Athabasca Valles Region: New Insights from THEMIS

1. Athabasca Valles Region: New Insights from THEMIS A. McEwen, L. Keszthelyi, M. Milazzo, D. Burr, P. Christensen, J. Rice, M. Malin, THEMIS team AGU, Dec. 9, 2002

2. Regional Map of SE Elysium

3. Athabasca Valles • Youngest outflow channels and large-scale volcanism on Mars • Was a candidate landing site for MER: • Possibly too rough at decimeter scales • Good MOC and THEMIS coverage • What are the ages and interrelations of recent floods and volcanism?

4. THEMIS daytime thermal IR mosaic. Dark = cold = rocky

5. Visible images have low contrast due to thin dust cover, but pre-dawn and afternoon thermal IR images have high contrast due to thermal inertia variations -- sees through thin dust coatings.

6. Nighttime THEMIS IR mosaic. • No topo-graphic shading • Inverse of afternoon temperature patterns • Dominated by thermal inertia

7. Daytime (left) vs. nighttime (right) • Cross-cutting relations: • 1.lavas • 2: cut E channel • 3. Lava in E channel • 4. Cut W channel • 5. Lava in W channel

8. 1. Bright dunes Morphologies seen in MOC images are strongly correlated with nighttime temperatures (thermal inertia). 2. Lava flows 3. Fluvial features 4. Secondary crater rays

9. Bright dunes have low thermal inertia: Lightly sintered dust or tephra? Nighttime IR

10. Lava flows: plates have moderate thermal inertia, polygonal ground between plates has high thermal inertia. MOC, ~1.5 km wide MOC, 3 km wide THEMIS, night

11. MOC compared with THEMIS: Fluvial Features THEMIS nighttime MOC, 3 km wide

12. MOC, 1.5 km wide Small bright-rayed craters occur in streaks and ejecta has low thermal inertia.

13. Rayed craters imaged at 1.5 m/pixel: craters are irregular and eject blankets are wind-eroded. (Craters superimposed over channel floor)

14. Bright ejecta may blend into visible background, but are still colder at night.

15. Regional distribution of crater rays (red) shows a systematic pattern, radial to a location ~400 km to the southeast.

16. A very fresh 10-km diameter crater at the center of the rays! • Pristine morphology; no superimposed craters. • Flow ejecta suggests water ice within a few hundred meters. • (THEMIS visible image at 18 m/pixel.)

17. Secondary crater chains

18. 10-km crater on Mars with 800-km rays! • 800-km rays are plausible: requires ejection at 1-3 km/sec. (Martian meteorites exceeded escape velocity of 5 km/sec.) • Youngest crater of this size on Mars? How old is this crater? • Expect ~1 crater 10 km or larger per 106 years on Mars (Ivanov, 2001). • Crater rays are thin and ephemeral and Mars has an active surface, so it may be << 106 years. • Crater is present in Viking images. • From 25 yrs to a few million years old.

19. What is the bright ray material? • Can’t be densely welded or cemented to have low thermal inertia (implies particle sizes of tens of microns). • Bright low-inertia dunes: must be sintered into particles large enough to saltate. • Bright dunes material may come from rays. • Lightly sintered dust or fine tephra?

20. Implications for formation of small craters on Mars • Most craters on youngest lavas of Athabasca Valles and Cerberus Plains are secondaries from this single event, so either • 1. Atmosphere screens out primary bodies (5-30 km/s) much more effectively than secondaries (1-5 km/s) • 2. Primary population includes fewer small bodies than expected. • 3. This impact was especially recent.

21. Athabasca Valles Geologic History • Lava flows emplaced from Elysium Mons, now moderately cratered. • Wrinkle ridges formed. • Recent eruption of lava from Cerberus Fossae. • Aqueous floods from Cerberus Fossae cut older lavas and carved and recarved channels. • Floods followed soon by more lavas from Cerberus Fossae; many rootless cones in channels. • Cerberus Fossae reactivated. • Rays of secondary craters formed.

22. THEMIS is fantastic for geology!