aurorae are produced by the impact of electrons originating from the solar wind and precipitating in the night-side atmosphere.

Like Venus, Mars is a planet with no intrinsic magnetic field. However, it was suggested that Mars could have aurorae as well, and this hypothesis was reinforced by Mars Global Surveyor's discovery of crustal magnetic anomalies, most likely the remnants of an old planetary magnetic field.

SPICAM detected light emissions in the southern hemisphere on Mars, in Terra Cimmeria, during night-time observations in the region of 177º East, 52º South. The total size of the emission region was about 30 km across, and possibly about 8 km high. By analyzing the map of crustal magnetic anomalies compiled with Mars Global Surveyor’s data, scientists observed that the region of the emissions corresponds to the area where the strongest magnetic field is localized. This correlation indicates that the origin of the light emission actually was a flux of electrons moving along the crust magnetic lines and exciting the upper atmosphere of Mars.

The above map shows the crustal magnetic field intensity in the Terra Cimmeria region. The aurora was located in an area where the crustal magnetic field is very strong (dark red in the image). The photo underneath is of the same region as the map.