[meteorite-list] Mars Spacecraft Reveal Comet Flyby Effects on Martian Atmosphere

[Ron Baalke]

November 7, 2014
     
Mars Spacecraft Reveal Comet Flyby Effects on Martian Atmosphere

Two NASA and one European spacecraft that obtained the first up-close 
observations of a comet flyby of Mars on Oct. 19, have gathered new 
information about the basic properties of the comet's nucleus and directly 
detected the effects on the Martian atmosphere.

Data from observations carried out by NASA's Mars Atmosphere and Volatile 
Evolution (MAVEN) mission, NASA's Mars Reconnaissance Orbiter (MRO), and a 
radar instrument on the European Space Agency's (ESA's) Mars Express 
spacecraft have revealed that debris from the comet added a temporary and 
very strong layer of ions to the ionosphere, the electrically charged layer 
high above Mars. In these observations, scientists were able to make a direct 
connection from the input of debris from a specific meteor shower to the 
formation of this kind of transient layer in response; that is a first on any 
planet, including Earth.

Comet C/2013 A1 Siding Spring traveled from the most distant region of our 
solar system, called the Oort Cloud, and made a close approach around 2:27 
p.m. EDT within about 87,000 miles (139,500 kilometers) of the Red Planet. 
This is less than half the distance between Earth and our moon and less than 
one-tenth the distance of any known comet flyby of Earth.

Dust from the comet impacted Mars and was vaporized high in the atmosphere, 
producing what was likely an impressive meteor shower. This debris resulted 
in significant temporary changes to the planet's upper atmosphere and 
possible longer-term perturbations. Earth-based and a host of space 
telescopes also observed the unique celestial object.

"This historic event allowed us to observe the details of this fast-moving 
Oort Cloud comet in a way never before possible using our existing Mars 
missions," said Jim Green, director of NASA's Planetary Science Division 
at the agency's Headquarters in Washington. "Observing the effects on 
Mars of the comet's dust slamming into the upper atmosphere makes me very 
happy that we decided to put our spacecraft on the other side of Mars at the 
peak of the dust tail passage and out of harm's way."

The MAVEN spacecraft, recently arrived at Mars, detected the comet encounter 
in two ways. The remote-sensing Imaging-Ultraviolet Spectrograph observed 
intense ultraviolet emission from magnesium and iron ions high in the 
atmosphere in the aftermath of the meteor shower. Not even the most intense 
meteor storms on Earth have produced as strong a response as this one. The 
emission dominated Mars' ultraviolet spectrum for several hours after the 
encounter and then "dissipated" over the next two days.

MAVEN also was able to directly sample and determine the composition of some 
of the comet dust in Mars' atmosphere. Analysis of these samples by the 
spacecraft's Neutral Gas and Ion Mass Spectrometer detected eight different 
types of metal ions, including sodium, magnesium and iron. These are the 
first direct measurements of the composition of dust from an Oort Cloud 
comet. The Oort Cloud, well beyond the outer-most planets that surround our 
sun, is a spherical region of icy objects believed to be material left over 
from the formation of the solar system.

Elsewhere above Mars, a joint U.S. and Italian instrument on Mars Express 
observed a huge increase in the density of electrons following the comet's 
close approach. This instrument, the Mars Advanced Radar for Subsurface and 
Ionospheric Sounding (MARSIS), saw a huge jump in the electron density in the 
ionosphere a few hours after the comet rendezvous. This spike occurred at a 
substantially lower altitude than the normal density peak in the Martian 
ionosphere. The increased ionization, like the effects observed by MAVEN, 
appears to be the result of fine particles from the comet burning up in the 
atmosphere.

MRO's Shallow Subsurface Radar (SHARAD) also detected the enhanced 
ionosphere. Images from the instrument were smeared by the passage of the 
radar signals through the temporary ion layer created by the comet's dust. 
SHARAD scientists used this smearing to determine that the electron density 
of the ionosphere on the planet's night side, where the observations were 
made, was five to 10 times higher than usual.

Studies of the comet itself, made with MRO's High Resolution Imaging Science 
Experiment (HiRISE) camera, revealed the nucleus is smaller than the expected 
1.2 miles (2 kilometers). The HiRISE images also indicate a rotation period 
for the nucleus of eight hours, which is consistent with recent preliminary 
observations by NASA's Hubble Space Telescope.

MRO's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) also 
observed the comet to see whether signs of any particular chemical 
constituents stood out in its spectrum. Team members said the spectrum 
appears to show a dusty comet with no strong emission lines at their 
instrument's sensitivity.

In addition to these immediate effects, MAVEN and the other missions will 
continue to look for long-term perturbations to Mars' atmosphere.

MAVEN's principal investigator is based at the University of Colorado's 
Laboratory for Atmospheric and Space Physics in Boulder, and NASA's Goddard 
Space Flight Center in Greenbelt, Maryland, manages the mission. NASA's Jet 
Propulsion Laboratory, a division of Caltech in Pasadena, manages the Mars 
Reconnaissance Orbiter. Mars Express is a project of the European Space 
Agency; NASA and the Italian Space Agency jointly funded the MARSIS 
instrument.

For more information about NASA's Mars missions, visit:

http://www.nasa.gov/mars

-end-

Dwayne Brown
Headquarters, Washington
202-358-1726
dwayne.c.brown at nasa.gov 

Guy Webster
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6278
guy.webster at jpl.nasa.gov 

Nancy Neal Jones/Elizabeth Zubritsky
Goddard Space Flight Center, Greenbelt, Md.
301-286-0039 / 301-614-5438
nancy.n.jones at nasa.gov  / elizabeth.a.zubritsky at nasa.gov